| //! Candidate selection. See the [rustc guide] for more information on how this works. |
| //! |
| //! [rustc guide]: https://rust-lang.github.io/rustc-guide/traits/resolution.html#selection |
| |
| use self::EvaluationResult::*; |
| use self::SelectionCandidate::*; |
| |
| use super::coherence::{self, Conflict}; |
| use super::project; |
| use super::project::{normalize_with_depth, Normalized, ProjectionCacheKey}; |
| use super::util; |
| use super::DerivedObligationCause; |
| use super::Selection; |
| use super::SelectionResult; |
| use super::TraitNotObjectSafe; |
| use super::{BuiltinDerivedObligation, ImplDerivedObligation, ObligationCauseCode}; |
| use super::{IntercrateMode, TraitQueryMode}; |
| use super::{ObjectCastObligation, Obligation}; |
| use super::{ObligationCause, PredicateObligation, TraitObligation}; |
| use super::{OutputTypeParameterMismatch, Overflow, SelectionError, Unimplemented}; |
| use super::{ |
| VtableAutoImpl, VtableBuiltin, VtableClosure, VtableFnPointer, VtableGenerator, VtableImpl, |
| VtableObject, VtableParam, VtableTraitAlias, |
| }; |
| use super::{ |
| VtableAutoImplData, VtableBuiltinData, VtableClosureData, VtableFnPointerData, |
| VtableGeneratorData, VtableImplData, VtableObjectData, VtableTraitAliasData, |
| }; |
| |
| use dep_graph::{DepKind, DepNodeIndex}; |
| use hir::def_id::DefId; |
| use infer::{InferCtxt, InferOk, TypeFreshener}; |
| use middle::lang_items; |
| use mir::interpret::GlobalId; |
| use ty::fast_reject; |
| use ty::relate::TypeRelation; |
| use ty::subst::{Subst, Substs}; |
| use ty::{self, ToPolyTraitRef, ToPredicate, Ty, TyCtxt, TypeFoldable}; |
| |
| use hir; |
| use rustc_data_structures::bit_set::GrowableBitSet; |
| use rustc_data_structures::sync::Lock; |
| use rustc_target::spec::abi::Abi; |
| use std::cmp; |
| use std::fmt; |
| use std::iter; |
| use std::rc::Rc; |
| use util::nodemap::{FxHashMap, FxHashSet}; |
| |
| pub struct SelectionContext<'cx, 'gcx: 'cx + 'tcx, 'tcx: 'cx> { |
| infcx: &'cx InferCtxt<'cx, 'gcx, 'tcx>, |
| |
| /// Freshener used specifically for entries on the obligation |
| /// stack. This ensures that all entries on the stack at one time |
| /// will have the same set of placeholder entries, which is |
| /// important for checking for trait bounds that recursively |
| /// require themselves. |
| freshener: TypeFreshener<'cx, 'gcx, 'tcx>, |
| |
| /// If `true`, indicates that the evaluation should be conservative |
| /// and consider the possibility of types outside this crate. |
| /// This comes up primarily when resolving ambiguity. Imagine |
| /// there is some trait reference `$0: Bar` where `$0` is an |
| /// inference variable. If `intercrate` is true, then we can never |
| /// say for sure that this reference is not implemented, even if |
| /// there are *no impls at all for `Bar`*, because `$0` could be |
| /// bound to some type that in a downstream crate that implements |
| /// `Bar`. This is the suitable mode for coherence. Elsewhere, |
| /// though, we set this to false, because we are only interested |
| /// in types that the user could actually have written --- in |
| /// other words, we consider `$0: Bar` to be unimplemented if |
| /// there is no type that the user could *actually name* that |
| /// would satisfy it. This avoids crippling inference, basically. |
| intercrate: Option<IntercrateMode>, |
| |
| intercrate_ambiguity_causes: Option<Vec<IntercrateAmbiguityCause>>, |
| |
| /// Controls whether or not to filter out negative impls when selecting. |
| /// This is used in librustdoc to distinguish between the lack of an impl |
| /// and a negative impl |
| allow_negative_impls: bool, |
| |
| /// The mode that trait queries run in, which informs our error handling |
| /// policy. In essence, canonicalized queries need their errors propagated |
| /// rather than immediately reported because we do not have accurate spans. |
| query_mode: TraitQueryMode, |
| } |
| |
| #[derive(Clone, Debug)] |
| pub enum IntercrateAmbiguityCause { |
| DownstreamCrate { |
| trait_desc: String, |
| self_desc: Option<String>, |
| }, |
| UpstreamCrateUpdate { |
| trait_desc: String, |
| self_desc: Option<String>, |
| }, |
| } |
| |
| impl IntercrateAmbiguityCause { |
| /// Emits notes when the overlap is caused by complex intercrate ambiguities. |
| /// See #23980 for details. |
| pub fn add_intercrate_ambiguity_hint<'a, 'tcx>( |
| &self, |
| err: &mut ::errors::DiagnosticBuilder<'_>, |
| ) { |
| err.note(&self.intercrate_ambiguity_hint()); |
| } |
| |
| pub fn intercrate_ambiguity_hint(&self) -> String { |
| match self { |
| &IntercrateAmbiguityCause::DownstreamCrate { |
| ref trait_desc, |
| ref self_desc, |
| } => { |
| let self_desc = if let &Some(ref ty) = self_desc { |
| format!(" for type `{}`", ty) |
| } else { |
| String::new() |
| }; |
| format!( |
| "downstream crates may implement trait `{}`{}", |
| trait_desc, self_desc |
| ) |
| } |
| &IntercrateAmbiguityCause::UpstreamCrateUpdate { |
| ref trait_desc, |
| ref self_desc, |
| } => { |
| let self_desc = if let &Some(ref ty) = self_desc { |
| format!(" for type `{}`", ty) |
| } else { |
| String::new() |
| }; |
| format!( |
| "upstream crates may add new impl of trait `{}`{} \ |
| in future versions", |
| trait_desc, self_desc |
| ) |
| } |
| } |
| } |
| } |
| |
| // A stack that walks back up the stack frame. |
| struct TraitObligationStack<'prev, 'tcx: 'prev> { |
| obligation: &'prev TraitObligation<'tcx>, |
| |
| /// Trait ref from `obligation` but "freshened" with the |
| /// selection-context's freshener. Used to check for recursion. |
| fresh_trait_ref: ty::PolyTraitRef<'tcx>, |
| |
| previous: TraitObligationStackList<'prev, 'tcx>, |
| } |
| |
| #[derive(Clone, Default)] |
| pub struct SelectionCache<'tcx> { |
| hashmap: Lock< |
| FxHashMap<ty::TraitRef<'tcx>, WithDepNode<SelectionResult<'tcx, SelectionCandidate<'tcx>>>>, |
| >, |
| } |
| |
| /// The selection process begins by considering all impls, where |
| /// clauses, and so forth that might resolve an obligation. Sometimes |
| /// we'll be able to say definitively that (e.g.) an impl does not |
| /// apply to the obligation: perhaps it is defined for `usize` but the |
| /// obligation is for `int`. In that case, we drop the impl out of the |
| /// list. But the other cases are considered *candidates*. |
| /// |
| /// For selection to succeed, there must be exactly one matching |
| /// candidate. If the obligation is fully known, this is guaranteed |
| /// by coherence. However, if the obligation contains type parameters |
| /// or variables, there may be multiple such impls. |
| /// |
| /// It is not a real problem if multiple matching impls exist because |
| /// of type variables - it just means the obligation isn't sufficiently |
| /// elaborated. In that case we report an ambiguity, and the caller can |
| /// try again after more type information has been gathered or report a |
| /// "type annotations required" error. |
| /// |
| /// However, with type parameters, this can be a real problem - type |
| /// parameters don't unify with regular types, but they *can* unify |
| /// with variables from blanket impls, and (unless we know its bounds |
| /// will always be satisfied) picking the blanket impl will be wrong |
| /// for at least *some* substitutions. To make this concrete, if we have |
| /// |
| /// trait AsDebug { type Out : fmt::Debug; fn debug(self) -> Self::Out; } |
| /// impl<T: fmt::Debug> AsDebug for T { |
| /// type Out = T; |
| /// fn debug(self) -> fmt::Debug { self } |
| /// } |
| /// fn foo<T: AsDebug>(t: T) { println!("{:?}", <T as AsDebug>::debug(t)); } |
| /// |
| /// we can't just use the impl to resolve the <T as AsDebug> obligation |
| /// - a type from another crate (that doesn't implement fmt::Debug) could |
| /// implement AsDebug. |
| /// |
| /// Because where-clauses match the type exactly, multiple clauses can |
| /// only match if there are unresolved variables, and we can mostly just |
| /// report this ambiguity in that case. This is still a problem - we can't |
| /// *do anything* with ambiguities that involve only regions. This is issue |
| /// #21974. |
| /// |
| /// If a single where-clause matches and there are no inference |
| /// variables left, then it definitely matches and we can just select |
| /// it. |
| /// |
| /// In fact, we even select the where-clause when the obligation contains |
| /// inference variables. The can lead to inference making "leaps of logic", |
| /// for example in this situation: |
| /// |
| /// pub trait Foo<T> { fn foo(&self) -> T; } |
| /// impl<T> Foo<()> for T { fn foo(&self) { } } |
| /// impl Foo<bool> for bool { fn foo(&self) -> bool { *self } } |
| /// |
| /// pub fn foo<T>(t: T) where T: Foo<bool> { |
| /// println!("{:?}", <T as Foo<_>>::foo(&t)); |
| /// } |
| /// fn main() { foo(false); } |
| /// |
| /// Here the obligation <T as Foo<$0>> can be matched by both the blanket |
| /// impl and the where-clause. We select the where-clause and unify $0=bool, |
| /// so the program prints "false". However, if the where-clause is omitted, |
| /// the blanket impl is selected, we unify $0=(), and the program prints |
| /// "()". |
| /// |
| /// Exactly the same issues apply to projection and object candidates, except |
| /// that we can have both a projection candidate and a where-clause candidate |
| /// for the same obligation. In that case either would do (except that |
| /// different "leaps of logic" would occur if inference variables are |
| /// present), and we just pick the where-clause. This is, for example, |
| /// required for associated types to work in default impls, as the bounds |
| /// are visible both as projection bounds and as where-clauses from the |
| /// parameter environment. |
| #[derive(PartialEq, Eq, Debug, Clone)] |
| enum SelectionCandidate<'tcx> { |
| /// If has_nested is false, there are no *further* obligations |
| BuiltinCandidate { |
| has_nested: bool, |
| }, |
| ParamCandidate(ty::PolyTraitRef<'tcx>), |
| ImplCandidate(DefId), |
| AutoImplCandidate(DefId), |
| |
| /// This is a trait matching with a projected type as `Self`, and |
| /// we found an applicable bound in the trait definition. |
| ProjectionCandidate, |
| |
| /// Implementation of a `Fn`-family trait by one of the anonymous types |
| /// generated for a `||` expression. |
| ClosureCandidate, |
| |
| /// Implementation of a `Generator` trait by one of the anonymous types |
| /// generated for a generator. |
| GeneratorCandidate, |
| |
| /// Implementation of a `Fn`-family trait by one of the anonymous |
| /// types generated for a fn pointer type (e.g., `fn(int)->int`) |
| FnPointerCandidate, |
| |
| TraitAliasCandidate(DefId), |
| |
| ObjectCandidate, |
| |
| BuiltinObjectCandidate, |
| |
| BuiltinUnsizeCandidate, |
| } |
| |
| impl<'a, 'tcx> ty::Lift<'tcx> for SelectionCandidate<'a> { |
| type Lifted = SelectionCandidate<'tcx>; |
| fn lift_to_tcx<'b, 'gcx>(&self, tcx: TyCtxt<'b, 'gcx, 'tcx>) -> Option<Self::Lifted> { |
| Some(match *self { |
| BuiltinCandidate { has_nested } => BuiltinCandidate { has_nested }, |
| ImplCandidate(def_id) => ImplCandidate(def_id), |
| AutoImplCandidate(def_id) => AutoImplCandidate(def_id), |
| ProjectionCandidate => ProjectionCandidate, |
| ClosureCandidate => ClosureCandidate, |
| GeneratorCandidate => GeneratorCandidate, |
| FnPointerCandidate => FnPointerCandidate, |
| TraitAliasCandidate(def_id) => TraitAliasCandidate(def_id), |
| ObjectCandidate => ObjectCandidate, |
| BuiltinObjectCandidate => BuiltinObjectCandidate, |
| BuiltinUnsizeCandidate => BuiltinUnsizeCandidate, |
| |
| ParamCandidate(ref trait_ref) => { |
| return tcx.lift(trait_ref).map(ParamCandidate); |
| } |
| }) |
| } |
| } |
| |
| struct SelectionCandidateSet<'tcx> { |
| // a list of candidates that definitely apply to the current |
| // obligation (meaning: types unify). |
| vec: Vec<SelectionCandidate<'tcx>>, |
| |
| // if this is true, then there were candidates that might or might |
| // not have applied, but we couldn't tell. This occurs when some |
| // of the input types are type variables, in which case there are |
| // various "builtin" rules that might or might not trigger. |
| ambiguous: bool, |
| } |
| |
| #[derive(PartialEq, Eq, Debug, Clone)] |
| struct EvaluatedCandidate<'tcx> { |
| candidate: SelectionCandidate<'tcx>, |
| evaluation: EvaluationResult, |
| } |
| |
| /// When does the builtin impl for `T: Trait` apply? |
| enum BuiltinImplConditions<'tcx> { |
| /// The impl is conditional on T1,T2,.. : Trait |
| Where(ty::Binder<Vec<Ty<'tcx>>>), |
| /// There is no built-in impl. There may be some other |
| /// candidate (a where-clause or user-defined impl). |
| None, |
| /// It is unknown whether there is an impl. |
| Ambiguous, |
| } |
| |
| #[derive(Copy, Clone, Debug, PartialOrd, Ord, PartialEq, Eq)] |
| /// The result of trait evaluation. The order is important |
| /// here as the evaluation of a list is the maximum of the |
| /// evaluations. |
| /// |
| /// The evaluation results are ordered: |
| /// - `EvaluatedToOk` implies `EvaluatedToOkModuloRegions` |
| /// implies `EvaluatedToAmbig` implies `EvaluatedToUnknown` |
| /// - `EvaluatedToErr` implies `EvaluatedToRecur` |
| /// - the "union" of evaluation results is equal to their maximum - |
| /// all the "potential success" candidates can potentially succeed, |
| /// so they are no-ops when unioned with a definite error, and within |
| /// the categories it's easy to see that the unions are correct. |
| pub enum EvaluationResult { |
| /// Evaluation successful |
| EvaluatedToOk, |
| /// Evaluation successful, but there were unevaluated region obligations |
| EvaluatedToOkModuloRegions, |
| /// Evaluation is known to be ambiguous - it *might* hold for some |
| /// assignment of inference variables, but it might not. |
| /// |
| /// While this has the same meaning as `EvaluatedToUnknown` - we can't |
| /// know whether this obligation holds or not - it is the result we |
| /// would get with an empty stack, and therefore is cacheable. |
| EvaluatedToAmbig, |
| /// Evaluation failed because of recursion involving inference |
| /// variables. We are somewhat imprecise there, so we don't actually |
| /// know the real result. |
| /// |
| /// This can't be trivially cached for the same reason as `EvaluatedToRecur`. |
| EvaluatedToUnknown, |
| /// Evaluation failed because we encountered an obligation we are already |
| /// trying to prove on this branch. |
| /// |
| /// We know this branch can't be a part of a minimal proof-tree for |
| /// the "root" of our cycle, because then we could cut out the recursion |
| /// and maintain a valid proof tree. However, this does not mean |
| /// that all the obligations on this branch do not hold - it's possible |
| /// that we entered this branch "speculatively", and that there |
| /// might be some other way to prove this obligation that does not |
| /// go through this cycle - so we can't cache this as a failure. |
| /// |
| /// For example, suppose we have this: |
| /// |
| /// ```rust,ignore (pseudo-Rust) |
| /// pub trait Trait { fn xyz(); } |
| /// // This impl is "useless", but we can still have |
| /// // an `impl Trait for SomeUnsizedType` somewhere. |
| /// impl<T: Trait + Sized> Trait for T { fn xyz() {} } |
| /// |
| /// pub fn foo<T: Trait + ?Sized>() { |
| /// <T as Trait>::xyz(); |
| /// } |
| /// ``` |
| /// |
| /// When checking `foo`, we have to prove `T: Trait`. This basically |
| /// translates into this: |
| /// |
| /// ```plain,ignore |
| /// (T: Trait + Sized →_\impl T: Trait), T: Trait ⊢ T: Trait |
| /// ``` |
| /// |
| /// When we try to prove it, we first go the first option, which |
| /// recurses. This shows us that the impl is "useless" - it won't |
| /// tell us that `T: Trait` unless it already implemented `Trait` |
| /// by some other means. However, that does not prevent `T: Trait` |
