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fuchsia / third_party / rust / b3827e4f3728b77bd3e60587487ebfc3cc8d901b / . / compiler / rustc_trait_selection / src / traits / select / mod.rs
blob: 2b563c5b8d5fa57e6b072a30835927416ae81006 [file] [log] [blame]
//! Candidate selection. See the [rustc dev guide] for more information on how this works.
//!
//! [rustc dev guide]: https://rustc-dev-guide.rust-lang.org/traits/resolution.html#selection
use std::assert_matches::assert_matches;
use std::cell::{Cell, RefCell};
use std::fmt::{self, Display};
use std::ops::ControlFlow;
use std::{cmp, iter};
use hir::def::DefKind;
use rustc_data_structures::fx::{FxIndexMap, FxIndexSet};
use rustc_data_structures::stack::ensure_sufficient_stack;
use rustc_errors::{Diag, EmissionGuarantee};
use rustc_hir as hir;
use rustc_hir::LangItem;
use rustc_hir::def_id::DefId;
use rustc_infer::infer::BoundRegionConversionTime::{self, HigherRankedType};
use rustc_infer::infer::DefineOpaqueTypes;
use rustc_infer::infer::at::ToTrace;
use rustc_infer::infer::relate::TypeRelation;
use rustc_infer::traits::{PredicateObligations, TraitObligation};
use rustc_macros::{TypeFoldable, TypeVisitable};
use rustc_middle::bug;
use rustc_middle::dep_graph::{DepNodeIndex, dep_kinds};
pub use rustc_middle::traits::select::*;
use rustc_middle::ty::abstract_const::NotConstEvaluatable;
use rustc_middle::ty::error::TypeErrorToStringExt;
use rustc_middle::ty::print::{PrintTraitRefExt as _, with_no_trimmed_paths};
use rustc_middle::ty::{
self, DeepRejectCtxt, GenericArgsRef, PolyProjectionPredicate, SizedTraitKind, Ty, TyCtxt,
TypeFoldable, TypeVisitableExt, TypingMode, Upcast, elaborate, may_use_unstable_feature,
};
use rustc_span::{Symbol, sym};
use tracing::{debug, instrument, trace};
use self::EvaluationResult::*;
use self::SelectionCandidate::*;
use super::coherence::{self, Conflict};
use super::project::ProjectionTermObligation;
use super::util::closure_trait_ref_and_return_type;
use super::{
ImplDerivedCause, Normalized, Obligation, ObligationCause, ObligationCauseCode,
PolyTraitObligation, PredicateObligation, Selection, SelectionError, SelectionResult,
TraitQueryMode, const_evaluatable, project, util, wf,
};
use crate::error_reporting::InferCtxtErrorExt;
use crate::infer::{InferCtxt, InferOk, TypeFreshener};
use crate::solve::InferCtxtSelectExt as _;
use crate::traits::normalize::{normalize_with_depth, normalize_with_depth_to};
use crate::traits::project::{ProjectAndUnifyResult, ProjectionCacheKeyExt};
use crate::traits::{EvaluateConstErr, ProjectionCacheKey, effects, sizedness_fast_path};
mod _match;
mod candidate_assembly;
mod confirmation;
#[derive(Clone, Debug, Eq, PartialEq, Hash)]
pub enum IntercrateAmbiguityCause<'tcx> {
DownstreamCrate { trait_ref: ty::TraitRef<'tcx>, self_ty: Option<Ty<'tcx>> },
UpstreamCrateUpdate { trait_ref: ty::TraitRef<'tcx>, self_ty: Option<Ty<'tcx>> },
ReservationImpl { message: Symbol },
}
impl<'tcx> IntercrateAmbiguityCause<'tcx> {
/// Emits notes when the overlap is caused by complex intercrate ambiguities.
/// See #23980 for details.
pub fn add_intercrate_ambiguity_hint<G: EmissionGuarantee>(&self, err: &mut Diag<'_, G>) {
err.note(self.intercrate_ambiguity_hint());
}
pub fn intercrate_ambiguity_hint(&self) -> String {
with_no_trimmed_paths!(match self {
IntercrateAmbiguityCause::DownstreamCrate { trait_ref, self_ty } => {
format!(
"downstream crates may implement trait `{trait_desc}`{self_desc}",
trait_desc = trait_ref.print_trait_sugared(),
self_desc = if let Some(self_ty) = self_ty {
format!(" for type `{self_ty}`")
} else {
String::new()
}
)
}
IntercrateAmbiguityCause::UpstreamCrateUpdate { trait_ref, self_ty } => {
format!(
"upstream crates may add a new impl of trait `{trait_desc}`{self_desc} \
in future versions",
trait_desc = trait_ref.print_trait_sugared(),
self_desc = if let Some(self_ty) = self_ty {
format!(" for type `{self_ty}`")
} else {
String::new()
}
)
}
IntercrateAmbiguityCause::ReservationImpl { message } => message.to_string(),
})
}
}
pub struct SelectionContext<'cx, 'tcx> {
pub infcx: &'cx InferCtxt<'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, 'tcx>,
/// If `intercrate` is set, we remember predicates which were
/// considered ambiguous because of impls potentially added in other crates.
/// This is used in coherence to give improved diagnostics.
/// We don't do his until we detect a coherence error because it can
/// lead to false overflow results (#47139) and because always
/// computing it may negatively impact performance.
intercrate_ambiguity_causes: Option<FxIndexSet<IntercrateAmbiguityCause<'tcx>>>,
/// 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,
}
// A stack that walks back up the stack frame.
struct TraitObligationStack<'prev, 'tcx> {
obligation: &'prev PolyTraitObligation<'tcx>,
/// The trait predicate from `obligation` but "freshened" with the
/// selection-context's freshener. Used to check for recursion.
fresh_trait_pred: ty::PolyTraitPredicate<'tcx>,
/// Starts out equal to `depth` -- if, during evaluation, we
/// encounter a cycle, then we will set this flag to the minimum
/// depth of that cycle for all participants in the cycle. These
/// participants will then forego caching their results. This is
/// not the most efficient solution, but it addresses #60010. The
/// problem we are trying to prevent:
///
/// - If you have `A: AutoTrait` requires `B: AutoTrait` and `C: NonAutoTrait`
/// - `B: AutoTrait` requires `A: AutoTrait` (coinductive cycle, ok)
/// - `C: NonAutoTrait` requires `A: AutoTrait` (non-coinductive cycle, not ok)
///
/// you don't want to cache that `B: AutoTrait` or `A: AutoTrait`
/// is `EvaluatedToOk`; this is because they were only considered
/// ok on the premise that if `A: AutoTrait` held, but we indeed
/// encountered a problem (later on) with `A: AutoTrait`. So we
/// currently set a flag on the stack node for `B: AutoTrait` (as
/// well as the second instance of `A: AutoTrait`) to suppress
/// caching.
///
/// This is a simple, targeted fix. A more-performant fix requires
/// deeper changes, but would permit more caching: we could
/// basically defer caching until we have fully evaluated the
/// tree, and then cache the entire tree at once. In any case, the
/// performance impact here shouldn't be so horrible: every time
/// this is hit, we do cache at least one trait, so we only
/// evaluate each member of a cycle up to N times, where N is the
/// length of the cycle. This means the performance impact is
/// bounded and we shouldn't have any terrible worst-cases.
reached_depth: Cell<usize>,
previous: TraitObligationStackList<'prev, 'tcx>,
/// The number of parent frames plus one (thus, the topmost frame has depth 1).
depth: usize,
/// The depth-first number of this node in the search graph -- a
/// pre-order index. Basically, a freshly incremented counter.
dfn: usize,
}
struct SelectionCandidateSet<'tcx> {
/// A list of candidates that definitely apply to the current
/// obligation (meaning: types unify).