| /// does not hold, because of the bound (which can indeed be satisfied |
| /// by `SomeUnsizedType` from another crate). |
| /// |
| /// FIXME: when an `EvaluatedToRecur` goes past its parent root, we |
| /// ought to convert it to an `EvaluatedToErr`, because we know |
| /// there definitely isn't a proof tree for that obligation. Not |
| /// doing so is still sound - there isn't any proof tree, so the |
| /// branch still can't be a part of a minimal one - but does not |
| /// re-enable caching. |
| EvaluatedToRecur, |
| /// Evaluation failed |
| EvaluatedToErr, |
| } |
| |
| impl EvaluationResult { |
| /// True if this evaluation result is known to apply, even |
| /// considering outlives constraints. |
| pub fn must_apply_considering_regions(self) -> bool { |
| self == EvaluatedToOk |
| } |
| |
| /// True if this evaluation result is known to apply, ignoring |
| /// outlives constraints. |
| pub fn must_apply_modulo_regions(self) -> bool { |
| self <= EvaluatedToOkModuloRegions |
| } |
| |
| pub fn may_apply(self) -> bool { |
| match self { |
| EvaluatedToOk | EvaluatedToOkModuloRegions | EvaluatedToAmbig | EvaluatedToUnknown => { |
| true |
| } |
| |
| EvaluatedToErr | EvaluatedToRecur => false, |
| } |
| } |
| |
| fn is_stack_dependent(self) -> bool { |
| match self { |
| EvaluatedToUnknown | EvaluatedToRecur => true, |
| |
| EvaluatedToOk | EvaluatedToOkModuloRegions | EvaluatedToAmbig | EvaluatedToErr => false, |
| } |
| } |
| } |
| |
| impl_stable_hash_for!(enum self::EvaluationResult { |
| EvaluatedToOk, |
| EvaluatedToOkModuloRegions, |
| EvaluatedToAmbig, |
| EvaluatedToUnknown, |
| EvaluatedToRecur, |
| EvaluatedToErr |
| }); |
| |
| #[derive(Copy, Clone, Debug, PartialEq, Eq)] |
| /// Indicates that trait evaluation caused overflow. |
| pub struct OverflowError; |
| |
| impl_stable_hash_for!(struct OverflowError {}); |
| |
| impl<'tcx> From<OverflowError> for SelectionError<'tcx> { |
| fn from(OverflowError: OverflowError) -> SelectionError<'tcx> { |
| SelectionError::Overflow |
| } |
| } |
| |
| #[derive(Clone, Default)] |
| pub struct EvaluationCache<'tcx> { |
| hashmap: Lock<FxHashMap<ty::PolyTraitRef<'tcx>, WithDepNode<EvaluationResult>>>, |
| } |
| |
| impl<'cx, 'gcx, 'tcx> SelectionContext<'cx, 'gcx, 'tcx> { |
| pub fn new(infcx: &'cx InferCtxt<'cx, 'gcx, 'tcx>) -> SelectionContext<'cx, 'gcx, 'tcx> { |
| SelectionContext { |
| infcx, |
| freshener: infcx.freshener(), |
| intercrate: None, |
| intercrate_ambiguity_causes: None, |
| allow_negative_impls: false, |
| query_mode: TraitQueryMode::Standard, |
| } |
| } |
| |
| pub fn intercrate( |
| infcx: &'cx InferCtxt<'cx, 'gcx, 'tcx>, |
| mode: IntercrateMode, |
| ) -> SelectionContext<'cx, 'gcx, 'tcx> { |
| debug!("intercrate({:?})", mode); |
| SelectionContext { |
| infcx, |
| freshener: infcx.freshener(), |
| intercrate: Some(mode), |
| intercrate_ambiguity_causes: None, |
| allow_negative_impls: false, |
| query_mode: TraitQueryMode::Standard, |
| } |
| } |
| |
| pub fn with_negative( |
| infcx: &'cx InferCtxt<'cx, 'gcx, 'tcx>, |
| allow_negative_impls: bool, |
| ) -> SelectionContext<'cx, 'gcx, 'tcx> { |
| debug!("with_negative({:?})", allow_negative_impls); |
| SelectionContext { |
| infcx, |
| freshener: infcx.freshener(), |
| intercrate: None, |
| intercrate_ambiguity_causes: None, |
| allow_negative_impls, |
| query_mode: TraitQueryMode::Standard, |
| } |
| } |
| |
| pub fn with_query_mode( |
| infcx: &'cx InferCtxt<'cx, 'gcx, 'tcx>, |
| query_mode: TraitQueryMode, |
| ) -> SelectionContext<'cx, 'gcx, 'tcx> { |
| debug!("with_query_mode({:?})", query_mode); |
| SelectionContext { |
| infcx, |
| freshener: infcx.freshener(), |
| intercrate: None, |
| intercrate_ambiguity_causes: None, |
| allow_negative_impls: false, |
| query_mode, |
| } |
| } |
| |
| /// Enables tracking of intercrate ambiguity causes. These are |
| /// used in coherence to give improved diagnostics. We don't do |
| /// this until we detect a coherence error because it can lead to |
| /// false overflow results (#47139) and because it costs |
| /// computation time. |
| pub fn enable_tracking_intercrate_ambiguity_causes(&mut self) { |
| assert!(self.intercrate.is_some()); |
| assert!(self.intercrate_ambiguity_causes.is_none()); |
| self.intercrate_ambiguity_causes = Some(vec![]); |
| debug!("selcx: enable_tracking_intercrate_ambiguity_causes"); |
| } |
| |
| /// Gets the intercrate ambiguity causes collected since tracking |
| /// was enabled and disables tracking at the same time. If |
| /// tracking is not enabled, just returns an empty vector. |
| pub fn take_intercrate_ambiguity_causes(&mut self) -> Vec<IntercrateAmbiguityCause> { |
| assert!(self.intercrate.is_some()); |
| self.intercrate_ambiguity_causes.take().unwrap_or(vec![]) |
| } |
| |
| pub fn infcx(&self) -> &'cx InferCtxt<'cx, 'gcx, 'tcx> { |
| self.infcx |
| } |
| |
| pub fn tcx(&self) -> TyCtxt<'cx, 'gcx, 'tcx> { |
| self.infcx.tcx |
| } |
| |
| pub fn closure_typer(&self) -> &'cx InferCtxt<'cx, 'gcx, 'tcx> { |
| self.infcx |
| } |
| |
| /////////////////////////////////////////////////////////////////////////// |
| // Selection |
| // |
| // The selection phase tries to identify *how* an obligation will |
| // be resolved. For example, it will identify which impl or |
| // parameter bound is to be used. The process can be inconclusive |
| // if the self type in the obligation is not fully inferred. Selection |
| // can result in an error in one of two ways: |
| // |
| // 1. If no applicable impl or parameter bound can be found. |
| // 2. If the output type parameters in the obligation do not match |
| // those specified by the impl/bound. For example, if the obligation |
| // is `Vec<Foo>:Iterable<Bar>`, but the impl specifies |
| // `impl<T> Iterable<T> for Vec<T>`, than an error would result. |
| |
| /// Attempts to satisfy the obligation. If successful, this will affect the surrounding |
| /// type environment by performing unification. |
| pub fn select( |
| &mut self, |
| obligation: &TraitObligation<'tcx>, |
| ) -> SelectionResult<'tcx, Selection<'tcx>> { |
| debug!("select({:?})", obligation); |
| debug_assert!(!obligation.predicate.has_escaping_bound_vars()); |
| |
| let stack = self.push_stack(TraitObligationStackList::empty(), obligation); |
| |
| let candidate = match self.candidate_from_obligation(&stack) { |
| Err(SelectionError::Overflow) => { |
| // In standard mode, overflow must have been caught and reported |
| // earlier. |
| assert!(self.query_mode == TraitQueryMode::Canonical); |
| return Err(SelectionError::Overflow); |
| } |
| Err(e) => { |
| return Err(e); |
| } |
| Ok(None) => { |
| return Ok(None); |
| } |
| Ok(Some(candidate)) => candidate, |
| }; |
| |
| match self.confirm_candidate(obligation, candidate) { |
| Err(SelectionError::Overflow) => { |
| assert!(self.query_mode == TraitQueryMode::Canonical); |
| Err(SelectionError::Overflow) |
| } |
| Err(e) => Err(e), |
| Ok(candidate) => Ok(Some(candidate)), |
| } |
| } |
| |
| /////////////////////////////////////////////////////////////////////////// |
| // EVALUATION |
| // |
| // Tests whether an obligation can be selected or whether an impl |
| // can be applied to particular types. It skips the "confirmation" |
| // step and hence completely ignores output type parameters. |
| // |
| // The result is "true" if the obligation *may* hold and "false" if |
| // we can be sure it does not. |
| |
| /// Evaluates whether the obligation `obligation` can be satisfied (by any means). |
| pub fn predicate_may_hold_fatal(&mut self, obligation: &PredicateObligation<'tcx>) -> bool { |
| debug!("predicate_may_hold_fatal({:?})", obligation); |
| |
| // This fatal query is a stopgap that should only be used in standard mode, |
| // where we do not expect overflow to be propagated. |
| assert!(self.query_mode == TraitQueryMode::Standard); |
| |
| self.evaluate_obligation_recursively(obligation) |
| .expect("Overflow should be caught earlier in standard query mode") |
| .may_apply() |
| } |
| |
| /// Evaluates whether the obligation `obligation` can be satisfied and returns |
| /// an `EvaluationResult`. |
| pub fn evaluate_obligation_recursively( |
| &mut self, |
| obligation: &PredicateObligation<'tcx>, |
| ) -> Result<EvaluationResult, OverflowError> { |
| self.evaluation_probe(|this| { |
| this.evaluate_predicate_recursively(TraitObligationStackList::empty(), obligation) |
| }) |
| } |
| |
| fn evaluation_probe( |
| &mut self, |
| op: impl FnOnce(&mut Self) -> Result<EvaluationResult, OverflowError>, |
| ) -> Result<EvaluationResult, OverflowError> { |
| self.infcx.probe(|snapshot| -> Result<EvaluationResult, OverflowError> { |
| let result = op(self)?; |
| match self.infcx.region_constraints_added_in_snapshot(snapshot) { |
| None => Ok(result), |
| Some(_) => Ok(result.max(EvaluatedToOkModuloRegions)), |
| } |
| }) |
| } |
| |
| /// Evaluates the predicates in `predicates` recursively. Note that |
| /// this applies projections in the predicates, and therefore |
| /// is run within an inference probe. |
| fn evaluate_predicates_recursively<'a, 'o, I>( |
| &mut self, |
| stack: TraitObligationStackList<'o, 'tcx>, |
| predicates: I, |
| ) -> Result<EvaluationResult, OverflowError> |
| where |
| I: IntoIterator<Item = &'a PredicateObligation<'tcx>>, |
| 'tcx: 'a, |
| { |
| let mut result = EvaluatedToOk; |
| for obligation in predicates { |
| let eval = self.evaluate_predicate_recursively(stack, obligation)?; |
| debug!( |
| "evaluate_predicate_recursively({:?}) = {:?}", |
| obligation, eval |
| ); |
| if let EvaluatedToErr = eval { |
| // fast-path - EvaluatedToErr is the top of the lattice, |
| // so we don't need to look on the other predicates. |
| return Ok(EvaluatedToErr); |
| } else { |
| result = cmp::max(result, eval); |
| } |
| } |
| Ok(result) |
| } |
| |
| fn evaluate_predicate_recursively<'o>( |
| &mut self, |
| previous_stack: TraitObligationStackList<'o, 'tcx>, |
| obligation: &PredicateObligation<'tcx>, |
| ) -> Result<EvaluationResult, OverflowError> { |
| debug!("evaluate_predicate_recursively({:?})", obligation); |
| |
| match obligation.predicate { |
| ty::Predicate::Trait(ref t) => { |
| debug_assert!(!t.has_escaping_bound_vars()); |
| let obligation = obligation.with(t.clone()); |
| self.evaluate_trait_predicate_recursively(previous_stack, obligation) |
| } |
| |
| ty::Predicate::Subtype(ref p) => { |
| // does this code ever run? |
| match self.infcx |
| .subtype_predicate(&obligation.cause, obligation.param_env, p) |
| { |
| Some(Ok(InferOk { obligations, .. })) => { |
| self.evaluate_predicates_recursively(previous_stack, &obligations) |
| } |
| Some(Err(_)) => Ok(EvaluatedToErr), |
| None => Ok(EvaluatedToAmbig), |
| } |
| } |
| |
| ty::Predicate::WellFormed(ty) => match ty::wf::obligations( |
| self.infcx, |
| obligation.param_env, |
| obligation.cause.body_id, |
| ty, |
| obligation.cause.span, |
| ) { |
| Some(obligations) => { |
| self.evaluate_predicates_recursively(previous_stack, obligations.iter()) |
| } |
| None => Ok(EvaluatedToAmbig), |
| }, |
| |
| ty::Predicate::TypeOutlives(..) | ty::Predicate::RegionOutlives(..) => { |
| // we do not consider region relationships when |
| // evaluating trait matches |
| Ok(EvaluatedToOkModuloRegions) |
| } |
| |
| ty::Predicate::ObjectSafe(trait_def_id) => { |
| if self.tcx().is_object_safe(trait_def_id) { |
| Ok(EvaluatedToOk) |
| } else { |
| Ok(EvaluatedToErr) |
| } |
| } |
| |
| ty::Predicate::Projection(ref data) => { |
| let project_obligation = obligation.with(data.clone()); |
| match project::poly_project_and_unify_type(self, &project_obligation) { |
| Ok(Some(subobligations)) => { |
| let result = self.evaluate_predicates_recursively( |
| previous_stack, |
| subobligations.iter(), |
| ); |
| if let Some(key) = |
| ProjectionCacheKey::from_poly_projection_predicate(self, data) |
| { |
| self.infcx.projection_cache.borrow_mut().complete(key); |
| } |
| result |
| } |
| Ok(None) => Ok(EvaluatedToAmbig), |
| Err(_) => Ok(EvaluatedToErr), |
| } |
| } |
| |
| ty::Predicate::ClosureKind(closure_def_id, closure_substs, kind) => { |
| match self.infcx.closure_kind(closure_def_id, closure_substs) { |
| Some(closure_kind) => { |
| if closure_kind.extends(kind) { |
| Ok(EvaluatedToOk) |
| } else { |
| Ok(EvaluatedToErr) |
| } |
| } |
| None => Ok(EvaluatedToAmbig), |
| } |
| } |
| |
| ty::Predicate::ConstEvaluatable(def_id, substs) => { |
| let tcx = self.tcx(); |
| match tcx.lift_to_global(&(obligation.param_env, substs)) { |
| Some((param_env, substs)) => { |
| let instance = |
| ty::Instance::resolve(tcx.global_tcx(), param_env, def_id, substs); |
| if let Some(instance) = instance { |
| let cid = GlobalId { |
| instance, |
| promoted: None, |
| }; |
| match self.tcx().const_eval(param_env.and(cid)) { |
| Ok(_) => Ok(EvaluatedToOk), |
| Err(_) => Ok(EvaluatedToErr), |
| } |
| } else { |
| Ok(EvaluatedToErr) |
| } |
| } |
| None => { |
| // Inference variables still left in param_env or substs. |
| Ok(EvaluatedToAmbig) |
| } |
| } |
| } |
| } |
| } |
| |
| fn evaluate_trait_predicate_recursively<'o>( |
| &mut self, |
| previous_stack: TraitObligationStackList<'o, 'tcx>, |
| mut obligation: TraitObligation<'tcx>, |
| ) -> Result<EvaluationResult, OverflowError> { |
| debug!("evaluate_trait_predicate_recursively({:?})", obligation); |
| |
| if self.intercrate.is_none() && obligation.is_global() |
| && obligation |
| .param_env |
| .caller_bounds |
| .iter() |
| .all(|bound| bound.needs_subst()) |
| { |
| // If a param env has no global bounds, global obligations do not |
| // depend on its particular value in order to work, so we can clear |
| // out the param env and get better caching. |
| debug!( |
| "evaluate_trait_predicate_recursively({:?}) - in global", |
| obligation |
| ); |
| obligation.param_env = obligation.param_env.without_caller_bounds(); |
| } |
| |
| let stack = self.push_stack(previous_stack, &obligation); |
| let fresh_trait_ref = stack.fresh_trait_ref; |
| if let Some(result) = self.check_evaluation_cache(obligation.param_env, fresh_trait_ref) { |
| debug!("CACHE HIT: EVAL({:?})={:?}", fresh_trait_ref, result); |
| return Ok(result); |
| } |
| |
| let (result, dep_node) = self.in_task(|this| this.evaluate_stack(&stack)); |
| let result = result?; |
| |
| debug!("CACHE MISS: EVAL({:?})={:?}", fresh_trait_ref, result); |
| self.insert_evaluation_cache(obligation.param_env, fresh_trait_ref, dep_node, result); |
| |
| Ok(result) |
| } |
| |
| fn evaluate_stack<'o>( |
| &mut self, |
| stack: &TraitObligationStack<'o, 'tcx>, |
| ) -> Result<EvaluationResult, OverflowError> { |
| // In intercrate mode, whenever any of the types are unbound, |
| // there can always be an impl. Even if there are no impls in |
| // this crate, perhaps the type would be unified with |
| // something from another crate that does provide an impl. |
| // |
| // In intra mode, we must still be conservative. The reason is |
| // that we want to avoid cycles. Imagine an impl like: |
| // |
| // impl<T:Eq> Eq for Vec<T> |
| // |
| // and a trait reference like `$0 : Eq` where `$0` is an |
| // unbound variable. When we evaluate this trait-reference, we |
| // will unify `$0` with `Vec<$1>` (for some fresh variable |
| // `$1`), on the condition that `$1 : Eq`. We will then wind |
| // up with many candidates (since that are other `Eq` impls |
| // that apply) and try to winnow things down. This results in |
| // a recursive evaluation that `$1 : Eq` -- as you can |
| // imagine, this is just where we started. To avoid that, we |
| // check for unbound variables and return an ambiguous (hence possible) |
| // match if we've seen this trait before. |
| // |
| // This suffices to allow chains like `FnMut` implemented in |
| // terms of `Fn` etc, but we could probably make this more |
| // precise still. |
| let unbound_input_types = stack |