vec: Vec<SelectionCandidate<'tcx>>,
/// If `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,
}
impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
pub fn new(infcx: &'cx InferCtxt<'tcx>) -> SelectionContext<'cx, 'tcx> {
SelectionContext {
infcx,
freshener: infcx.freshener(),
intercrate_ambiguity_causes: None,
query_mode: TraitQueryMode::Standard,
}
}
pub fn with_query_mode(
infcx: &'cx InferCtxt<'tcx>,
query_mode: TraitQueryMode,
) -> SelectionContext<'cx, 'tcx> {
debug!(?query_mode, "with_query_mode");
SelectionContext { query_mode, ..SelectionContext::new(infcx) }
}
/// Enables tracking of intercrate ambiguity causes. See
/// the documentation of [`Self::intercrate_ambiguity_causes`] for more.
pub fn enable_tracking_intercrate_ambiguity_causes(&mut self) {
assert_matches!(self.infcx.typing_mode(), TypingMode::Coherence);
assert!(self.intercrate_ambiguity_causes.is_none());
self.intercrate_ambiguity_causes = Some(FxIndexSet::default());
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,
) -> FxIndexSet<IntercrateAmbiguityCause<'tcx>> {
assert_matches!(self.infcx.typing_mode(), TypingMode::Coherence);
self.intercrate_ambiguity_causes.take().unwrap_or_default()
}
pub fn tcx(&self) -> TyCtxt<'tcx> {
self.infcx.tcx
}
///////////////////////////////////////////////////////////////////////////
// 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.
#[instrument(level = "debug", skip(self), ret)]
pub fn poly_select(
&mut self,
obligation: &PolyTraitObligation<'tcx>,
) -> SelectionResult<'tcx, Selection<'tcx>> {
assert!(!self.infcx.next_trait_solver());
let candidate = match self.select_from_obligation(obligation) {
Err(SelectionError::Overflow(OverflowError::Canonical)) => {
// In standard mode, overflow must have been caught and reported
// earlier.
assert!(self.query_mode == TraitQueryMode::Canonical);
return Err(SelectionError::Overflow(OverflowError::Canonical));
}
Err(e) => {
return Err(e);
}
Ok(None) => {
return Ok(None);
}
Ok(Some(candidate)) => candidate,
};
match self.confirm_candidate(obligation, candidate) {
Err(SelectionError::Overflow(OverflowError::Canonical)) => {
assert!(self.query_mode == TraitQueryMode::Canonical);
Err(SelectionError::Overflow(OverflowError::Canonical))
}
Err(e) => Err(e),
Ok(candidate) => Ok(Some(candidate)),
}
}
pub fn select(
&mut self,
obligation: &TraitObligation<'tcx>,
) -> SelectionResult<'tcx, Selection<'tcx>> {
if self.infcx.next_trait_solver() {
return self.infcx.select_in_new_trait_solver(obligation);
}
self.poly_select(&Obligation {
cause: obligation.cause.clone(),
param_env: obligation.param_env,
predicate: ty::Binder::dummy(obligation.predicate),
recursion_depth: obligation.recursion_depth,
})
}
fn select_from_obligation(
&mut self,
obligation: &PolyTraitObligation<'tcx>,
) -> SelectionResult<'tcx, SelectionCandidate<'tcx>> {
debug_assert!(!obligation.predicate.has_escaping_bound_vars());
let pec = &ProvisionalEvaluationCache::default();
let stack = self.push_stack(TraitObligationStackList::empty(pec), obligation);
self.candidate_from_obligation(&stack)
}
#[instrument(level = "debug", skip(self), ret)]
fn candidate_from_obligation<'o>(
&mut self,
stack: &TraitObligationStack<'o, 'tcx>,
) -> SelectionResult<'tcx, SelectionCandidate<'tcx>> {
debug_assert!(!self.infcx.next_trait_solver());
// Watch out for overflow. This intentionally bypasses (and does
// not update) the cache.
self.check_recursion_limit(stack.obligation, stack.obligation)?;
// 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);
debug!(?cache_fresh_trait_pred);
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");
return c;
}
// If no match, compute result and insert into cache.
//
// FIXME(nikomatsakis) -- this cache is not taking into
// account cycles that may have occurred in forming the
// candidate. I don't know of any specific problems that
// result but it seems awfully suspicious.
let (candidate, dep_node) =
self.in_task(|this| this.candidate_from_obligation_no_cache(stack));
debug!("CACHE MISS");
self.insert_candidate_cache(
stack.obligation.param_env,
cache_fresh_trait_pred,
dep_node,
candidate.clone(),
);
candidate
}
fn candidate_from_obligation_no_cache<'o>(
&mut self,
stack: &TraitObligationStack<'o, 'tcx>,
) -> SelectionResult<'tcx, SelectionCandidate<'tcx>> {
if let Err(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;
for c in candidate_set.vec.iter() {
if self.evaluate_candidate(stack, c)?.may_apply() {
no_candidates_apply = false;
break;
}
}
if !candidate_set.ambiguous && no_candidates_apply {
let trait_ref = self.infcx.resolve_vars_if_possible(
stack.obligation.predicate.skip_binder().trait_ref,
);
if !trait_ref.references_error() {
let self_ty = trait_ref.self_ty();
let self_ty = self_ty.has_concrete_skeleton().then(|| self_ty);
let cause = if let Conflict::Upstream = conflict {
IntercrateAmbiguityCause::UpstreamCrateUpdate { trait_ref, self_ty }
} else {
IntercrateAmbiguityCause::DownstreamCrate { trait_ref, self_ty }
};
debug!(?cause, "evaluate_stack: pushing cause");
self.intercrate_ambiguity_causes.as_mut().unwrap().insert(cause);
}
}
}
}
return Ok(None);
}
let candidate_set = self.assemble_candidates(stack)?;
if candidate_set.ambiguous {
debug!("candidate set contains ambig");
return Ok(None);
}
let candidates = candidate_set.vec;
debug!(?stack, ?candidates, "assembled {} candidates", candidates.len());
// 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.
let mut candidates = self.filter_impls(candidates, stack.obligation);
// 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_reservation_impls(candidates.pop().unwrap());
}
// Winnow, but record the exact outcome of evaluation, which
// is needed for specialization. Propagate overflow if it occurs.
let 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::Canonical) => {
Err(SelectionError::Overflow(OverflowError::Canonical))
}
Err(OverflowError::Error(e)) => {
Err(SelectionError::Overflow(OverflowError::Error(e)))
}
})
.flat_map(Result::transpose)
.collect::<Result<Vec<_>, _>>()?;
debug!(?stack, ?candidates, "{} potentially applicable candidates", candidates.len());
// 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() {
// If there's an error type, 'downgrade' our result from
// `Err(Unimplemented)` to `Ok(None)`. This helps us avoid
// emitting additional spurious errors, since we're guaranteed
// to have emitted at least one.
if stack.obligation.predicate.references_error() {
debug!(?stack.obligation.predicate, "found error type in predicate, treating as ambiguous");
Ok(None)
} else {
Err(SelectionError::Unimplemented)
}
} else {
let has_non_region_infer = stack.obligation.predicate.has_non_region_infer();
if let Some(candidate) = self.winnow_candidates(has_non_region_infer, candidates) {
self.filter_reservation_impls(candidate)
} else {
Ok(None)
}
}
}
///////////////////////////////////////////////////////////////////////////
// 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
/// and returns an `EvaluationResult`. This is meant for the
/// *initial* call.
///
/// Do not use this directly, use `infcx.evaluate_obligation` instead.
pub fn evaluate_root_obligation(
&mut self,
obligation: &PredicateObligation<'tcx>,
) -> Result<EvaluationResult, OverflowError> {
debug_assert!(!self.infcx.next_trait_solver());
self.evaluation_probe(|this| {
let goal =
this.infcx.resolve_vars_if_possible((obligation.predicate, obligation.param_env));
let mut result = this.evaluate_predicate_recursively(
TraitObligationStackList::empty(&ProvisionalEvaluationCache::default()),
obligation.clone(),
)?;
// If the predicate has done any inference, then downgrade the
// result to ambiguous.
if this.infcx.resolve_vars_if_possible(goal) != goal {
result = result.max(EvaluatedToAmbig);
}
Ok(result)
})
}
/// Computes the evaluation result of `op`, discarding any constraints.