| .fresh_trait_ref |
| .skip_binder() |
| .input_types() |
| .any(|ty| ty.is_fresh()); |
| // this check was an imperfect workaround for a bug n the old |
| // intercrate mode, it should be removed when that goes away. |
| if unbound_input_types && self.intercrate == Some(IntercrateMode::Issue43355) { |
| debug!( |
| "evaluate_stack({:?}) --> unbound argument, intercrate --> ambiguous", |
| stack.fresh_trait_ref |
| ); |
| // Heuristics: show the diagnostics when there are no candidates in crate. |
| if self.intercrate_ambiguity_causes.is_some() { |
| debug!("evaluate_stack: intercrate_ambiguity_causes is some"); |
| if let Ok(candidate_set) = self.assemble_candidates(stack) { |
| if !candidate_set.ambiguous && candidate_set.vec.is_empty() { |
| let trait_ref = stack.obligation.predicate.skip_binder().trait_ref; |
| let self_ty = trait_ref.self_ty(); |
| let cause = IntercrateAmbiguityCause::DownstreamCrate { |
| trait_desc: trait_ref.to_string(), |
| self_desc: if self_ty.has_concrete_skeleton() { |
| Some(self_ty.to_string()) |
| } else { |
| None |
| }, |
| }; |
| debug!("evaluate_stack: pushing cause = {:?}", cause); |
| self.intercrate_ambiguity_causes |
| .as_mut() |
| .unwrap() |
| .push(cause); |
| } |
| } |
| } |
| return Ok(EvaluatedToAmbig); |
| } |
| if unbound_input_types && stack.iter().skip(1).any(|prev| { |
| stack.obligation.param_env == prev.obligation.param_env |
| && self.match_fresh_trait_refs(&stack.fresh_trait_ref, &prev.fresh_trait_ref) |
| }) { |
| debug!( |
| "evaluate_stack({:?}) --> unbound argument, recursive --> giving up", |
| stack.fresh_trait_ref |
| ); |
| return Ok(EvaluatedToUnknown); |
| } |
| |
| // If there is any previous entry on the stack that precisely |
| // matches this obligation, then we can assume that the |
| // obligation is satisfied for now (still all other conditions |
| // must be met of course). One obvious case this comes up is |
| // marker traits like `Send`. Think of a linked list: |
| // |
| // struct List<T> { data: T, next: Option<Box<List<T>>> } |
| // |
| // `Box<List<T>>` will be `Send` if `T` is `Send` and |
| // `Option<Box<List<T>>>` is `Send`, and in turn |
| // `Option<Box<List<T>>>` is `Send` if `Box<List<T>>` is |
| // `Send`. |
| // |
| // Note that we do this comparison using the `fresh_trait_ref` |
| // fields. Because these have all been freshened using |
| // `self.freshener`, we can be sure that (a) this will not |
| // affect the inferencer state and (b) that if we see two |
| // fresh regions with the same index, they refer to the same |
| // unbound type variable. |
| if let Some(rec_index) = stack.iter() |
| .skip(1) // skip top-most frame |
| .position(|prev| stack.obligation.param_env == prev.obligation.param_env && |
| stack.fresh_trait_ref == prev.fresh_trait_ref) |
| { |
| debug!("evaluate_stack({:?}) --> recursive", stack.fresh_trait_ref); |
| |
| // Subtle: when checking for a coinductive cycle, we do |
| // not compare using the "freshened trait refs" (which |
| // have erased regions) but rather the fully explicit |
| // trait refs. This is important because it's only a cycle |
| // if the regions match exactly. |
| let cycle = stack.iter().skip(1).take(rec_index + 1); |
| let cycle = cycle.map(|stack| ty::Predicate::Trait(stack.obligation.predicate)); |
| if self.coinductive_match(cycle) { |
| debug!( |
| "evaluate_stack({:?}) --> recursive, coinductive", |
| stack.fresh_trait_ref |
| ); |
| return Ok(EvaluatedToOk); |
| } else { |
| debug!( |
| "evaluate_stack({:?}) --> recursive, inductive", |
| stack.fresh_trait_ref |
| ); |
| return Ok(EvaluatedToRecur); |
| } |
| } |
| |
| match self.candidate_from_obligation(stack) { |
| Ok(Some(c)) => self.evaluate_candidate(stack, &c), |
| Ok(None) => Ok(EvaluatedToAmbig), |
| Err(Overflow) => Err(OverflowError), |
| Err(..) => Ok(EvaluatedToErr), |
| } |
| } |
| |
| /// For defaulted traits, we use a co-inductive strategy to solve, so |
| /// that recursion is ok. This routine returns true if the top of the |
| /// stack (`cycle[0]`): |
| /// |
| /// - is a defaulted trait, and |
| /// - it also appears in the backtrace at some position `X`; and, |
| /// - all the predicates at positions `X..` between `X` an the top are |
| /// also defaulted traits. |
| pub fn coinductive_match<I>(&mut self, cycle: I) -> bool |
| where |
| I: Iterator<Item = ty::Predicate<'tcx>>, |
| { |
| let mut cycle = cycle; |
| cycle.all(|predicate| self.coinductive_predicate(predicate)) |
| } |
| |
| fn coinductive_predicate(&self, predicate: ty::Predicate<'tcx>) -> bool { |
| let result = match predicate { |
| ty::Predicate::Trait(ref data) => self.tcx().trait_is_auto(data.def_id()), |
| _ => false, |
| }; |
| debug!("coinductive_predicate({:?}) = {:?}", predicate, result); |
| result |
| } |
| |
| /// Further evaluate `candidate` to decide whether all type parameters match and whether nested |
| /// obligations are met. Returns true if `candidate` remains viable after this further |
| /// scrutiny. |
| fn evaluate_candidate<'o>( |
| &mut self, |
| stack: &TraitObligationStack<'o, 'tcx>, |
| candidate: &SelectionCandidate<'tcx>, |
| ) -> Result<EvaluationResult, OverflowError> { |
| debug!( |
| "evaluate_candidate: depth={} candidate={:?}", |
| stack.obligation.recursion_depth, candidate |
| ); |
| let result = self.evaluation_probe(|this| { |
| let candidate = (*candidate).clone(); |
| match this.confirm_candidate(stack.obligation, candidate) { |
| Ok(selection) => this.evaluate_predicates_recursively( |
| stack.list(), |
| selection.nested_obligations().iter(), |
| ), |
| Err(..) => Ok(EvaluatedToErr), |
| } |
| })?; |
| debug!( |
| "evaluate_candidate: depth={} result={:?}", |
| stack.obligation.recursion_depth, result |
| ); |
| Ok(result) |
| } |
| |
| fn check_evaluation_cache( |
| &self, |
| param_env: ty::ParamEnv<'tcx>, |
| trait_ref: ty::PolyTraitRef<'tcx>, |
| ) -> Option<EvaluationResult> { |
| let tcx = self.tcx(); |
| if self.can_use_global_caches(param_env) { |
| let cache = tcx.evaluation_cache.hashmap.borrow(); |
| if let Some(cached) = cache.get(&trait_ref) { |
| return Some(cached.get(tcx)); |
| } |
| } |
| self.infcx |
| .evaluation_cache |
| .hashmap |
| .borrow() |
| .get(&trait_ref) |
| .map(|v| v.get(tcx)) |
| } |
| |
| fn insert_evaluation_cache( |
| &mut self, |
| param_env: ty::ParamEnv<'tcx>, |
| trait_ref: ty::PolyTraitRef<'tcx>, |
| dep_node: DepNodeIndex, |
| result: EvaluationResult, |
| ) { |
| // Avoid caching results that depend on more than just the trait-ref |
| // - the stack can create recursion. |
| if result.is_stack_dependent() { |
| return; |
| } |
| |
| if self.can_use_global_caches(param_env) { |
| if let Some(trait_ref) = self.tcx().lift_to_global(&trait_ref) { |
| debug!( |
| "insert_evaluation_cache(trait_ref={:?}, candidate={:?}) global", |
| trait_ref, result, |
| ); |
| // This may overwrite the cache with the same value |
| // FIXME: Due to #50507 this overwrites the different values |
| // This should be changed to use HashMapExt::insert_same |
| // when that is fixed |
| self.tcx() |
| .evaluation_cache |
| .hashmap |
| .borrow_mut() |
| .insert(trait_ref, WithDepNode::new(dep_node, result)); |
| return; |
| } |
| } |
| |
| debug!( |
| "insert_evaluation_cache(trait_ref={:?}, candidate={:?})", |
| trait_ref, result, |
| ); |
| self.infcx |
| .evaluation_cache |
| .hashmap |
| .borrow_mut() |
| .insert(trait_ref, WithDepNode::new(dep_node, result)); |
| } |
| |
| /////////////////////////////////////////////////////////////////////////// |
| // CANDIDATE ASSEMBLY |
| // |
| // The selection process begins by examining all in-scope impls, |
| // caller obligations, and so forth and assembling a list of |
| // candidates. See the [rustc guide] for more details. |
| // |
| // [rustc guide]: |
| // https://rust-lang.github.io/rustc-guide/traits/resolution.html#candidate-assembly |
| |
| fn candidate_from_obligation<'o>( |
| &mut self, |
| stack: &TraitObligationStack<'o, 'tcx>, |
| ) -> SelectionResult<'tcx, SelectionCandidate<'tcx>> { |
| // Watch out for overflow. This intentionally bypasses (and does |
| // not update) the cache. |
| let recursion_limit = *self.infcx.tcx.sess.recursion_limit.get(); |
| if stack.obligation.recursion_depth >= recursion_limit { |
| match self.query_mode { |
| TraitQueryMode::Standard => { |
| self.infcx().report_overflow_error(&stack.obligation, true); |
| } |
| TraitQueryMode::Canonical => { |
| return Err(Overflow); |
| } |
| } |
| } |
| |
| // Check the cache. Note that we freshen the trait-ref |
| // separately rather than using `stack.fresh_trait_ref` -- |
| // this is because we want the unbound variables to be |
| // replaced with fresh types starting from index 0. |
| let cache_fresh_trait_pred = self.infcx.freshen(stack.obligation.predicate.clone()); |
| debug!( |
| "candidate_from_obligation(cache_fresh_trait_pred={:?}, obligation={:?})", |
| cache_fresh_trait_pred, stack |
| ); |
| debug_assert!(!stack.obligation.predicate.has_escaping_bound_vars()); |
| |
| if let Some(c) = |
| self.check_candidate_cache(stack.obligation.param_env, &cache_fresh_trait_pred) |
| { |
| debug!("CACHE HIT: SELECT({:?})={:?}", cache_fresh_trait_pred, c); |
| return c; |
| } |
| |
| // If no match, compute result and insert into cache. |
| let (candidate, dep_node) = |
| self.in_task(|this| this.candidate_from_obligation_no_cache(stack)); |
| |
| debug!( |
| "CACHE MISS: SELECT({:?})={:?}", |
| cache_fresh_trait_pred, candidate |
| ); |
| self.insert_candidate_cache( |
| stack.obligation.param_env, |
| cache_fresh_trait_pred, |
| dep_node, |
| candidate.clone(), |
| ); |
| candidate |
| } |
| |
| fn in_task<OP, R>(&mut self, op: OP) -> (R, DepNodeIndex) |
| where |
| OP: FnOnce(&mut Self) -> R, |
| { |
| let (result, dep_node) = self.tcx() |
| .dep_graph |
| .with_anon_task(DepKind::TraitSelect, || op(self)); |
| self.tcx().dep_graph.read_index(dep_node); |
| (result, dep_node) |
| } |
| |
| // Treat negative impls as unimplemented |
| fn filter_negative_impls( |
| &self, |
| candidate: SelectionCandidate<'tcx>, |
| ) -> SelectionResult<'tcx, SelectionCandidate<'tcx>> { |
| if let ImplCandidate(def_id) = candidate { |
| if !self.allow_negative_impls |
| && self.tcx().impl_polarity(def_id) == hir::ImplPolarity::Negative |
| { |
| return Err(Unimplemented); |
| } |
| } |
| Ok(Some(candidate)) |
| } |
| |
| fn candidate_from_obligation_no_cache<'o>( |
| &mut self, |
| stack: &TraitObligationStack<'o, 'tcx>, |
| ) -> SelectionResult<'tcx, SelectionCandidate<'tcx>> { |
| if stack.obligation.predicate.references_error() { |
| // If we encounter a `Error`, we generally prefer the |
| // most "optimistic" result in response -- that is, the |
| // one least likely to report downstream errors. But |
| // because this routine is shared by coherence and by |
| // trait selection, there isn't an obvious "right" choice |
| // here in that respect, so we opt to just return |
| // ambiguity and let the upstream clients sort it out. |
| return Ok(None); |
| } |
| |
| if let Some(conflict) = self.is_knowable(stack) { |
| debug!("coherence stage: not knowable"); |
| if self.intercrate_ambiguity_causes.is_some() { |
| debug!("evaluate_stack: intercrate_ambiguity_causes is some"); |
| // Heuristics: show the diagnostics when there are no candidates in crate. |
| if let Ok(candidate_set) = self.assemble_candidates(stack) { |
| let mut no_candidates_apply = true; |
| { |
| let evaluated_candidates = candidate_set |
| .vec |
| .iter() |
| .map(|c| self.evaluate_candidate(stack, &c)); |
| |
| for ec in evaluated_candidates { |
| match ec { |
| Ok(c) => { |
| if c.may_apply() { |
| no_candidates_apply = false; |
| break; |
| } |
| } |
| Err(e) => return Err(e.into()), |
| } |
| } |
| } |
| |
| if !candidate_set.ambiguous && no_candidates_apply { |
| let trait_ref = stack.obligation.predicate.skip_binder().trait_ref; |
| let self_ty = trait_ref.self_ty(); |
| let trait_desc = trait_ref.to_string(); |
| let self_desc = if self_ty.has_concrete_skeleton() { |
| Some(self_ty.to_string()) |
| } else { |
| None |
| }; |
| let cause = if let Conflict::Upstream = conflict { |
| IntercrateAmbiguityCause::UpstreamCrateUpdate { |
| trait_desc, |
| self_desc, |
| } |
| } else { |
| IntercrateAmbiguityCause::DownstreamCrate { |
| trait_desc, |
| self_desc, |
| } |
| }; |
| debug!("evaluate_stack: pushing cause = {:?}", cause); |
| self.intercrate_ambiguity_causes |
| .as_mut() |
| .unwrap() |
| .push(cause); |
| } |
| } |
| } |
| return Ok(None); |
| } |
| |
| let candidate_set = self.assemble_candidates(stack)?; |
| |
| if candidate_set.ambiguous { |
| debug!("candidate set contains ambig"); |
| return Ok(None); |
| } |
| |
| let mut candidates = candidate_set.vec; |
| |
| debug!( |
| "assembled {} candidates for {:?}: {:?}", |
| candidates.len(), |
| stack, |
| candidates |
| ); |
| |
| // At this point, we know that each of the entries in the |
| // candidate set is *individually* applicable. Now we have to |
| // figure out if they contain mutual incompatibilities. This |
| // frequently arises if we have an unconstrained input type -- |
| // for example, we are looking for $0:Eq where $0 is some |
| // unconstrained type variable. In that case, we'll get a |
| // candidate which assumes $0 == int, one that assumes $0 == |
| // usize, etc. This spells an ambiguity. |
| |
| // If there is more than one candidate, first winnow them down |
| // by considering extra conditions (nested obligations and so |
| // forth). We don't winnow if there is exactly one |
| // candidate. This is a relatively minor distinction but it |
| // can lead to better inference and error-reporting. An |
| // example would be if there was an impl: |
| // |
| // impl<T:Clone> Vec<T> { fn push_clone(...) { ... } } |
| // |
| // and we were to see some code `foo.push_clone()` where `boo` |
| // is a `Vec<Bar>` and `Bar` does not implement `Clone`. If |
| // we were to winnow, we'd wind up with zero candidates. |
| // Instead, we select the right impl now but report `Bar does |
| // not implement Clone`. |
| if candidates.len() == 1 { |
| return self.filter_negative_impls(candidates.pop().unwrap()); |
| } |
| |
| // Winnow, but record the exact outcome of evaluation, which |
| // is needed for specialization. Propagate overflow if it occurs. |
| let mut candidates = candidates |
| .into_iter() |
| .map(|c| match self.evaluate_candidate(stack, &c) { |
| Ok(eval) if eval.may_apply() => Ok(Some(EvaluatedCandidate { |
| candidate: c, |
| evaluation: eval, |
| })), |
| Ok(_) => Ok(None), |
| Err(OverflowError) => Err(Overflow), |
| }) |
| .flat_map(Result::transpose) |
| .collect::<Result<Vec<_>, _>>()?; |
| |
| debug!( |
| "winnowed to {} candidates for {:?}: {:?}", |
| candidates.len(), |
| stack, |
| candidates |
| ); |
| |
| // If there are STILL multiple candidates, we can further |
| // reduce the list by dropping duplicates -- including |
| // resolving specializations. |
| if candidates.len() > 1 { |
| let mut i = 0; |
| while i < candidates.len() { |
| let is_dup = (0..candidates.len()).filter(|&j| i != j).any(|j| { |
| self.candidate_should_be_dropped_in_favor_of(&candidates[i], &candidates[j]) |
| }); |
| if is_dup { |
| debug!( |
| "Dropping candidate #{}/{}: {:?}", |
| i, |
| candidates.len(), |
| candidates[i] |
| ); |
| candidates.swap_remove(i); |
| } else { |
| debug!( |
| "Retaining candidate #{}/{}: {:?}", |
| i, |
| candidates.len(), |
| candidates[i] |
| ); |
| i += 1; |
| |
| // If there are *STILL* multiple candidates, give up |
| // and report ambiguity. |
| if i > 1 { |
| debug!("multiple matches, ambig"); |
| return Ok(None); |
| } |
| } |
| } |
| } |
| |
| // If there are *NO* candidates, then there are no impls -- |
| // that we know of, anyway. Note that in the case where there |
| // are unbound type variables within the obligation, it might |