///
/// This also runs for leak check to allow higher ranked region errors to impact
/// selection. By default it checks for leaks from all universes created inside of
/// `op`, but this can be overwritten if necessary.
fn evaluation_probe(
&mut self,
op: impl FnOnce(&mut Self) -> Result<EvaluationResult, OverflowError>,
) -> Result<EvaluationResult, OverflowError> {
self.infcx.probe(|snapshot| -> Result<EvaluationResult, OverflowError> {
let outer_universe = self.infcx.universe();
let result = op(self)?;
match self.infcx.leak_check(outer_universe, Some(snapshot)) {
Ok(()) => {}
Err(_) => return Ok(EvaluatedToErr),
}
if self.infcx.opaque_types_added_in_snapshot(snapshot) {
return Ok(result.max(EvaluatedToOkModuloOpaqueTypes));
}
if self.infcx.region_constraints_added_in_snapshot(snapshot) {
Ok(result.max(EvaluatedToOkModuloRegions))
} else {
Ok(result)
}
})
}
/// Evaluates the predicates in `predicates` recursively. This may
/// guide inference. If this is not desired, run it inside of a
/// is run within an inference probe.
/// `probe`.
#[instrument(skip(self, stack), level = "debug")]
fn evaluate_predicates_recursively<'o, I>(
&mut self,
stack: TraitObligationStackList<'o, 'tcx>,
predicates: I,
) -> Result<EvaluationResult, OverflowError>
where
I: IntoIterator<Item = PredicateObligation<'tcx>> + std::fmt::Debug,
{
let mut result = EvaluatedToOk;
for mut obligation in predicates {
obligation.set_depth_from_parent(stack.depth());
let eval = self.evaluate_predicate_recursively(stack, obligation.clone())?;
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)
}
#[instrument(
level = "debug",
skip(self, previous_stack),
fields(previous_stack = ?previous_stack.head())
ret,
)]
fn evaluate_predicate_recursively<'o>(
&mut self,
previous_stack: TraitObligationStackList<'o, 'tcx>,
obligation: PredicateObligation<'tcx>,
) -> Result<EvaluationResult, OverflowError> {
debug_assert!(!self.infcx.next_trait_solver());
// `previous_stack` stores a `PolyTraitObligation`, while `obligation` is
// a `PredicateObligation`. These are distinct types, so we can't
// use any `Option` combinator method that would force them to be
// the same.
match previous_stack.head() {
Some(h) => self.check_recursion_limit(&obligation, h.obligation)?,
None => self.check_recursion_limit(&obligation, &obligation)?,
}
if sizedness_fast_path(self.tcx(), obligation.predicate) {
return Ok(EvaluatedToOk);
}
ensure_sufficient_stack(|| {
let bound_predicate = obligation.predicate.kind();
match bound_predicate.skip_binder() {
ty::PredicateKind::Clause(ty::ClauseKind::Trait(t)) => {
let t = bound_predicate.rebind(t);
debug_assert!(!t.has_escaping_bound_vars());
let obligation = obligation.with(self.tcx(), t);
self.evaluate_trait_predicate_recursively(previous_stack, obligation)
}
ty::PredicateKind::Clause(ty::ClauseKind::HostEffect(data)) => {
self.infcx.enter_forall(bound_predicate.rebind(data), |data| {
match effects::evaluate_host_effect_obligation(
self,
&obligation.with(self.tcx(), data),
) {
Ok(nested) => {
self.evaluate_predicates_recursively(previous_stack, nested)
}
Err(effects::EvaluationFailure::Ambiguous) => Ok(EvaluatedToAmbig),
Err(effects::EvaluationFailure::NoSolution) => Ok(EvaluatedToErr),
}
})
}
ty::PredicateKind::Subtype(p) => {
let p = bound_predicate.rebind(p);
// Does this code ever run?
match self.infcx.subtype_predicate(&obligation.cause, obligation.param_env, p) {
Ok(Ok(InferOk { obligations, .. })) => {
self.evaluate_predicates_recursively(previous_stack, obligations)
}
Ok(Err(_)) => Ok(EvaluatedToErr),
Err(..) => Ok(EvaluatedToAmbig),
}
}
ty::PredicateKind::Coerce(p) => {
let p = bound_predicate.rebind(p);
// Does this code ever run?
match self.infcx.coerce_predicate(&obligation.cause, obligation.param_env, p) {
Ok(Ok(InferOk { obligations, .. })) => {
self.evaluate_predicates_recursively(previous_stack, obligations)
}
Ok(Err(_)) => Ok(EvaluatedToErr),
Err(..) => Ok(EvaluatedToAmbig),
}
}
ty::PredicateKind::Clause(ty::ClauseKind::WellFormed(term)) => {
if term.is_trivially_wf(self.tcx()) {
return Ok(EvaluatedToOk);
}
// So, there is a bit going on here. First, `WellFormed` predicates
// are coinductive, like trait predicates with auto traits.
// This means that we need to detect if we have recursively
// evaluated `WellFormed(X)`. Otherwise, we would run into
// a "natural" overflow error.
//
// Now, the next question is whether we need to do anything
// special with caching. Considering the following tree:
// - `WF(Foo<T>)`
// - `Bar<T>: Send`
// - `WF(Foo<T>)`
// - `Foo<T>: Trait`
// In this case, the innermost `WF(Foo<T>)` should return
// `EvaluatedToOk`, since it's coinductive. Then if
// `Bar<T>: Send` is resolved to `EvaluatedToOk`, it can be
// inserted into a cache (because without thinking about `WF`
// goals, it isn't in a cycle). If `Foo<T>: Trait` later doesn't
// hold, then `Bar<T>: Send` shouldn't hold. Therefore, we
// *do* need to keep track of coinductive cycles.
let cache = previous_stack.cache;
let dfn = cache.next_dfn();
for stack_term in previous_stack.cache.wf_args.borrow().iter().rev() {
if stack_term.0 != term {
continue;
}
debug!("WellFormed({:?}) on stack", term);
if let Some(stack) = previous_stack.head {
// Okay, let's imagine we have two different stacks:
// `T: NonAutoTrait -> WF(T) -> T: NonAutoTrait`
// `WF(T) -> T: NonAutoTrait -> WF(T)`
// Because of this, we need to check that all
// predicates between the WF goals are coinductive.
// Otherwise, we can say that `T: NonAutoTrait` is
// true.
// Let's imagine we have a predicate stack like
// `Foo: Bar -> WF(T) -> T: NonAutoTrait -> T: Auto`
// depth ^1 ^2 ^3
// and the current predicate is `WF(T)`. `wf_args`
// would contain `(T, 1)`. We want to check all
// trait predicates greater than `1`. The previous
// stack would be `T: Auto`.
let cycle = stack.iter().take_while(|s| s.depth > stack_term.1);
let tcx = self.tcx();
let cycle = cycle.map(|stack| stack.obligation.predicate.upcast(tcx));
if self.coinductive_match(cycle) {
stack.update_reached_depth(stack_term.1);
return Ok(EvaluatedToOk);
} else {
return Ok(EvaluatedToAmbigStackDependent);
}
}
return Ok(EvaluatedToOk);
}
match wf::obligations(
self.infcx,
obligation.param_env,
obligation.cause.body_id,
obligation.recursion_depth + 1,
term,
obligation.cause.span,
) {
Some(obligations) => {
cache.wf_args.borrow_mut().push((term, previous_stack.depth()));
let result =
self.evaluate_predicates_recursively(previous_stack, obligations);
cache.wf_args.borrow_mut().pop();
let result = result?;
if !result.must_apply_modulo_regions() {
cache.on_failure(dfn);
}
cache.on_completion(dfn);
Ok(result)
}
None => Ok(EvaluatedToAmbig),
}
}
ty::PredicateKind::Clause(ty::ClauseKind::TypeOutlives(pred)) => {
// A global type with no free lifetimes or generic parameters
// outlives anything.