| // be the case that you could still satisfy the obligation |
| // from another crate by instantiating the type variables with |
| // a type from another crate that does have an impl. This case |
| // is checked for in `evaluate_stack` (and hence users |
| // who might care about this case, like coherence, should use |
| // that function). |
| if candidates.is_empty() { |
| return Err(Unimplemented); |
| } |
| |
| // Just one candidate left. |
| self.filter_negative_impls(candidates.pop().unwrap().candidate) |
| } |
| |
| fn is_knowable<'o>(&mut self, stack: &TraitObligationStack<'o, 'tcx>) -> Option<Conflict> { |
| debug!("is_knowable(intercrate={:?})", self.intercrate); |
| |
| if !self.intercrate.is_some() { |
| return None; |
| } |
| |
| let obligation = &stack.obligation; |
| let predicate = self.infcx() |
| .resolve_type_vars_if_possible(&obligation.predicate); |
| |
| // OK to skip binder because of the nature of the |
| // trait-ref-is-knowable check, which does not care about |
| // bound regions |
| let trait_ref = predicate.skip_binder().trait_ref; |
| |
| let result = coherence::trait_ref_is_knowable(self.tcx(), trait_ref); |
| if let ( |
| Some(Conflict::Downstream { |
| used_to_be_broken: true, |
| }), |
| Some(IntercrateMode::Issue43355), |
| ) = (result, self.intercrate) |
| { |
| debug!("is_knowable: IGNORING conflict to be bug-compatible with #43355"); |
| None |
| } else { |
| result |
| } |
| } |
| |
| /// Returns true if the global caches can be used. |
| /// Do note that if the type itself is not in the |
| /// global tcx, the local caches will be used. |
| fn can_use_global_caches(&self, param_env: ty::ParamEnv<'tcx>) -> bool { |
| // If there are any where-clauses in scope, then we always use |
| // a cache local to this particular scope. Otherwise, we |
| // switch to a global cache. We used to try and draw |
| // finer-grained distinctions, but that led to a serious of |
| // annoying and weird bugs like #22019 and #18290. This simple |
| // rule seems to be pretty clearly safe and also still retains |
| // a very high hit rate (~95% when compiling rustc). |
| if !param_env.caller_bounds.is_empty() { |
| return false; |
| } |
| |
| // Avoid using the master cache during coherence and just rely |
| // on the local cache. This effectively disables caching |
| // during coherence. It is really just a simplification to |
| // avoid us having to fear that coherence results "pollute" |
| // the master cache. Since coherence executes pretty quickly, |
| // it's not worth going to more trouble to increase the |
| // hit-rate I don't think. |
| if self.intercrate.is_some() { |
| return false; |
| } |
| |
| // Otherwise, we can use the global cache. |
| true |
| } |
| |
| fn check_candidate_cache( |
| &mut self, |
| param_env: ty::ParamEnv<'tcx>, |
| cache_fresh_trait_pred: &ty::PolyTraitPredicate<'tcx>, |
| ) -> Option<SelectionResult<'tcx, SelectionCandidate<'tcx>>> { |
| let tcx = self.tcx(); |
| let trait_ref = &cache_fresh_trait_pred.skip_binder().trait_ref; |
| if self.can_use_global_caches(param_env) { |
| let cache = tcx.selection_cache.hashmap.borrow(); |
| if let Some(cached) = cache.get(&trait_ref) { |
| return Some(cached.get(tcx)); |
| } |
| } |
| self.infcx |
| .selection_cache |
| .hashmap |
| .borrow() |
| .get(trait_ref) |
| .map(|v| v.get(tcx)) |
| } |
| |
| /// Determines whether can we safely cache the result |
| /// of selecting an obligation. This is almost always 'true', |
| /// except when dealing with certain ParamCandidates. |
| /// |
| /// Ordinarily, a ParamCandidate will contain no inference variables, |
| /// since it was usually produced directly from a DefId. However, |
| /// certain cases (currently only librustdoc's blanket impl finder), |
| /// a ParamEnv may be explicitly constructed with inference types. |
| /// When this is the case, we do *not* want to cache the resulting selection |
| /// candidate. This is due to the fact that it might not always be possible |
| /// to equate the obligation's trait ref and the candidate's trait ref, |
| /// if more constraints end up getting added to an inference variable. |
| /// |
| /// Because of this, we always want to re-run the full selection |
| /// process for our obligation the next time we see it, since |
| /// we might end up picking a different SelectionCandidate (or none at all) |
| fn can_cache_candidate(&self, |
| result: &SelectionResult<'tcx, SelectionCandidate<'tcx>> |
| ) -> bool { |
| match result { |
| Ok(Some(SelectionCandidate::ParamCandidate(trait_ref))) => { |
| !trait_ref.skip_binder().input_types().any(|t| t.walk().any(|t_| t_.is_ty_infer())) |
| }, |
| _ => true |
| } |
| } |
| |
| fn insert_candidate_cache( |
| &mut self, |
| param_env: ty::ParamEnv<'tcx>, |
| cache_fresh_trait_pred: ty::PolyTraitPredicate<'tcx>, |
| dep_node: DepNodeIndex, |
| candidate: SelectionResult<'tcx, SelectionCandidate<'tcx>>, |
| ) { |
| let tcx = self.tcx(); |
| let trait_ref = cache_fresh_trait_pred.skip_binder().trait_ref; |
| |
| if !self.can_cache_candidate(&candidate) { |
| debug!("insert_candidate_cache(trait_ref={:?}, candidate={:?} -\ |
| candidate is not cacheable", trait_ref, candidate); |
| return; |
| |
| } |
| |
| if self.can_use_global_caches(param_env) { |
| if let Err(Overflow) = candidate { |
| // Don't cache overflow globally; we only produce this |
| // in certain modes. |
| } else if let Some(trait_ref) = tcx.lift_to_global(&trait_ref) { |
| if let Some(candidate) = tcx.lift_to_global(&candidate) { |
| debug!( |
| "insert_candidate_cache(trait_ref={:?}, candidate={:?}) global", |
| trait_ref, candidate, |
| ); |
| // This may overwrite the cache with the same value |
| tcx.selection_cache |
| .hashmap |
| .borrow_mut() |
| .insert(trait_ref, WithDepNode::new(dep_node, candidate)); |
| return; |
| } |
| } |
| } |
| |
| debug!( |
| "insert_candidate_cache(trait_ref={:?}, candidate={:?}) local", |
| trait_ref, candidate, |
| ); |
| self.infcx |
| .selection_cache |
| .hashmap |
| .borrow_mut() |
| .insert(trait_ref, WithDepNode::new(dep_node, candidate)); |
| } |
| |
| fn assemble_candidates<'o>( |
| &mut self, |
| stack: &TraitObligationStack<'o, 'tcx>, |
| ) -> Result<SelectionCandidateSet<'tcx>, SelectionError<'tcx>> { |
| let TraitObligationStack { obligation, .. } = *stack; |
| let ref obligation = Obligation { |
| param_env: obligation.param_env, |
| cause: obligation.cause.clone(), |
| recursion_depth: obligation.recursion_depth, |
| predicate: self.infcx() |
| .resolve_type_vars_if_possible(&obligation.predicate), |
| }; |
| |
| if obligation.predicate.skip_binder().self_ty().is_ty_var() { |
| // Self is a type variable (e.g., `_: AsRef<str>`). |
| // |
| // This is somewhat problematic, as the current scheme can't really |
| // handle it turning to be a projection. This does end up as truly |
| // ambiguous in most cases anyway. |
| // |
| // Take the fast path out - this also improves |
| // performance by preventing assemble_candidates_from_impls from |
| // matching every impl for this trait. |
| return Ok(SelectionCandidateSet { |
| vec: vec![], |
| ambiguous: true, |
| }); |
| } |
| |
| let mut candidates = SelectionCandidateSet { |
| vec: Vec::new(), |
| ambiguous: false, |
| }; |
| |
| self.assemble_candidates_for_trait_alias(obligation, &mut candidates)?; |
| |
| // Other bounds. Consider both in-scope bounds from fn decl |
| // and applicable impls. There is a certain set of precedence rules here. |
| let def_id = obligation.predicate.def_id(); |
| let lang_items = self.tcx().lang_items(); |
| |
| if lang_items.copy_trait() == Some(def_id) { |
| debug!( |
| "obligation self ty is {:?}", |
| obligation.predicate.skip_binder().self_ty() |
| ); |
| |
| // User-defined copy impls are permitted, but only for |
| // structs and enums. |
| self.assemble_candidates_from_impls(obligation, &mut candidates)?; |
| |
| // For other types, we'll use the builtin rules. |
| let copy_conditions = self.copy_clone_conditions(obligation); |
| self.assemble_builtin_bound_candidates(copy_conditions, &mut candidates)?; |
| } else if lang_items.sized_trait() == Some(def_id) { |
| // Sized is never implementable by end-users, it is |
| // always automatically computed. |
| let sized_conditions = self.sized_conditions(obligation); |
| self.assemble_builtin_bound_candidates(sized_conditions, &mut candidates)?; |
| } else if lang_items.unsize_trait() == Some(def_id) { |
| self.assemble_candidates_for_unsizing(obligation, &mut candidates); |
| } else { |
| if lang_items.clone_trait() == Some(def_id) { |
| // Same builtin conditions as `Copy`, i.e., every type which has builtin support |
| // for `Copy` also has builtin support for `Clone`, + tuples and arrays of `Clone` |
| // types have builtin support for `Clone`. |
| let clone_conditions = self.copy_clone_conditions(obligation); |
| self.assemble_builtin_bound_candidates(clone_conditions, &mut candidates)?; |
| } |
| |
| self.assemble_generator_candidates(obligation, &mut candidates)?; |
| self.assemble_closure_candidates(obligation, &mut candidates)?; |
| self.assemble_fn_pointer_candidates(obligation, &mut candidates)?; |
| self.assemble_candidates_from_impls(obligation, &mut candidates)?; |
| self.assemble_candidates_from_object_ty(obligation, &mut candidates); |
| } |
| |
| self.assemble_candidates_from_projected_tys(obligation, &mut candidates); |
| self.assemble_candidates_from_caller_bounds(stack, &mut candidates)?; |
| // Auto implementations have lower priority, so we only |
| // consider triggering a default if there is no other impl that can apply. |
| if candidates.vec.is_empty() { |
| self.assemble_candidates_from_auto_impls(obligation, &mut candidates)?; |
| } |
| debug!("candidate list size: {}", candidates.vec.len()); |
| Ok(candidates) |
| } |
| |
| fn assemble_candidates_from_projected_tys( |
| &mut self, |
| obligation: &TraitObligation<'tcx>, |
| candidates: &mut SelectionCandidateSet<'tcx>, |
| ) { |
| debug!("assemble_candidates_for_projected_tys({:?})", obligation); |
| |
| // before we go into the whole placeholder thing, just |
| // quickly check if the self-type is a projection at all. |
| match obligation.predicate.skip_binder().trait_ref.self_ty().sty { |
| ty::Projection(_) | ty::Opaque(..) => {} |
| ty::Infer(ty::TyVar(_)) => { |
| span_bug!( |
| obligation.cause.span, |
| "Self=_ should have been handled by assemble_candidates" |
| ); |
| } |
| _ => return, |
| } |
| |
| let result = self.infcx.probe(|_| { |
| self.match_projection_obligation_against_definition_bounds(obligation) |
| }); |
| |
| if result { |
| candidates.vec.push(ProjectionCandidate); |
| } |
| } |
| |
| fn match_projection_obligation_against_definition_bounds( |
| &mut self, |
| obligation: &TraitObligation<'tcx>, |
| ) -> bool { |
| let poly_trait_predicate = self.infcx() |
| .resolve_type_vars_if_possible(&obligation.predicate); |
| let (skol_trait_predicate, _) = self.infcx() |
| .replace_bound_vars_with_placeholders(&poly_trait_predicate); |
| debug!( |
| "match_projection_obligation_against_definition_bounds: \ |
| skol_trait_predicate={:?}", |
| skol_trait_predicate, |
| ); |
| |
| let (def_id, substs) = match skol_trait_predicate.trait_ref.self_ty().sty { |
| ty::Projection(ref data) => (data.trait_ref(self.tcx()).def_id, data.substs), |
| ty::Opaque(def_id, substs) => (def_id, substs), |
| _ => { |
| span_bug!( |
| obligation.cause.span, |
| "match_projection_obligation_against_definition_bounds() called \ |
| but self-ty is not a projection: {:?}", |
| skol_trait_predicate.trait_ref.self_ty() |
| ); |
| } |
| }; |
| debug!( |
| "match_projection_obligation_against_definition_bounds: \ |
| def_id={:?}, substs={:?}", |
| def_id, substs |
| ); |
| |
| let predicates_of = self.tcx().predicates_of(def_id); |
| let bounds = predicates_of.instantiate(self.tcx(), substs); |
| debug!( |
| "match_projection_obligation_against_definition_bounds: \ |
| bounds={:?}", |
| bounds |
| ); |
| |
| let matching_bound = util::elaborate_predicates(self.tcx(), bounds.predicates) |
| .filter_to_traits() |
| .find(|bound| { |
| self.infcx.probe(|_| { |
| self.match_projection( |
| obligation, |
| bound.clone(), |
| skol_trait_predicate.trait_ref.clone(), |
| ) |
| }) |
| }); |
| |
| debug!( |
| "match_projection_obligation_against_definition_bounds: \ |
| matching_bound={:?}", |
| matching_bound |
| ); |
| match matching_bound { |
| None => false, |
| Some(bound) => { |
| // Repeat the successful match, if any, this time outside of a probe. |
| let result = self.match_projection( |
| obligation, |
| bound, |
| skol_trait_predicate.trait_ref.clone(), |
| ); |
| |
| assert!(result); |
| true |
| } |
| } |
| } |
| |
| fn match_projection( |
| &mut self, |
| obligation: &TraitObligation<'tcx>, |
| trait_bound: ty::PolyTraitRef<'tcx>, |
| skol_trait_ref: ty::TraitRef<'tcx>, |
| ) -> bool { |
| debug_assert!(!skol_trait_ref.has_escaping_bound_vars()); |
| self.infcx |
| .at(&obligation.cause, obligation.param_env) |
| .sup(ty::Binder::dummy(skol_trait_ref), trait_bound) |
| .is_ok() |
| } |
| |
| /// Given an obligation like `<SomeTrait for T>`, search the obligations that the caller |
| /// supplied to find out whether it is listed among them. |
| /// |
| /// Never affects inference environment. |
| fn assemble_candidates_from_caller_bounds<'o>( |
| &mut self, |
| stack: &TraitObligationStack<'o, 'tcx>, |
| candidates: &mut SelectionCandidateSet<'tcx>, |
| ) -> Result<(), SelectionError<'tcx>> { |
| debug!( |
| "assemble_candidates_from_caller_bounds({:?})", |
| stack.obligation |
| ); |
| |
| let all_bounds = stack |
| .obligation |
| .param_env |
| .caller_bounds |
| .iter() |
| .filter_map(|o| o.to_opt_poly_trait_ref()); |
| |
| // micro-optimization: filter out predicates relating to different |
| // traits. |
| let matching_bounds = |
| all_bounds.filter(|p| p.def_id() == stack.obligation.predicate.def_id()); |
| |
| // keep only those bounds which may apply, and propagate overflow if it occurs |
| let mut param_candidates = vec![]; |
| for bound in matching_bounds { |
| let wc = self.evaluate_where_clause(stack, bound.clone())?; |
| if wc.may_apply() { |
| param_candidates.push(ParamCandidate(bound)); |
| } |
| } |
| |
| candidates.vec.extend(param_candidates); |
| |
| Ok(()) |
| } |
| |
| fn evaluate_where_clause<'o>( |
| &mut self, |
| stack: &TraitObligationStack<'o, 'tcx>, |
| where_clause_trait_ref: ty::PolyTraitRef<'tcx>, |
| ) -> Result<EvaluationResult, OverflowError> { |
| self.evaluation_probe(|this| { |
| match this.match_where_clause_trait_ref(stack.obligation, where_clause_trait_ref) { |
| Ok(obligations) => { |
| this.evaluate_predicates_recursively(stack.list(), obligations.iter()) |
| } |
| Err(()) => Ok(EvaluatedToErr), |
| } |
| }) |
| } |
| |
| fn assemble_generator_candidates( |
| &mut self, |
| obligation: &TraitObligation<'tcx>, |
| candidates: &mut SelectionCandidateSet<'tcx>, |
| ) -> Result<(), SelectionError<'tcx>> { |
| if self.tcx().lang_items().gen_trait() != Some(obligation.predicate.def_id()) { |
| return Ok(()); |
| } |
| |
| // OK to skip binder because the substs on generator types never |
| // touch bound regions, they just capture the in-scope |
| // type/region parameters |
| let self_ty = *obligation.self_ty().skip_binder(); |
| match self_ty.sty { |
| ty::Generator(..) => { |
| debug!( |
| "assemble_generator_candidates: self_ty={:?} obligation={:?}", |
| self_ty, obligation |
| ); |
| |
| candidates.vec.push(GeneratorCandidate); |
| } |
| ty::Infer(ty::TyVar(_)) => { |
| debug!("assemble_generator_candidates: ambiguous self-type"); |
| candidates.ambiguous = true; |
| } |
| _ => {} |
| } |
| |
| Ok(()) |
| } |
| |
| /// Check for the artificial impl that the compiler will create for an obligation like `X : |
| /// FnMut<..>` where `X` is a closure type. |
| /// |
| /// Note: the type parameters on a closure candidate are modeled as *output* type |
| /// parameters and hence do not affect whether this trait is a match or not. They will be |
| /// unified during the confirmation step. |
| fn assemble_closure_candidates( |
| &mut self, |
| obligation: &TraitObligation<'tcx>, |
| candidates: &mut SelectionCandidateSet<'tcx>, |
| ) -> Result<(), SelectionError<'tcx>> { |
| let kind = match self.tcx() |
| .lang_items() |
| .fn_trait_kind(obligation.predicate.def_id()) |
| { |
| Some(k) => k, |
| None => { |
| return Ok(()); |
| } |
| }; |
| |
| // OK to skip binder because the substs on closure types never |
| // touch bound regions, they just capture the in-scope |
| // type/region parameters |
| match obligation.self_ty().skip_binder().sty { |
| ty::Closure(closure_def_id, closure_substs) => { |
| debug!( |
| "assemble_unboxed_candidates: kind={:?} obligation={:?}", |
| kind, obligation |
| ); |
| match self.infcx.closure_kind(closure_def_id, closure_substs) { |
| Some(closure_kind) => { |
| debug!( |
| "assemble_unboxed_candidates: closure_kind = {:?}", |
| closure_kind |
| ); |
| if closure_kind.extends(kind) { |
| candidates.vec.push(ClosureCandidate); |
| } |
| } |
| None => { |
| debug!("assemble_unboxed_candidates: closure_kind not yet known"); |
| candidates.vec.push(ClosureCandidate); |
| } |
| } |
| } |
| ty::Infer(ty::TyVar(_)) => { |
| debug!("assemble_unboxed_closure_candidates: ambiguous self-type"); |
| candidates.ambiguous = true; |
| } |
| _ => {} |
| } |
| |
| Ok(()) |
| } |
| |
| /// Implement one of the `Fn()` family for a fn pointer. |
| fn assemble_fn_pointer_candidates( |
| &mut self, |
| obligation: &TraitObligation<'tcx>, |
| candidates: &mut SelectionCandidateSet<'tcx>, |
| ) -> Result<(), SelectionError<'tcx>> { |
| // We provide impl of all fn traits for fn pointers. |
| if self.tcx() |
| .lang_items() |
| .fn_trait_kind(obligation.predicate.def_id()) |
| .is_none() |
| { |
| return Ok(()); |
| } |
| |
| // OK to skip binder because what we are inspecting doesn't involve bound regions |
| let self_ty = *obligation.self_ty().skip_binder(); |
| match self_ty.sty { |
| ty::Infer(ty::TyVar(_)) => { |
| debug!("assemble_fn_pointer_candidates: ambiguous self-type"); |
| candidates.ambiguous = true; // could wind up being a fn() type |
| } |
| // provide an impl, but only for suitable `fn` pointers |
| ty::FnDef(..) | ty::FnPtr(_) => { |
| if let ty::FnSig { |
| unsafety: hir::Unsafety::Normal, |
| abi: Abi::Rust, |
| variadic: false, |
| .. |
| } = self_ty.fn_sig(self.tcx()).skip_binder() |
| { |
| candidates.vec.push(FnPointerCandidate); |
| } |
| } |
| _ => {} |
| } |
| |
| Ok(()) |
| } |
| |
| /// Search for impls that might apply to `obligation`. |
| fn assemble_candidates_from_impls( |
| &mut self, |
| obligation: &TraitObligation<'tcx>, |
| candidates: &mut SelectionCandidateSet<'tcx>, |
| ) -> Result<(), SelectionError<'tcx>> { |
| debug!( |
| "assemble_candidates_from_impls(obligation={:?})", |
| obligation |
| ); |
| |
| self.tcx().for_each_relevant_impl( |
| obligation.predicate.def_id(), |
| obligation.predicate.skip_binder().trait_ref.self_ty(), |
| |impl_def_id| { |
| self.infcx.probe(|_| { |
| if let Ok(_substs) = self.match_impl(impl_def_id, obligation) |
| { |
| candidates.vec.push(ImplCandidate(impl_def_id)); |
| } |
| }); |
| }, |
| ); |
| |
| Ok(()) |
| } |
| |
| fn assemble_candidates_from_auto_impls( |
| &mut self, |
| obligation: &TraitObligation<'tcx>, |
| candidates: &mut SelectionCandidateSet<'tcx>, |
| ) -> Result<(), SelectionError<'tcx>> { |
| // OK to skip binder here because the tests we do below do not involve bound regions |
| let self_ty = *obligation.self_ty().skip_binder(); |
| debug!("assemble_candidates_from_auto_impls(self_ty={:?})", self_ty); |
| |
| let def_id = obligation.predicate.def_id(); |
| |
| if self.tcx().trait_is_auto(def_id) { |
| match self_ty.sty { |
| ty::Dynamic(..) => { |
| // For object types, we don't know what the closed |
| // over types are. This means we conservatively |
| // say nothing; a candidate may be added by |
| // `assemble_candidates_from_object_ty`. |
| } |
| ty::Foreign(..) => { |
| // Since the contents of foreign types is unknown, |
| // we don't add any `..` impl. Default traits could |
| // still be provided by a manual implementation for |
| // this trait and type. |
| } |
| ty::Param(..) | ty::Projection(..) => { |
| // In these cases, we don't know what the actual |
| // type is. Therefore, we cannot break it down |
| // into its constituent types. So we don't |
| // consider the `..` impl but instead just add no |
| // candidates: this means that typeck will only |
| // succeed if there is another reason to believe |
| // that this obligation holds. That could be a |
| // where-clause or, in the case of an object type, |
| // it could be that the object type lists the |
| // trait (e.g., `Foo+Send : Send`). See |
| // `compile-fail/typeck-default-trait-impl-send-param.rs` |
| // for an example of a test case that exercises |
| // this path. |
| } |
| ty::Infer(ty::TyVar(_)) => { |
| // the auto impl might apply, we don't know |
| candidates.ambiguous = true; |
| } |
| _ => candidates.vec.push(AutoImplCandidate(def_id.clone())), |
| } |
| } |
| |
| Ok(()) |
| } |
| |
| /// Search for impls that might apply to `obligation`. |
| fn assemble_candidates_from_object_ty( |
| &mut self, |
| obligation: &TraitObligation<'tcx>, |
| candidates: &mut SelectionCandidateSet<'tcx>, |
| ) { |
| debug!( |
| "assemble_candidates_from_object_ty(self_ty={:?})", |
| obligation.self_ty().skip_binder() |
| ); |
| |
| self.infcx.probe(|_snapshot| { |
| // The code below doesn't care about regions, and the |
| // self-ty here doesn't escape this probe, so just erase |
| // any LBR. |
| let self_ty = self.tcx().erase_late_bound_regions(&obligation.self_ty()); |
| let poly_trait_ref = match self_ty.sty { |
| ty::Dynamic(ref data, ..) => { |
| if data.auto_traits() |
| .any(|did| did == obligation.predicate.def_id()) |
| { |
| debug!( |
| "assemble_candidates_from_object_ty: matched builtin bound, \ |
| pushing candidate" |
| ); |
| candidates.vec.push(BuiltinObjectCandidate); |
| return; |
| } |
| |
| if let Some(principal) = data.principal() { |
| principal.with_self_ty(self.tcx(), self_ty) |
| } else { |
| // Only auto-trait bounds exist. |
| return; |
| } |
| } |
| ty::Infer(ty::TyVar(_)) => { |
| debug!("assemble_candidates_from_object_ty: ambiguous"); |
| candidates.ambiguous = true; // could wind up being an object type |
| return; |
| } |
| _ => return, |
| }; |
| |
| debug!( |
| "assemble_candidates_from_object_ty: poly_trait_ref={:?}", |
| poly_trait_ref |
| ); |
| |
| // Count only those upcast versions that match the trait-ref |
| // we are looking for. Specifically, do not only check for the |
| // correct trait, but also the correct type parameters. |
| // For example, we may be trying to upcast `Foo` to `Bar<i32>`, |
| // but `Foo` is declared as `trait Foo : Bar<u32>`. |
| let upcast_trait_refs = util::supertraits(self.tcx(), poly_trait_ref) |
| .filter(|upcast_trait_ref| { |
| self.infcx.probe(|_| { |
| let upcast_trait_ref = upcast_trait_ref.clone(); |
| self.match_poly_trait_ref(obligation, upcast_trait_ref) |
| .is_ok() |
| }) |
| }) |
| .count(); |
| |
| if upcast_trait_refs > 1 { |
| // Can be upcast in many ways; need more type information. |
| candidates.ambiguous = true; |
| } else if upcast_trait_refs == 1 { |
| candidates.vec.push(ObjectCandidate); |
| } |
| }) |
| } |
| |
| /// Search for unsizing that might apply to `obligation`. |
| fn assemble_candidates_for_unsizing( |
| &mut self, |
| obligation: &TraitObligation<'tcx>, |
| candidates: &mut SelectionCandidateSet<'tcx>, |
| ) { |
| // We currently never consider higher-ranked obligations e.g. |
| // `for<'a> &'a T: Unsize<Trait+'a>` to be implemented. This is not |
| // because they are a priori invalid, and we could potentially add support |
| // for them later, it's just that there isn't really a strong need for it. |
| // A `T: Unsize<U>` obligation is always used as part of a `T: CoerceUnsize<U>` |
| // impl, and those are generally applied to concrete types. |
| // |
| // That said, one might try to write a fn with a where clause like |
| // for<'a> Foo<'a, T>: Unsize<Foo<'a, Trait>> |
| // where the `'a` is kind of orthogonal to the relevant part of the `Unsize`. |
| // Still, you'd be more likely to write that where clause as |
| // T: Trait |
| // so it seems ok if we (conservatively) fail to accept that `Unsize` |
| // obligation above. Should be possible to extend this in the future. |
| let source = match obligation.self_ty().no_bound_vars() { |
| Some(t) => t, |
| None => { |
| // Don't add any candidates if there are bound regions. |
| return; |
| } |
| }; |
| let target = obligation |
| .predicate |
| .skip_binder() |
| .trait_ref |
| .substs |
| .type_at(1); |
| |
| debug!( |
| "assemble_candidates_for_unsizing(source={:?}, target={:?})", |
| source, target |
| ); |
| |
| let may_apply = match (&source.sty, &target.sty) { |
| // Trait+Kx+'a -> Trait+Ky+'b (upcasts). |
| (&ty::Dynamic(ref data_a, ..), &ty::Dynamic(ref data_b, ..)) => { |
| // Upcasts permit two things: |
| // |
| // 1. Dropping builtin bounds, e.g., `Foo+Send` to `Foo` |
| // 2. Tightening the region bound, e.g., `Foo+'a` to `Foo+'b` if `'a : 'b` |
| // |
| // Note that neither of these changes requires any |
| // change at runtime. Eventually this will be |
| // generalized. |
| // |
| // We always upcast when we can because of reason |
| // #2 (region bounds). |
| data_a.principal_def_id() == data_b.principal_def_id() |
| && data_b.auto_traits() |
| // All of a's auto traits need to be in b's auto traits. |
| .all(|b| data_a.auto_traits().any(|a| a == b)) |
| } |
| |
| // T -> Trait. |
| (_, &ty::Dynamic(..)) => true, |
| |
| // Ambiguous handling is below T -> Trait, because inference |
| // variables can still implement Unsize<Trait> and nested |
| // obligations will have the final say (likely deferred). |
| (&ty::Infer(ty::TyVar(_)), _) | (_, &ty::Infer(ty::TyVar(_))) => { |
| debug!("assemble_candidates_for_unsizing: ambiguous"); |
| candidates.ambiguous = true; |
| false |
| } |
| |
| // [T; n] -> [T]. |
| (&ty::Array(..), &ty::Slice(_)) => true, |
| |
| // Struct<T> -> Struct<U>. |
| (&ty::Adt(def_id_a, _), &ty::Adt(def_id_b, _)) if def_id_a.is_struct() => { |
| def_id_a == def_id_b |
| } |
| |
| // (.., T) -> (.., U). |
| (&ty::Tuple(tys_a), &ty::Tuple(tys_b)) => tys_a.len() == tys_b.len(), |
| |
| _ => false, |
| }; |
| |
| if may_apply { |
| candidates.vec.push(BuiltinUnsizeCandidate); |
| } |
| } |
| |
| fn assemble_candidates_for_trait_alias( |
| &mut self, |
| obligation: &TraitObligation<'tcx>, |
| candidates: &mut SelectionCandidateSet<'tcx>, |
| ) -> Result<(), SelectionError<'tcx>> { |
| // OK to skip binder here because the tests we do below do not involve bound regions |
| let self_ty = *obligation.self_ty().skip_binder(); |
| debug!("assemble_candidates_for_trait_alias(self_ty={:?})", self_ty); |
| |
| let def_id = obligation.predicate.def_id(); |
| |
| if ty::is_trait_alias(self.tcx(), def_id) { |
| candidates.vec.push(TraitAliasCandidate(def_id.clone())); |
| } |
| |
| Ok(()) |
| } |
| |
| /////////////////////////////////////////////////////////////////////////// |
| // WINNOW |
| // |
| // Winnowing is the process of attempting to resolve ambiguity by |
| // probing further. During the winnowing process, we unify all |
| // type variables and then we also attempt to evaluate recursive |
| // bounds to see if they are satisfied. |
| |
| /// Returns true if `victim` should be dropped in favor of |
| /// `other`. Generally speaking we will drop duplicate |
| /// candidates and prefer where-clause candidates. |
| /// |
| /// See the comment for "SelectionCandidate" for more details. |
| fn candidate_should_be_dropped_in_favor_of<'o>( |
| &mut self, |
| victim: &EvaluatedCandidate<'tcx>, |
| other: &EvaluatedCandidate<'tcx>, |
| ) -> bool { |
| if victim.candidate == other.candidate { |
| return true; |
| } |
| |
| // Check if a bound would previously have been removed when normalizing |
| // the param_env so that it can be given the lowest priority. See |
| // #50825 for the motivation for this. |
| let is_global = |
| |cand: &ty::PolyTraitRef<'_>| cand.is_global() && !cand.has_late_bound_regions(); |
| |
| match other.candidate { |
| // Prefer BuiltinCandidate { has_nested: false } to anything else. |
| // This is a fix for #53123 and prevents winnowing from accidentally extending the |
| // lifetime of a variable. |
| BuiltinCandidate { has_nested: false } => true, |
| ParamCandidate(ref cand) => match victim.candidate { |
| AutoImplCandidate(..) => { |
| bug!( |
| "default implementations shouldn't be recorded \ |
| when there are other valid candidates" |
| ); |
| } |
| // Prefer BuiltinCandidate { has_nested: false } to anything else. |
| // This is a fix for #53123 and prevents winnowing from accidentally extending the |
| // lifetime of a variable. |
| BuiltinCandidate { has_nested: false } => false, |
| ImplCandidate(..) |
| | ClosureCandidate |
| | GeneratorCandidate |
| | FnPointerCandidate |
| | BuiltinObjectCandidate |
| | BuiltinUnsizeCandidate |
| | BuiltinCandidate { .. } |
| | TraitAliasCandidate(..) => { |
| // Global bounds from the where clause should be ignored |
| // here (see issue #50825). Otherwise, we have a where |
| // clause so don't go around looking for impls. |
| !is_global(cand) |
| } |
| ObjectCandidate | ProjectionCandidate => { |
| // Arbitrarily give param candidates priority |
| // over projection and object candidates. |
| !is_global(cand) |
| } |
| ParamCandidate(..) => false, |
| }, |
| ObjectCandidate | ProjectionCandidate => match victim.candidate { |
| AutoImplCandidate(..) => { |
| bug!( |
| "default implementations shouldn't be recorded \ |
| when there are other valid candidates" |
| ); |
| } |
| // Prefer BuiltinCandidate { has_nested: false } to anything else. |
| // This is a fix for #53123 and prevents winnowing from accidentally extending the |
| // lifetime of a variable. |
| BuiltinCandidate { has_nested: false } => false, |
| ImplCandidate(..) |
| | ClosureCandidate |
| | GeneratorCandidate |
| | FnPointerCandidate |
| | BuiltinObjectCandidate |
| | BuiltinUnsizeCandidate |
| | BuiltinCandidate { .. } |
| | TraitAliasCandidate(..) => true, |
| ObjectCandidate | ProjectionCandidate => { |
| // Arbitrarily give param candidates priority |
| // over projection and object candidates. |
| true |
| } |
| ParamCandidate(ref cand) => is_global(cand), |
| }, |
| ImplCandidate(other_def) => { |
| // See if we can toss out `victim` based on specialization. |
| // This requires us to know *for sure* that the `other` impl applies |
| // i.e., EvaluatedToOk: |
| if other.evaluation.must_apply_modulo_regions() { |
| match victim.candidate { |
| ImplCandidate(victim_def) => { |
| let tcx = self.tcx().global_tcx(); |
| return tcx.specializes((other_def, victim_def)) |
| || tcx.impls_are_allowed_to_overlap( |
| other_def, victim_def).is_some(); |
| } |
| ParamCandidate(ref cand) => { |
| // Prefer the impl to a global where clause candidate. |
| return is_global(cand); |
| } |
| _ => (), |
| } |
| } |
| |
| false |
| } |
| ClosureCandidate |
| | GeneratorCandidate |
| | FnPointerCandidate |
| | BuiltinObjectCandidate |
| | BuiltinUnsizeCandidate |
| | BuiltinCandidate { has_nested: true } => { |
| match victim.candidate { |
| ParamCandidate(ref cand) => { |
| // Prefer these to a global where-clause bound |
| // (see issue #50825) |
| is_global(cand) && other.evaluation.must_apply_modulo_regions() |
| } |
| _ => false, |
| } |
| } |
| _ => false, |
| } |
| } |
| |
| /////////////////////////////////////////////////////////////////////////// |
| // BUILTIN BOUNDS |
| // |
| // These cover the traits that are built-in to the language |