if pred.0.has_free_regions()
|| pred.0.has_bound_regions()
|| pred.0.has_non_region_infer()
|| pred.0.has_non_region_infer()
{
Ok(EvaluatedToOkModuloRegions)
} else {
Ok(EvaluatedToOk)
}
}
ty::PredicateKind::Clause(ty::ClauseKind::RegionOutlives(..)) => {
// We do not consider region relationships when evaluating trait matches.
Ok(EvaluatedToOkModuloRegions)
}
ty::PredicateKind::DynCompatible(trait_def_id) => {
if self.tcx().is_dyn_compatible(trait_def_id) {
Ok(EvaluatedToOk)
} else {
Ok(EvaluatedToErr)
}
}
ty::PredicateKind::Clause(ty::ClauseKind::Projection(data)) => {
let data = bound_predicate.rebind(data);
let project_obligation = obligation.with(self.tcx(), data);
match project::poly_project_and_unify_term(self, &project_obligation) {
ProjectAndUnifyResult::Holds(mut subobligations) => {
'compute_res: {
// If we've previously marked this projection as 'complete', then
// use the final cached result (either `EvaluatedToOk` or
// `EvaluatedToOkModuloRegions`), and skip re-evaluating the
// sub-obligations.
if let Some(key) =
ProjectionCacheKey::from_poly_projection_obligation(
self,
&project_obligation,
)
{
if let Some(cached_res) = self
.infcx
.inner
.borrow_mut()
.projection_cache()
.is_complete(key)
{
break 'compute_res Ok(cached_res);
}
}
// Need to explicitly set the depth of nested goals here as
// projection obligations can cycle by themselves and in
// `evaluate_predicates_recursively` we only add the depth
// for parent trait goals because only these get added to the
// `TraitObligationStackList`.
for subobligation in subobligations.iter_mut() {
subobligation.set_depth_from_parent(obligation.recursion_depth);
}
let res = self.evaluate_predicates_recursively(
previous_stack,
subobligations,
);
if let Ok(eval_rslt) = res
&& (eval_rslt == EvaluatedToOk
|| eval_rslt == EvaluatedToOkModuloRegions)
&& let Some(key) =
ProjectionCacheKey::from_poly_projection_obligation(
self,
&project_obligation,
)
{
// If the result is something that we can cache, then mark this
// entry as 'complete'. This will allow us to skip evaluating the
// subobligations at all the next time we evaluate the projection
// predicate.
self.infcx
.inner
.borrow_mut()
.projection_cache()
.complete(key, eval_rslt);
}
res
}
}
ProjectAndUnifyResult::FailedNormalization => Ok(EvaluatedToAmbig),
ProjectAndUnifyResult::Recursive => Ok(EvaluatedToAmbigStackDependent),
ProjectAndUnifyResult::MismatchedProjectionTypes(_) => Ok(EvaluatedToErr),
}
}
ty::PredicateKind::Clause(ty::ClauseKind::UnstableFeature(symbol)) => {
if may_use_unstable_feature(self.infcx, obligation.param_env, symbol) {
Ok(EvaluatedToOk)
} else {
Ok(EvaluatedToAmbig)
}
}
ty::PredicateKind::Clause(ty::ClauseKind::ConstEvaluatable(uv)) => {
match const_evaluatable::is_const_evaluatable(
self.infcx,
uv,
obligation.param_env,
obligation.cause.span,
) {
Ok(()) => Ok(EvaluatedToOk),
Err(NotConstEvaluatable::MentionsInfer) => Ok(EvaluatedToAmbig),
Err(NotConstEvaluatable::MentionsParam) => Ok(EvaluatedToErr),
Err(_) => Ok(EvaluatedToErr),
}
}