| // itself: `Copy`, `Clone` and `Sized`. |
| |
| fn assemble_builtin_bound_candidates<'o>( |
| &mut self, |
| conditions: BuiltinImplConditions<'tcx>, |
| candidates: &mut SelectionCandidateSet<'tcx>, |
| ) -> Result<(), SelectionError<'tcx>> { |
| match conditions { |
| BuiltinImplConditions::Where(nested) => { |
| debug!("builtin_bound: nested={:?}", nested); |
| candidates.vec.push(BuiltinCandidate { |
| has_nested: nested.skip_binder().len() > 0, |
| }); |
| } |
| BuiltinImplConditions::None => {} |
| BuiltinImplConditions::Ambiguous => { |
| debug!("assemble_builtin_bound_candidates: ambiguous builtin"); |
| candidates.ambiguous = true; |
| } |
| } |
| |
| Ok(()) |
| } |
| |
| fn sized_conditions( |
| &mut self, |
| obligation: &TraitObligation<'tcx>, |
| ) -> BuiltinImplConditions<'tcx> { |
| use self::BuiltinImplConditions::{Ambiguous, None, Where}; |
| |
| // NOTE: binder moved to (*) |
| let self_ty = self.infcx |
| .shallow_resolve(obligation.predicate.skip_binder().self_ty()); |
| |
| match self_ty.sty { |
| ty::Infer(ty::IntVar(_)) |
| | ty::Infer(ty::FloatVar(_)) |
| | ty::Uint(_) |
| | ty::Int(_) |
| | ty::Bool |
| | ty::Float(_) |
| | ty::FnDef(..) |
| | ty::FnPtr(_) |
| | ty::RawPtr(..) |
| | ty::Char |
| | ty::Ref(..) |
| | ty::Generator(..) |
| | ty::GeneratorWitness(..) |
| | ty::Array(..) |
| | ty::Closure(..) |
| | ty::Never |
| | ty::Error => { |
| // safe for everything |
| Where(ty::Binder::dummy(Vec::new())) |
| } |
| |
| ty::Str | ty::Slice(_) | ty::Dynamic(..) | ty::Foreign(..) => None, |
| |
| ty::Tuple(tys) => Where(ty::Binder::bind(tys.last().into_iter().cloned().collect())), |
| |
| ty::Adt(def, substs) => { |
| let sized_crit = def.sized_constraint(self.tcx()); |
| // (*) binder moved here |
| Where(ty::Binder::bind( |
| sized_crit |
| .iter() |
| .map(|ty| ty.subst(self.tcx(), substs)) |
| .collect(), |
| )) |
| } |
| |
| ty::Projection(_) | ty::Param(_) | ty::Opaque(..) => None, |
| ty::Infer(ty::TyVar(_)) => Ambiguous, |
| |
| ty::UnnormalizedProjection(..) |
| | ty::Placeholder(..) |
| | ty::Bound(..) |
| | ty::Infer(ty::FreshTy(_)) |
| | ty::Infer(ty::FreshIntTy(_)) |
| | ty::Infer(ty::FreshFloatTy(_)) => { |
| bug!( |
| "asked to assemble builtin bounds of unexpected type: {:?}", |
| self_ty |
| ); |
| } |
| } |
| } |
| |
| fn copy_clone_conditions( |
| &mut self, |
| obligation: &TraitObligation<'tcx>, |
| ) -> BuiltinImplConditions<'tcx> { |
| // NOTE: binder moved to (*) |
| let self_ty = self.infcx |
| .shallow_resolve(obligation.predicate.skip_binder().self_ty()); |
| |
| use self::BuiltinImplConditions::{Ambiguous, None, Where}; |
| |
| match self_ty.sty { |
| ty::Infer(ty::IntVar(_)) |
| | ty::Infer(ty::FloatVar(_)) |
| | ty::FnDef(..) |
| | ty::FnPtr(_) |
| | ty::Error => Where(ty::Binder::dummy(Vec::new())), |
| |
| ty::Uint(_) |
| | ty::Int(_) |
| | ty::Bool |
| | ty::Float(_) |
| | ty::Char |
| | ty::RawPtr(..) |
| | ty::Never |
| | ty::Ref(_, _, hir::MutImmutable) => { |
| // Implementations provided in libcore |
| None |
| } |
| |
| ty::Dynamic(..) |
| | ty::Str |
| | ty::Slice(..) |
| | ty::Generator(..) |
| | ty::GeneratorWitness(..) |
| | ty::Foreign(..) |
| | ty::Ref(_, _, hir::MutMutable) => None, |
| |
| ty::Array(element_ty, _) => { |
| // (*) binder moved here |
| Where(ty::Binder::bind(vec![element_ty])) |
| } |
| |
| ty::Tuple(tys) => { |
| // (*) binder moved here |
| Where(ty::Binder::bind(tys.to_vec())) |
| } |
| |
| ty::Closure(def_id, substs) => { |
| let trait_id = obligation.predicate.def_id(); |
| let is_copy_trait = Some(trait_id) == self.tcx().lang_items().copy_trait(); |
| let is_clone_trait = Some(trait_id) == self.tcx().lang_items().clone_trait(); |
| if is_copy_trait || is_clone_trait { |
| Where(ty::Binder::bind( |
| substs.upvar_tys(def_id, self.tcx()).collect(), |
| )) |
| } else { |
| None |
| } |
| } |
| |
| ty::Adt(..) | ty::Projection(..) | ty::Param(..) | ty::Opaque(..) => { |
| // Fallback to whatever user-defined impls exist in this case. |
| None |
| } |
| |
| ty::Infer(ty::TyVar(_)) => { |
| // Unbound type variable. Might or might not have |
| // applicable impls and so forth, depending on what |
| // those type variables wind up being bound to. |
| Ambiguous |
| } |
| |
| ty::UnnormalizedProjection(..) |
| | ty::Placeholder(..) |
| | ty::Bound(..) |
| | ty::Infer(ty::FreshTy(_)) |
| | ty::Infer(ty::FreshIntTy(_)) |
| | ty::Infer(ty::FreshFloatTy(_)) => { |
| bug!( |
| "asked to assemble builtin bounds of unexpected type: {:?}", |
| self_ty |
| ); |
| } |
| } |
| } |
| |
| /// For default impls, we need to break apart a type into its |
| /// "constituent types" -- meaning, the types that it contains. |
| /// |
| /// Here are some (simple) examples: |
| /// |
| /// ``` |
| /// (i32, u32) -> [i32, u32] |
| /// Foo where struct Foo { x: i32, y: u32 } -> [i32, u32] |
| /// Bar<i32> where struct Bar<T> { x: T, y: u32 } -> [i32, u32] |
| /// Zed<i32> where enum Zed { A(T), B(u32) } -> [i32, u32] |
| /// ``` |
| fn constituent_types_for_ty(&self, t: Ty<'tcx>) -> Vec<Ty<'tcx>> { |
| match t.sty { |
| ty::Uint(_) |
| | ty::Int(_) |
| | ty::Bool |
| | ty::Float(_) |
| | ty::FnDef(..) |
| | ty::FnPtr(_) |
| | ty::Str |
| | ty::Error |
| | ty::Infer(ty::IntVar(_)) |
| | ty::Infer(ty::FloatVar(_)) |
| | ty::Never |
| | ty::Char => Vec::new(), |
| |
| ty::UnnormalizedProjection(..) |
| | ty::Placeholder(..) |
| | ty::Dynamic(..) |
| | ty::Param(..) |
| | ty::Foreign(..) |
| | ty::Projection(..) |
| | ty::Bound(..) |
| | ty::Infer(ty::TyVar(_)) |
| | ty::Infer(ty::FreshTy(_)) |
| | ty::Infer(ty::FreshIntTy(_)) |
| | ty::Infer(ty::FreshFloatTy(_)) => { |
| bug!( |
| "asked to assemble constituent types of unexpected type: {:?}", |
| t |
| ); |
| } |
| |
| ty::RawPtr(ty::TypeAndMut { ty: element_ty, .. }) | ty::Ref(_, element_ty, _) => { |
| vec![element_ty] |
| } |
| |
| ty::Array(element_ty, _) | ty::Slice(element_ty) => vec![element_ty], |
| |
| ty::Tuple(ref tys) => { |
| // (T1, ..., Tn) -- meets any bound that all of T1...Tn meet |
| tys.to_vec() |
| } |
| |
| ty::Closure(def_id, ref substs) => substs.upvar_tys(def_id, self.tcx()).collect(), |
| |
| ty::Generator(def_id, ref substs, _) => { |
| let witness = substs.witness(def_id, self.tcx()); |
| substs |
| .upvar_tys(def_id, self.tcx()) |
| .chain(iter::once(witness)) |
| .collect() |
| } |
| |
| ty::GeneratorWitness(types) => { |
| // This is sound because no regions in the witness can refer to |
| // the binder outside the witness. So we'll effectivly reuse |
| // the implicit binder around the witness. |
| types.skip_binder().to_vec() |
| } |
| |
| // for `PhantomData<T>`, we pass `T` |
| ty::Adt(def, substs) if def.is_phantom_data() => substs.types().collect(), |
| |
| ty::Adt(def, substs) => def.all_fields().map(|f| f.ty(self.tcx(), substs)).collect(), |
| |
| ty::Opaque(def_id, substs) => { |
| // We can resolve the `impl Trait` to its concrete type, |
| // which enforces a DAG between the functions requiring |
| // the auto trait bounds in question. |
| vec![self.tcx().type_of(def_id).subst(self.tcx(), substs)] |
| } |
| } |
| } |
| |
| fn collect_predicates_for_types( |
| &mut self, |
| param_env: ty::ParamEnv<'tcx>, |
| cause: ObligationCause<'tcx>, |
| recursion_depth: usize, |
| trait_def_id: DefId, |
| types: ty::Binder<Vec<Ty<'tcx>>>, |
| ) -> Vec<PredicateObligation<'tcx>> { |
| // Because the types were potentially derived from |
| // higher-ranked obligations they may reference late-bound |
| // regions. For example, `for<'a> Foo<&'a int> : Copy` would |
| // yield a type like `for<'a> &'a int`. In general, we |
| // maintain the invariant that we never manipulate bound |
| // regions, so we have to process these bound regions somehow. |
| // |
| // The strategy is to: |
| // |
| // 1. Instantiate those regions to placeholder regions (e.g., |
| // `for<'a> &'a int` becomes `&0 int`. |
| // 2. Produce something like `&'0 int : Copy` |
| // 3. Re-bind the regions back to `for<'a> &'a int : Copy` |
| |
| types |
| .skip_binder() |
| .into_iter() |
| .flat_map(|ty| { |
| // binder moved -\ |
| let ty: ty::Binder<Ty<'tcx>> = ty::Binder::bind(ty); // <----/ |
| |
| self.infcx.in_snapshot(|_| { |
| let (skol_ty, _) = self.infcx |
| .replace_bound_vars_with_placeholders(&ty); |
| let Normalized { |
| value: normalized_ty, |
| mut obligations, |
| } = project::normalize_with_depth( |
| self, |
| param_env, |
| cause.clone(), |
| recursion_depth, |
| &skol_ty, |
| ); |
| let skol_obligation = self.tcx().predicate_for_trait_def( |
| param_env, |
| cause.clone(), |
| trait_def_id, |
| recursion_depth, |
| normalized_ty, |
| &[], |
| ); |
| obligations.push(skol_obligation); |
| obligations |
| }) |
| }) |
| .collect() |
| } |
| |
| /////////////////////////////////////////////////////////////////////////// |
| // CONFIRMATION |
| // |
| // Confirmation unifies the output type parameters of the trait |
| // with the values found in the obligation, possibly yielding a |
| // type error. See the [rustc guide] for more details. |
| // |
| // [rustc guide]: |
| // https://rust-lang.github.io/rustc-guide/traits/resolution.html#confirmation |
| |
| fn confirm_candidate( |
| &mut self, |
| obligation: &TraitObligation<'tcx>, |
| candidate: SelectionCandidate<'tcx>, |
| ) -> Result<Selection<'tcx>, SelectionError<'tcx>> { |
| debug!("confirm_candidate({:?}, {:?})", obligation, candidate); |
| |
| match candidate { |
| BuiltinCandidate { has_nested } => { |
| let data = self.confirm_builtin_candidate(obligation, has_nested); |
| Ok(VtableBuiltin(data)) |
| } |
| |
| ParamCandidate(param) => { |
| let obligations = self.confirm_param_candidate(obligation, param); |
| Ok(VtableParam(obligations)) |
| } |
| |
| ImplCandidate(impl_def_id) => Ok(VtableImpl(self.confirm_impl_candidate( |
| obligation, |
| impl_def_id, |
| ))), |
| |
| AutoImplCandidate(trait_def_id) => { |
| let data = self.confirm_auto_impl_candidate(obligation, trait_def_id); |
| Ok(VtableAutoImpl(data)) |
| } |
| |
| ProjectionCandidate => { |
| self.confirm_projection_candidate(obligation); |
| Ok(VtableParam(Vec::new())) |
| } |
| |
| ClosureCandidate => { |
| let vtable_closure = self.confirm_closure_candidate(obligation)?; |
| Ok(VtableClosure(vtable_closure)) |
| } |
| |
| GeneratorCandidate => { |
| let vtable_generator = self.confirm_generator_candidate(obligation)?; |
| Ok(VtableGenerator(vtable_generator)) |
| } |
| |
| FnPointerCandidate => { |
| let data = self.confirm_fn_pointer_candidate(obligation)?; |
| Ok(VtableFnPointer(data)) |
| } |
| |
| TraitAliasCandidate(alias_def_id) => { |
| let data = self.confirm_trait_alias_candidate(obligation, alias_def_id); |
| Ok(VtableTraitAlias(data)) |
| } |
| |
| ObjectCandidate => { |
| let data = self.confirm_object_candidate(obligation); |
| Ok(VtableObject(data)) |
| } |
| |
| BuiltinObjectCandidate => { |
| // This indicates something like `(Trait+Send) : |
| // Send`. In this case, we know that this holds |
| // because that's what the object type is telling us, |
| // and there's really no additional obligations to |
| // prove and no types in particular to unify etc. |
| Ok(VtableParam(Vec::new())) |
| } |
| |
| BuiltinUnsizeCandidate => { |
| let data = self.confirm_builtin_unsize_candidate(obligation)?; |
| Ok(VtableBuiltin(data)) |
| } |
| } |
| } |
| |
| fn confirm_projection_candidate(&mut self, obligation: &TraitObligation<'tcx>) { |
| self.infcx.in_snapshot(|_| { |
| let result = |
| self.match_projection_obligation_against_definition_bounds(obligation); |
| assert!(result); |
| }) |
| } |
| |
| fn confirm_param_candidate( |
| &mut self, |
| obligation: &TraitObligation<'tcx>, |
| param: ty::PolyTraitRef<'tcx>, |
| ) -> Vec<PredicateObligation<'tcx>> { |
| debug!("confirm_param_candidate({:?},{:?})", obligation, param); |
| |
| // During evaluation, we already checked that this |
| // where-clause trait-ref could be unified with the obligation |
| // trait-ref. Repeat that unification now without any |
| // transactional boundary; it should not fail. |
| match self.match_where_clause_trait_ref(obligation, param.clone()) { |
| Ok(obligations) => obligations, |
| Err(()) => { |
| bug!( |
| "Where clause `{:?}` was applicable to `{:?}` but now is not", |
| param, |
| obligation |
| ); |
| } |
| } |
| } |
| |
| fn confirm_builtin_candidate( |
| &mut self, |
| obligation: &TraitObligation<'tcx>, |
| has_nested: bool, |
| ) -> VtableBuiltinData<PredicateObligation<'tcx>> { |
| debug!( |
| "confirm_builtin_candidate({:?}, {:?})", |
| obligation, has_nested |
| ); |
| |
| let lang_items = self.tcx().lang_items(); |
| let obligations = if has_nested { |
| let trait_def = obligation.predicate.def_id(); |
| let conditions = if Some(trait_def) == lang_items.sized_trait() { |
| self.sized_conditions(obligation) |
| } else if Some(trait_def) == lang_items.copy_trait() { |
| self.copy_clone_conditions(obligation) |
| } else if Some(trait_def) == lang_items.clone_trait() { |
| self.copy_clone_conditions(obligation) |
| } else { |
| bug!("unexpected builtin trait {:?}", trait_def) |
| }; |
| let nested = match conditions { |
| BuiltinImplConditions::Where(nested) => nested, |
| _ => bug!( |
| "obligation {:?} had matched a builtin impl but now doesn't", |
| obligation |
| ), |
| }; |
| |
| let cause = obligation.derived_cause(BuiltinDerivedObligation); |
| self.collect_predicates_for_types( |
| obligation.param_env, |
| cause, |
| obligation.recursion_depth + 1, |
| trait_def, |
| nested, |
| ) |
| } else { |
| vec![] |
| }; |
| |
| debug!("confirm_builtin_candidate: obligations={:?}", obligations); |
| |
| VtableBuiltinData { |
| nested: obligations, |
| } |
| } |
| |
| /// This handles the case where a `auto trait Foo` impl is being used. |
| /// The idea is that the impl applies to `X : Foo` if the following conditions are met: |
| /// |
| /// 1. For each constituent type `Y` in `X`, `Y : Foo` holds |
| /// 2. For each where-clause `C` declared on `Foo`, `[Self => X] C` holds. |
| fn confirm_auto_impl_candidate( |
| &mut self, |
| obligation: &TraitObligation<'tcx>, |
| trait_def_id: DefId, |
| ) -> VtableAutoImplData<PredicateObligation<'tcx>> { |
| debug!( |
| "confirm_auto_impl_candidate({:?}, {:?})", |
| obligation, trait_def_id |
| ); |
| |
| let types = obligation.predicate.map_bound(|inner| { |
| let self_ty = self.infcx.shallow_resolve(inner.self_ty()); |
| self.constituent_types_for_ty(self_ty) |
| }); |
| self.vtable_auto_impl(obligation, trait_def_id, types) |
| } |
| |
| /// See `confirm_auto_impl_candidate`. |
| fn vtable_auto_impl( |
| &mut self, |
| obligation: &TraitObligation<'tcx>, |
| trait_def_id: DefId, |
| nested: ty::Binder<Vec<Ty<'tcx>>>, |
| ) -> VtableAutoImplData<PredicateObligation<'tcx>> { |
| debug!("vtable_auto_impl: nested={:?}", nested); |
| |
| let cause = obligation.derived_cause(BuiltinDerivedObligation); |
| let mut obligations = self.collect_predicates_for_types( |
| obligation.param_env, |
| cause, |
| obligation.recursion_depth + 1, |
| trait_def_id, |
| nested, |
| ); |
| |
| let trait_obligations: Vec<PredicateObligation<'_>> = self.infcx.in_snapshot(|_| { |
| let poly_trait_ref = obligation.predicate.to_poly_trait_ref(); |
| let (trait_ref, _) = self.infcx |
| .replace_bound_vars_with_placeholders(&poly_trait_ref); |
| let cause = obligation.derived_cause(ImplDerivedObligation); |
| self.impl_or_trait_obligations( |
| cause, |
| obligation.recursion_depth + 1, |
| obligation.param_env, |
| trait_def_id, |
| &trait_ref.substs, |
| ) |
| }); |
| |
| // Adds the predicates from the trait. Note that this contains a `Self: Trait` |
| // predicate as usual. It won't have any effect since auto traits are coinductive. |
| obligations.extend(trait_obligations); |
| |
| debug!("vtable_auto_impl: obligations={:?}", obligations); |
| |
| VtableAutoImplData { |
| trait_def_id, |
| nested: obligations, |
| } |
| } |
| |
| fn confirm_impl_candidate( |
| &mut self, |
| obligation: &TraitObligation<'tcx>, |
| impl_def_id: DefId, |
| ) -> VtableImplData<'tcx, PredicateObligation<'tcx>> { |
| debug!("confirm_impl_candidate({:?},{:?})", obligation, impl_def_id); |
| |
| // First, create the substitutions by matching the impl again, |
| // this time not in a probe. |
| self.infcx.in_snapshot(|_| { |
| let substs = self.rematch_impl(impl_def_id, obligation); |
| debug!("confirm_impl_candidate: substs={:?}", substs); |
| let cause = obligation.derived_cause(ImplDerivedObligation); |
| self.vtable_impl( |
| impl_def_id, |
| substs, |
| cause, |
| obligation.recursion_depth + 1, |
| obligation.param_env, |
| ) |
| }) |
| } |
| |
| fn vtable_impl( |
| &mut self, |
| impl_def_id: DefId, |
| mut substs: Normalized<'tcx, &'tcx Substs<'tcx>>, |
| cause: ObligationCause<'tcx>, |
| recursion_depth: usize, |
| param_env: ty::ParamEnv<'tcx>, |
| ) -> VtableImplData<'tcx, PredicateObligation<'tcx>> { |
| debug!( |
| "vtable_impl(impl_def_id={:?}, substs={:?}, recursion_depth={})", |
| impl_def_id, substs, recursion_depth, |
| ); |
| |
| let mut impl_obligations = self.impl_or_trait_obligations( |
| cause, |
| recursion_depth, |
| param_env, |
| impl_def_id, |
| &substs.value, |
| ); |
| |
| debug!( |
| "vtable_impl: impl_def_id={:?} impl_obligations={:?}", |
| impl_def_id, impl_obligations |
| ); |
| |
| // Because of RFC447, the impl-trait-ref and obligations |
| // are sufficient to determine the impl substs, without |
| // relying on projections in the impl-trait-ref. |
| // |
| // e.g., `impl<U: Tr, V: Iterator<Item=U>> Foo<<U as Tr>::T> for V` |
| impl_obligations.append(&mut substs.obligations); |
| |
| VtableImplData { |
| impl_def_id, |
| substs: substs.value, |
| nested: impl_obligations, |
| } |
| } |
| |
| fn confirm_object_candidate( |
| &mut self, |
| obligation: &TraitObligation<'tcx>, |
| ) -> VtableObjectData<'tcx, PredicateObligation<'tcx>> { |
| debug!("confirm_object_candidate({:?})", obligation); |
| |
| // FIXME(nmatsakis) skipping binder here seems wrong -- we should |
| // probably flatten the binder from the obligation and the binder |
| // from the object. Have to try to make a broken test case that |
| // results. |
| let self_ty = self.infcx |
| .shallow_resolve(*obligation.self_ty().skip_binder()); |
| let poly_trait_ref = match self_ty.sty { |
| ty::Dynamic(ref data, ..) => |
| data.principal().unwrap_or_else(|| { |
| span_bug!(obligation.cause.span, "object candidate with no principal") |
| }).with_self_ty(self.tcx(), self_ty), |
| _ => span_bug!(obligation.cause.span, "object candidate with non-object"), |
| }; |
| |
| let mut upcast_trait_ref = None; |
| let mut nested = vec![]; |
| let vtable_base; |
| |
| { |
| let tcx = self.tcx(); |
| |
| // We want to find the first supertrait in the list of |
| // supertraits that we can unify with, and do that |
| // unification. We know that there is exactly one in the list |
| // where we can unify because otherwise select would have |
| // reported an ambiguity. (When we do find a match, also |
| // record it for later.) |
| let nonmatching = util::supertraits(tcx, poly_trait_ref).take_while( |
| |&t| match self.infcx.commit_if_ok(|_| self.match_poly_trait_ref(obligation, t)) { |
| Ok(obligations) => { |
| upcast_trait_ref = Some(t); |
| nested.extend(obligations); |
| false |
| } |
| Err(_) => true, |
| }, |
| ); |
| |
| // Additionally, for each of the nonmatching predicates that |
| // we pass over, we sum up the set of number of vtable |
| // entries, so that we can compute the offset for the selected |
| // trait. |
| vtable_base = nonmatching.map(|t| tcx.count_own_vtable_entries(t)).sum(); |
| } |
| |
| VtableObjectData { |
| upcast_trait_ref: upcast_trait_ref.unwrap(), |
| vtable_base, |
| nested, |
| } |
| } |
| |
| fn confirm_fn_pointer_candidate( |
| &mut self, |
| obligation: &TraitObligation<'tcx>, |
| ) -> Result<VtableFnPointerData<'tcx, PredicateObligation<'tcx>>, SelectionError<'tcx>> { |
| debug!("confirm_fn_pointer_candidate({:?})", obligation); |
| |
| // OK to skip binder; it is reintroduced below |
| let self_ty = self.infcx |
| .shallow_resolve(*obligation.self_ty().skip_binder()); |
| let sig = self_ty.fn_sig(self.tcx()); |
| let trait_ref = self.tcx() |
| .closure_trait_ref_and_return_type( |
| obligation.predicate.def_id(), |
| self_ty, |
| sig, |
| util::TupleArgumentsFlag::Yes, |
| ) |
| .map_bound(|(trait_ref, _)| trait_ref); |
| |
| let Normalized { |
| value: trait_ref, |
| obligations, |
| } = project::normalize_with_depth( |
| self, |
| obligation.param_env, |
| obligation.cause.clone(), |
| obligation.recursion_depth + 1, |
| &trait_ref, |
| ); |
| |
| self.confirm_poly_trait_refs( |
| obligation.cause.clone(), |
| obligation.param_env, |
| obligation.predicate.to_poly_trait_ref(), |
| trait_ref, |
| )?; |
| Ok(VtableFnPointerData { |
| fn_ty: self_ty, |
| nested: obligations, |
| }) |
| } |
| |
| fn confirm_trait_alias_candidate( |
| &mut self, |
| obligation: &TraitObligation<'tcx>, |
| alias_def_id: DefId, |
| ) -> VtableTraitAliasData<'tcx, PredicateObligation<'tcx>> { |
| debug!( |
| "confirm_trait_alias_candidate({:?}, {:?})", |
| obligation, alias_def_id |
| ); |
| |
| self.infcx.in_snapshot(|_| { |
| let (predicate, _) = self.infcx() |
| .replace_bound_vars_with_placeholders(&obligation.predicate); |
| let trait_ref = predicate.trait_ref; |
| let trait_def_id = trait_ref.def_id; |
| let substs = trait_ref.substs; |
| |
| let trait_obligations = self.impl_or_trait_obligations( |
| obligation.cause.clone(), |
| obligation.recursion_depth, |
| obligation.param_env, |
| trait_def_id, |
| &substs, |
| ); |
| |
| debug!( |
| "confirm_trait_alias_candidate: trait_def_id={:?} trait_obligations={:?}", |
| trait_def_id, trait_obligations |
| ); |
| |
| VtableTraitAliasData { |
| alias_def_id, |
| substs: substs, |
| nested: trait_obligations, |
| } |
| }) |
| } |
| |
| fn confirm_generator_candidate( |
| &mut self, |
| obligation: &TraitObligation<'tcx>, |
| ) -> Result<VtableGeneratorData<'tcx, PredicateObligation<'tcx>>, SelectionError<'tcx>> { |
| // OK to skip binder because the substs on generator types never |
| // touch bound regions, they just capture the in-scope |
| // type/region parameters |
| let self_ty = self.infcx |
| .shallow_resolve(obligation.self_ty().skip_binder()); |
| let (generator_def_id, substs) = match self_ty.sty { |
| ty::Generator(id, substs, _) => (id, substs), |
| _ => bug!("closure candidate for non-closure {:?}", obligation), |
| }; |
| |
| debug!( |
| "confirm_generator_candidate({:?},{:?},{:?})", |
| obligation, generator_def_id, substs |
| ); |
| |
| let trait_ref = self.generator_trait_ref_unnormalized(obligation, generator_def_id, substs); |
| let Normalized { |
| value: trait_ref, |
| mut obligations, |
| } = normalize_with_depth( |
| self, |
| obligation.param_env, |
| obligation.cause.clone(), |
| obligation.recursion_depth + 1, |
| &trait_ref, |
| ); |
| |
| debug!( |
| "confirm_generator_candidate(generator_def_id={:?}, \ |
| trait_ref={:?}, obligations={:?})", |
| generator_def_id, trait_ref, obligations |
| ); |
| |
| obligations.extend(self.confirm_poly_trait_refs( |
| obligation.cause.clone(), |
| obligation.param_env, |
| obligation.predicate.to_poly_trait_ref(), |
| trait_ref, |
| )?); |
| |
| Ok(VtableGeneratorData { |
| generator_def_id: generator_def_id, |
| substs: substs.clone(), |
| nested: obligations, |
| }) |
| } |
| |
| fn confirm_closure_candidate( |
| &mut self, |
| obligation: &TraitObligation<'tcx>, |
| ) -> Result<VtableClosureData<'tcx, PredicateObligation<'tcx>>, SelectionError<'tcx>> { |
| debug!("confirm_closure_candidate({:?})", obligation); |
| |
| let kind = self.tcx() |
| .lang_items() |
| .fn_trait_kind(obligation.predicate.def_id()) |
| .unwrap_or_else(|| bug!("closure candidate for non-fn trait {:?}", obligation)); |
| |
| // OK to skip binder because the substs on closure types never |
| // touch bound regions, they just capture the in-scope |
| // type/region parameters |
| let self_ty = self.infcx |
| .shallow_resolve(obligation.self_ty().skip_binder()); |
| let (closure_def_id, substs) = match self_ty.sty { |
| ty::Closure(id, substs) => (id, substs), |
| _ => bug!("closure candidate for non-closure {:?}", obligation), |
| }; |
| |
| let trait_ref = self.closure_trait_ref_unnormalized(obligation, closure_def_id, substs); |
| let Normalized { |
| value: trait_ref, |
| mut obligations, |
| } = normalize_with_depth( |
| self, |
| obligation.param_env, |
| obligation.cause.clone(), |
| obligation.recursion_depth + 1, |
| &trait_ref, |
| ); |
| |
| debug!( |
| "confirm_closure_candidate(closure_def_id={:?}, trait_ref={:?}, obligations={:?})", |
| closure_def_id, trait_ref, obligations |
| ); |
| |
| obligations.extend(self.confirm_poly_trait_refs( |
| obligation.cause.clone(), |
| obligation.param_env, |
| obligation.predicate.to_poly_trait_ref(), |
| trait_ref, |
| )?); |
| |
| // FIXME: chalk |
| if !self.tcx().sess.opts.debugging_opts.chalk { |
| obligations.push(Obligation::new( |
| obligation.cause.clone(), |
| obligation.param_env, |
| ty::Predicate::ClosureKind(closure_def_id, substs, kind), |
| )); |
| } |
| |
| Ok(VtableClosureData { |
| closure_def_id, |
| substs: substs.clone(), |
| nested: obligations, |
| }) |
| } |
| |
| /// In the case of closure types and fn pointers, |
| /// we currently treat the input type parameters on the trait as |
| /// outputs. This means that when we have a match we have only |
| /// considered the self type, so we have to go back and make sure |
| /// to relate the argument types too. This is kind of wrong, but |
| /// since we control the full set of impls, also not that wrong, |
| /// and it DOES yield better error messages (since we don't report |
| /// errors as if there is no applicable impl, but rather report |
| /// errors are about mismatched argument types. |
| /// |
| /// Here is an example. Imagine we have a closure expression |
| /// and we desugared it so that the type of the expression is |
| /// `Closure`, and `Closure` expects an int as argument. Then it |
| /// is "as if" the compiler generated this impl: |
| /// |
| /// impl Fn(int) for Closure { ... } |
| /// |
| /// Now imagine our obligation is `Fn(usize) for Closure`. So far |
| /// we have matched the self-type `Closure`. At this point we'll |
| /// compare the `int` to `usize` and generate an error. |
| /// |
| /// Note that this checking occurs *after* the impl has selected, |
| /// because these output type parameters should not affect the |
| /// selection of the impl. Therefore, if there is a mismatch, we |
| /// report an error to the user. |
| fn confirm_poly_trait_refs( |
| &mut self, |
| obligation_cause: ObligationCause<'tcx>, |
| obligation_param_env: ty::ParamEnv<'tcx>, |
| obligation_trait_ref: ty::PolyTraitRef<'tcx>, |
| expected_trait_ref: ty::PolyTraitRef<'tcx>, |
| ) -> Result<Vec<PredicateObligation<'tcx>>, SelectionError<'tcx>> { |
| let obligation_trait_ref = obligation_trait_ref.clone(); |
| self.infcx |
| .at(&obligation_cause, obligation_param_env) |
| .sup(obligation_trait_ref, expected_trait_ref) |
| .map(|InferOk { obligations, .. }| obligations) |
| .map_err(|e| OutputTypeParameterMismatch(expected_trait_ref, obligation_trait_ref, e)) |
| } |
| |
| fn confirm_builtin_unsize_candidate( |
| &mut self, |
| obligation: &TraitObligation<'tcx>, |
| ) -> Result<VtableBuiltinData<PredicateObligation<'tcx>>, SelectionError<'tcx>> { |
| let tcx = self.tcx(); |
| |
| // assemble_candidates_for_unsizing should ensure there are no late bound |
| // regions here. See the comment there for more details. |
| let source = self.infcx |
| .shallow_resolve(obligation.self_ty().no_bound_vars().unwrap()); |
| let target = obligation |
| .predicate |
| .skip_binder() |
| .trait_ref |
| .substs |
| .type_at(1); |
| let target = self.infcx.shallow_resolve(target); |
| |
| debug!( |
| "confirm_builtin_unsize_candidate(source={:?}, target={:?})", |
| source, target |
| ); |
| |
| let mut nested = vec![]; |
| match (&source.sty, &target.sty) { |
| // Trait+Kx+'a -> Trait+Ky+'b (upcasts). |
| (&ty::Dynamic(ref data_a, r_a), &ty::Dynamic(ref data_b, r_b)) => { |
| // See assemble_candidates_for_unsizing for more info. |
| let existential_predicates = data_a.map_bound(|data_a| { |
| let iter = |
| data_a.principal().map(|x| ty::ExistentialPredicate::Trait(x)) |
| .into_iter().chain( |
| data_a |
| .projection_bounds() |
| .map(|x| ty::ExistentialPredicate::Projection(x)), |
| ) |
| .chain( |
| data_b |
| .auto_traits() |
| .map(ty::ExistentialPredicate::AutoTrait), |
| ); |
| tcx.mk_existential_predicates(iter) |
| }); |
| let source_trait = tcx.mk_dynamic(existential_predicates, r_b); |
| let InferOk { obligations, .. } = self.infcx |
| .at(&obligation.cause, obligation.param_env) |
| .sup(target, source_trait) |
| .map_err(|_| Unimplemented)?; |
| nested.extend(obligations); |
| |
| // Register one obligation for 'a: 'b. |
| let cause = ObligationCause::new( |
| obligation.cause.span, |
| obligation.cause.body_id, |
| ObjectCastObligation(target), |
| ); |
| let outlives = ty::OutlivesPredicate(r_a, r_b); |
| nested.push(Obligation::with_depth( |
| cause, |
| obligation.recursion_depth + 1, |
| obligation.param_env, |
| ty::Binder::bind(outlives).to_predicate(), |
| )); |
| } |
| |
| // T -> Trait. |
| (_, &ty::Dynamic(ref data, r)) => { |
| let mut object_dids = data.auto_traits() |
| .chain(data.principal_def_id()); |
| if let Some(did) = object_dids.find(|did| !tcx.is_object_safe(*did)) { |
| return Err(TraitNotObjectSafe(did)); |
| } |
| |
| let cause = ObligationCause::new( |
| obligation.cause.span, |
| obligation.cause.body_id, |
| ObjectCastObligation(target), |
| ); |
| |
| let predicate_to_obligation = |predicate| { |
| Obligation::with_depth( |
| cause.clone(), |
| obligation.recursion_depth + 1, |
| obligation.param_env, |
| predicate, |
| ) |
| }; |
| |
| // Create obligations: |
| // - Casting T to Trait |
| // - For all the various builtin bounds attached to the object cast. (In other |
| // words, if the object type is Foo+Send, this would create an obligation for the |
| // Send check.) |
| // - Projection predicates |
| nested.extend( |
| data.iter() |
| .map(|d| predicate_to_obligation(d.with_self_ty(tcx, source))), |
| ); |
| |
| // We can only make objects from sized types. |
| let tr = ty::TraitRef { |
| def_id: tcx.require_lang_item(lang_items::SizedTraitLangItem), |
| substs: tcx.mk_substs_trait(source, &[]), |
| }; |
| nested.push(predicate_to_obligation(tr.to_predicate())); |
| |
| // If the type is `Foo+'a`, ensures that the type |
| // being cast to `Foo+'a` outlives `'a`: |
| let outlives = ty::OutlivesPredicate(source, r); |
| nested.push(predicate_to_obligation( |
| ty::Binder::dummy(outlives).to_predicate(), |
| )); |
| } |
| |
| // [T; n] -> [T]. |
| (&ty::Array(a, _), &ty::Slice(b)) => { |
| let InferOk { obligations, .. } = self.infcx |
| .at(&obligation.cause, obligation.param_env) |
| .eq(b, a) |
| .map_err(|_| Unimplemented)?; |
| nested.extend(obligations); |
| } |
| |
| // Struct<T> -> Struct<U>. |
| (&ty::Adt(def, substs_a), &ty::Adt(_, substs_b)) => { |
| let fields = def.all_fields() |
| .map(|f| tcx.type_of(f.did)) |
| .collect::<Vec<_>>(); |
| |
| // The last field of the structure has to exist and contain type parameters. |
| let field = if let Some(&field) = fields.last() { |
| field |
| } else { |
| return Err(Unimplemented); |
| }; |
| let mut ty_params = GrowableBitSet::new_empty(); |
| let mut found = false; |
| for ty in field.walk() { |
| if let ty::Param(p) = ty.sty { |
| ty_params.insert(p.idx as usize); |
| found = true; |
| } |
| } |
| if !found { |
| return Err(Unimplemented); |
| } |
| |
| // Replace type parameters used in unsizing with |
| // Error and ensure they do not affect any other fields. |
| // This could be checked after type collection for any struct |
| // with a potentially unsized trailing field. |
| let params = substs_a.iter().enumerate().map(|(i, &k)| { |
| if ty_params.contains(i) { |
| tcx.types.err.into() |
| } else { |
| k |
| } |
| }); |
| let substs = tcx.mk_substs(params); |
| for &ty in fields.split_last().unwrap().1 { |
| if ty.subst(tcx, substs).references_error() { |
| return Err(Unimplemented); |
| } |
| } |
| |
| // Extract Field<T> and Field<U> from Struct<T> and Struct<U>. |
| let inner_source = field.subst(tcx, substs_a); |
| let inner_target = field.subst(tcx, substs_b); |
| |
| // Check that the source struct with the target's |
| // unsized parameters is equal to the target. |
| let params = substs_a.iter().enumerate().map(|(i, &k)| { |
| if ty_params.contains(i) { |
| substs_b.type_at(i).into() |
| } else { |
| k |
| } |
| }); |
| let new_struct = tcx.mk_adt(def, tcx.mk_substs(params)); |
| let InferOk { obligations, .. } = self.infcx |
| .at(&obligation.cause, obligation.param_env) |
| .eq(target, new_struct) |
| .map_err(|_| Unimplemented)?; |
| nested.extend(obligations); |
| |
| // Construct the nested Field<T>: Unsize<Field<U>> predicate. |
| nested.push(tcx.predicate_for_trait_def( |
| obligation.param_env, |
| obligation.cause.clone(), |
| obligation.predicate.def_id(), |
| obligation.recursion_depth + 1, |
| inner_source, |
| &[inner_target.into()], |
| )); |
| } |
| |
| // (.., T) -> (.., U). |
| (&ty::Tuple(tys_a), &ty::Tuple(tys_b)) => { |
| assert_eq!(tys_a.len(), tys_b.len()); |
| |
| // The last field of the tuple has to exist. |
| let (&a_last, a_mid) = if let Some(x) = tys_a.split_last() { |
| x |
| } else { |
| return Err(Unimplemented); |
| }; |
| let &b_last = tys_b.last().unwrap(); |
| |
| // Check that the source tuple with the target's |
| // last element is equal to the target. |
| let new_tuple = tcx.mk_tup(a_mid.iter().cloned().chain(iter::once(b_last))); |
| let InferOk { obligations, .. } = self.infcx |
| .at(&obligation.cause, obligation.param_env) |
| .eq(target, new_tuple) |
| .map_err(|_| Unimplemented)?; |
| nested.extend(obligations); |
| |
| // Construct the nested T: Unsize<U> predicate. |
| nested.push(tcx.predicate_for_trait_def( |
| obligation.param_env, |
| obligation.cause.clone(), |
| obligation.predicate.def_id(), |
| obligation.recursion_depth + 1, |
| a_last, |
| &[b_last.into()], |
| )); |
| } |
| |
| _ => bug!(), |
| }; |
| |
| Ok(VtableBuiltinData { nested }) |
| } |
| |
| /////////////////////////////////////////////////////////////////////////// |
| // Matching |
| // |
| // Matching is a common path used for both evaluation and |
| // confirmation. It basically unifies types that appear in impls |
| // and traits. This does affect the surrounding environment; |
| // therefore, when used during evaluation, match routines must be |
| // run inside of a `probe()` so that their side-effects are |
| // contained. |
| |
| fn rematch_impl( |
| &mut self, |
| impl_def_id: DefId, |
| obligation: &TraitObligation<'tcx>, |
| ) -> Normalized<'tcx, &'tcx Substs<'tcx>> { |
| match self.match_impl(impl_def_id, obligation) { |
| Ok(substs) => substs, |
| Err(()) => { |
| bug!( |
| "Impl {:?} was matchable against {:?} but now is not", |
| impl_def_id, |
| obligation |
| ); |
| } |
| } |
| } |
| |
| fn match_impl( |
| &mut self, |
| impl_def_id: DefId, |
| obligation: &TraitObligation<'tcx>, |
| ) -> Result<Normalized<'tcx, &'tcx Substs<'tcx>>, ()> { |
| let impl_trait_ref = self.tcx().impl_trait_ref(impl_def_id).unwrap(); |
| |
| // Before we create the substitutions and everything, first |
| // consider a "quick reject". This avoids creating more types |
| // and so forth that we need to. |
| if self.fast_reject_trait_refs(obligation, &impl_trait_ref) { |
| return Err(()); |
| } |
| |
| let (skol_obligation, _) = self.infcx() |
| .replace_bound_vars_with_placeholders(&obligation.predicate); |
| let skol_obligation_trait_ref = skol_obligation.trait_ref; |
| |
| let impl_substs = self.infcx |
| .fresh_substs_for_item(obligation.cause.span, impl_def_id); |
| |
| let impl_trait_ref = impl_trait_ref.subst(self.tcx(), impl_substs); |
| |
| let Normalized { |
| value: impl_trait_ref, |
| obligations: mut nested_obligations, |
| } = project::normalize_with_depth( |
| self, |
| obligation.param_env, |
| obligation.cause.clone(), |
| obligation.recursion_depth + 1, |
| &impl_trait_ref, |
| ); |
| |
| debug!( |
| "match_impl(impl_def_id={:?}, obligation={:?}, \ |
| impl_trait_ref={:?}, skol_obligation_trait_ref={:?})", |
| impl_def_id, obligation, impl_trait_ref, skol_obligation_trait_ref |
| ); |
| |
| let InferOk { obligations, .. } = self.infcx |
| .at(&obligation.cause, obligation.param_env) |
| .eq(skol_obligation_trait_ref, impl_trait_ref) |
| .map_err(|e| debug!("match_impl: failed eq_trait_refs due to `{}`", e))?; |
| nested_obligations.extend(obligations); |
| |
| debug!("match_impl: success impl_substs={:?}", impl_substs); |
| Ok(Normalized { |
| value: impl_substs, |
| obligations: nested_obligations, |
| }) |
| } |
| |
| fn fast_reject_trait_refs( |
| &mut self, |
| obligation: &TraitObligation<'_>, |
| impl_trait_ref: &ty::TraitRef<'_>, |
| ) -> bool { |
| // We can avoid creating type variables and doing the full |
| // substitution if we find that any of the input types, when |
| // simplified, do not match. |
| |
| obligation |
| .predicate |
| .skip_binder() |
| .input_types() |
| .zip(impl_trait_ref.input_types()) |
| .any(|(obligation_ty, impl_ty)| { |
| let simplified_obligation_ty = |
| fast_reject::simplify_type(self.tcx(), obligation_ty, true); |
| let simplified_impl_ty = fast_reject::simplify_type(self.tcx(), impl_ty, false); |
| |
| simplified_obligation_ty.is_some() |
| && simplified_impl_ty.is_some() |
| && simplified_obligation_ty != simplified_impl_ty |
| }) |
| } |
| |
| /// Normalize `where_clause_trait_ref` and try to match it against |
| /// `obligation`. If successful, return any predicates that |
| /// result from the normalization. Normalization is necessary |
| /// because where-clauses are stored in the parameter environment |
| /// unnormalized. |
| fn match_where_clause_trait_ref( |
| &mut self, |
| obligation: &TraitObligation<'tcx>, |
| where_clause_trait_ref: ty::PolyTraitRef<'tcx>, |
| ) -> Result<Vec<PredicateObligation<'tcx>>, ()> { |
| self.match_poly_trait_ref(obligation, where_clause_trait_ref) |
| } |
| |
| /// Returns `Ok` if `poly_trait_ref` being true implies that the |
| /// obligation is satisfied. |
| fn match_poly_trait_ref( |
| &mut self, |
| obligation: &TraitObligation<'tcx>, |
| poly_trait_ref: ty::PolyTraitRef<'tcx>, |
| ) -> Result<Vec<PredicateObligation<'tcx>>, ()> { |
| debug!( |
| "match_poly_trait_ref: obligation={:?} poly_trait_ref={:?}", |
| obligation, poly_trait_ref |
| ); |
| |
| self.infcx |
| .at(&obligation.cause, obligation.param_env) |
| .sup(obligation.predicate.to_poly_trait_ref(), poly_trait_ref) |
| .map(|InferOk { obligations, .. }| obligations) |
| .map_err(|_| ()) |
| } |
| |
| /////////////////////////////////////////////////////////////////////////// |
| // Miscellany |
| |
| fn match_fresh_trait_refs( |
| &self, |
| previous: &ty::PolyTraitRef<'tcx>, |
| current: &ty::PolyTraitRef<'tcx>, |
| ) -> bool { |
| let mut matcher = ty::_match::Match::new(self.tcx()); |
| matcher.relate(previous, current).is_ok() |
| } |
| |
| fn push_stack<'o, 's: 'o>( |
| &mut self, |
| previous_stack: TraitObligationStackList<'s, 'tcx>, |
| obligation: &'o TraitObligation<'tcx>, |
| ) -> TraitObligationStack<'o, 'tcx> { |
| let fresh_trait_ref = obligation |
| .predicate |
| .to_poly_trait_ref() |
| .fold_with(&mut self.freshener); |
| |
| TraitObligationStack { |
| obligation, |
| fresh_trait_ref, |
| previous: previous_stack, |
| } |
| } |
| |
| fn closure_trait_ref_unnormalized( |
| &mut self, |
| obligation: &TraitObligation<'tcx>, |
| closure_def_id: DefId, |
| substs: ty::ClosureSubsts<'tcx>, |
| ) -> ty::PolyTraitRef<'tcx> { |
| debug!( |
| "closure_trait_ref_unnormalized(obligation={:?}, closure_def_id={:?}, substs={:?})", |
| obligation, closure_def_id, substs, |
| ); |
| let closure_type = self.infcx.closure_sig(closure_def_id, substs); |
| |
| debug!( |
| "closure_trait_ref_unnormalized: closure_type = {:?}", |
| closure_type |
| ); |
| |
| // (1) Feels icky to skip the binder here, but OTOH we know |
| // that the self-type is an unboxed closure type and hence is |
| // in fact unparameterized (or at least does not reference any |
| // regions bound in the obligation). Still probably some |
| // refactoring could make this nicer. |
| self.tcx() |
| .closure_trait_ref_and_return_type( |
| obligation.predicate.def_id(), |
| obligation.predicate.skip_binder().self_ty(), // (1) |
| closure_type, |
| util::TupleArgumentsFlag::No, |
| ) |
| .map_bound(|(trait_ref, _)| trait_ref) |
| } |
| |
| fn generator_trait_ref_unnormalized( |
| &mut self, |
| obligation: &TraitObligation<'tcx>, |
| closure_def_id: DefId, |
| substs: ty::GeneratorSubsts<'tcx>, |
| ) -> ty::PolyTraitRef<'tcx> { |
| let gen_sig = substs.poly_sig(closure_def_id, self.tcx()); |
| |
| // (1) Feels icky to skip the binder here, but OTOH we know |
| // that the self-type is an generator type and hence is |
| // in fact unparameterized (or at least does not reference any |
| // regions bound in the obligation). Still probably some |
| // refactoring could make this nicer. |
| |
| self.tcx() |
| .generator_trait_ref_and_outputs( |
| obligation.predicate.def_id(), |
| obligation.predicate.skip_binder().self_ty(), // (1) |
| gen_sig, |
| ) |
| .map_bound(|(trait_ref, ..)| trait_ref) |
| } |
| |
| /// Returns the obligations that are implied by instantiating an |
| /// impl or trait. The obligations are substituted and fully |
| /// normalized. This is used when confirming an impl or default |
| /// impl. |
| fn impl_or_trait_obligations( |
| &mut self, |
| cause: ObligationCause<'tcx>, |
| recursion_depth: usize, |
| param_env: ty::ParamEnv<'tcx>, |
| def_id: DefId, // of impl or trait |
| substs: &Substs<'tcx>, // for impl or trait |
| ) -> Vec<PredicateObligation<'tcx>> { |
| debug!("impl_or_trait_obligations(def_id={:?})", def_id); |
| let tcx = self.tcx(); |
| |
| // To allow for one-pass evaluation of the nested obligation, |
| // each predicate must be preceded by the obligations required |
| // to normalize it. |
| // for example, if we have: |
| // impl<U: Iterator, V: Iterator<Item=U>> Foo for V where U::Item: Copy |
| // the impl will have the following predicates: |
| // <V as Iterator>::Item = U, |
| // U: Iterator, U: Sized, |
| // V: Iterator, V: Sized, |
| // <U as Iterator>::Item: Copy |
| // When we substitute, say, `V => IntoIter<u32>, U => $0`, the last |
| // obligation will normalize to `<$0 as Iterator>::Item = $1` and |
| // `$1: Copy`, so we must ensure the obligations are emitted in |
| // that order. |
| let predicates = tcx.predicates_of(def_id); |
| assert_eq!(predicates.parent, None); |
| let mut predicates: Vec<_> = predicates |
| .predicates |
| .iter() |
| .flat_map(|(predicate, _)| { |
| let predicate = normalize_with_depth( |
| self, |
| param_env, |
| cause.clone(), |
| recursion_depth, |
| &predicate.subst(tcx, substs), |
| ); |
| predicate.obligations.into_iter().chain(Some(Obligation { |
| cause: cause.clone(), |
| recursion_depth, |
| param_env, |
| predicate: predicate.value, |
| })) |
| }) |
| .collect(); |
| |
| // We are performing deduplication here to avoid exponential blowups |
| // (#38528) from happening, but the real cause of the duplication is |
| // unknown. What we know is that the deduplication avoids exponential |
| // amount of predicates being propagated when processing deeply nested |
| // types. |
| // |
| // This code is hot enough that it's worth avoiding the allocation |
| // required for the FxHashSet when possible. Special-casing lengths 0, |
| // 1 and 2 covers roughly 75--80% of the cases. |
| if predicates.len() <= 1 { |
| // No possibility of duplicates. |
| } else if predicates.len() == 2 { |
| // Only two elements. Drop the second if they are equal. |
| if predicates[0] == predicates[1] { |
| predicates.truncate(1); |
| } |
| } else { |
| // Three or more elements. Use a general deduplication process. |
| let mut seen = FxHashSet::default(); |
| predicates.retain(|i| seen.insert(i.clone())); |
| } |
| |
| predicates |
| } |
| } |
| |
| impl<'tcx> TraitObligation<'tcx> { |
| #[allow(unused_comparisons)] |
| pub fn derived_cause( |
| &self, |
| variant: fn(DerivedObligationCause<'tcx>) -> ObligationCauseCode<'tcx>, |
| ) -> ObligationCause<'tcx> { |
| /*! |
| * Creates a cause for obligations that are derived from |
| * `obligation` by a recursive search (e.g., for a builtin |
| * bound, or eventually a `auto trait Foo`). If `obligation` |
| * is itself a derived obligation, this is just a clone, but |
| * otherwise we create a "derived obligation" cause so as to |
| * keep track of the original root obligation for error |
| * reporting. |
| */ |
| |
| let obligation = self; |
| |
| // NOTE(flaper87): As of now, it keeps track of the whole error |
| // chain. Ideally, we should have a way to configure this either |
| // by using -Z verbose or just a CLI argument. |
| if obligation.recursion_depth >= 0 { |
| let derived_cause = DerivedObligationCause { |
| parent_trait_ref: obligation.predicate.to_poly_trait_ref(), |
| parent_code: Rc::new(obligation.cause.code.clone()), |
| }; |
| let derived_code = variant(derived_cause); |
| ObligationCause::new( |
| obligation.cause.span, |
| obligation.cause.body_id, |
| derived_code, |
| ) |
| } else { |
| obligation.cause.clone() |
| } |
| } |
| } |
| |
| impl<'tcx> SelectionCache<'tcx> { |
| /// Actually frees the underlying memory in contrast to what stdlib containers do on `clear` |
| pub fn clear(&self) { |
| *self.hashmap.borrow_mut() = Default::default(); |
| } |
| } |
| |
| impl<'tcx> EvaluationCache<'tcx> { |
| /// Actually frees the underlying memory in contrast to what stdlib containers do on `clear` |
| pub fn clear(&self) { |
| *self.hashmap.borrow_mut() = Default::default(); |
| } |
| } |
| |
| impl<'o, 'tcx> TraitObligationStack<'o, 'tcx> { |
| fn list(&'o self) -> TraitObligationStackList<'o, 'tcx> { |
| TraitObligationStackList::with(self) |
| } |
| |
| fn iter(&'o self) -> TraitObligationStackList<'o, 'tcx> { |
| self.list() |
| } |
| } |
| |
| #[derive(Copy, Clone)] |
| struct TraitObligationStackList<'o, 'tcx: 'o> { |
| head: Option<&'o TraitObligationStack<'o, 'tcx>>, |
| } |
| |
| impl<'o, 'tcx> TraitObligationStackList<'o, 'tcx> { |
| fn empty() -> TraitObligationStackList<'o, 'tcx> { |
| TraitObligationStackList { head: None } |
| } |
| |
| fn with(r: &'o TraitObligationStack<'o, 'tcx>) -> TraitObligationStackList<'o, 'tcx> { |
| TraitObligationStackList { head: Some(r) } |
| } |
| } |
| |
| impl<'o, 'tcx> Iterator for TraitObligationStackList<'o, 'tcx> { |
| type Item = &'o TraitObligationStack<'o, 'tcx>; |
| |
| fn next(&mut self) -> Option<&'o TraitObligationStack<'o, 'tcx>> { |
| match self.head { |
| Some(o) => { |
| *self = o.previous; |
| Some(o) |
| } |
| None => None, |
| } |
| } |
| } |
| |
| impl<'o, 'tcx> fmt::Debug for TraitObligationStack<'o, 'tcx> { |
| fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { |
| write!(f, "TraitObligationStack({:?})", self.obligation) |
| } |
| } |
| |
| #[derive(Clone, Eq, PartialEq)] |
| pub struct WithDepNode<T> { |
| dep_node: DepNodeIndex, |
| cached_value: T, |
| } |
| |
| impl<T: Clone> WithDepNode<T> { |
| pub fn new(dep_node: DepNodeIndex, cached_value: T) -> Self { |
| WithDepNode { |
| dep_node, |
| cached_value, |
| } |
| } |
| |
| pub fn get(&self, tcx: TyCtxt<'_, '_, '_>) -> T { |
| tcx.dep_graph.read_index(self.dep_node); |
| self.cached_value.clone() |
| } |
| } |