ty::PredicateKind::ConstEquate(c1, c2) => {
let tcx = self.tcx();
assert!(
tcx.features().generic_const_exprs(),
"`ConstEquate` without a feature gate: {c1:?} {c2:?}",
);
{
let c1 = tcx.expand_abstract_consts(c1);
let c2 = tcx.expand_abstract_consts(c2);
debug!(
"evaluate_predicate_recursively: equating consts:\nc1= {:?}\nc2= {:?}",
c1, c2
);
use rustc_hir::def::DefKind;
match (c1.kind(), c2.kind()) {
(ty::ConstKind::Unevaluated(a), ty::ConstKind::Unevaluated(b))
if a.def == b.def && tcx.def_kind(a.def) == DefKind::AssocConst =>
{
if let Ok(InferOk { obligations, value: () }) = self
.infcx
.at(&obligation.cause, obligation.param_env)
// Can define opaque types as this is only reachable with
// `generic_const_exprs`
.eq(
DefineOpaqueTypes::Yes,
ty::AliasTerm::from(a),
ty::AliasTerm::from(b),
)
{
return self.evaluate_predicates_recursively(
previous_stack,
obligations,
);
}
}
(_, ty::ConstKind::Unevaluated(_))
| (ty::ConstKind::Unevaluated(_), _) => (),
(_, _) => {
if let Ok(InferOk { obligations, value: () }) = self
.infcx
.at(&obligation.cause, obligation.param_env)
// Can define opaque types as this is only reachable with
// `generic_const_exprs`
.eq(DefineOpaqueTypes::Yes, c1, c2)
{
return self.evaluate_predicates_recursively(
previous_stack,
obligations,
);
}
}
}
}
let evaluate = |c: ty::Const<'tcx>| {
if let ty::ConstKind::Unevaluated(_) = c.kind() {
match crate::traits::try_evaluate_const(
self.infcx,
c,
obligation.param_env,
) {
Ok(val) => Ok(val),
Err(e) => Err(e),
}
} else {
Ok(c)
}
};
match (evaluate(c1), evaluate(c2)) {
(Ok(c1), Ok(c2)) => {
match self.infcx.at(&obligation.cause, obligation.param_env).eq(
// Can define opaque types as this is only reachable with
// `generic_const_exprs`
DefineOpaqueTypes::Yes,
c1,
c2,
) {
Ok(inf_ok) => self.evaluate_predicates_recursively(
previous_stack,
inf_ok.into_obligations(),
),
Err(_) => Ok(EvaluatedToErr),
}
}
(Err(EvaluateConstErr::InvalidConstParamTy(..)), _)
| (_, Err(EvaluateConstErr::InvalidConstParamTy(..))) => Ok(EvaluatedToErr),
(Err(EvaluateConstErr::EvaluationFailure(..)), _)
| (_, Err(EvaluateConstErr::EvaluationFailure(..))) => Ok(EvaluatedToErr),
(Err(EvaluateConstErr::HasGenericsOrInfers), _)
| (_, Err(EvaluateConstErr::HasGenericsOrInfers)) => {
if c1.has_non_region_infer() || c2.has_non_region_infer() {
Ok(EvaluatedToAmbig)
} else {
// Two different constants using generic parameters ~> error.
Ok(EvaluatedToErr)
}
}
}
}
ty::PredicateKind::NormalizesTo(..) => {
bug!("NormalizesTo is only used by the new solver")
}
ty::PredicateKind::AliasRelate(..) => {
bug!("AliasRelate is only used by the new solver")
}
ty::PredicateKind::Ambiguous => Ok(EvaluatedToAmbig),
ty::PredicateKind::Clause(ty::ClauseKind::ConstArgHasType(ct, ty)) => {
let ct = self.infcx.shallow_resolve_const(ct);
let ct_ty = match ct.kind() {
ty::ConstKind::Infer(_) => {
return Ok(EvaluatedToAmbig);
}
ty::ConstKind::Error(_) => return Ok(EvaluatedToOk),
ty::ConstKind::Value(cv) => cv.ty,
ty::ConstKind::Unevaluated(uv) => {
self.tcx().type_of(uv.def).instantiate(self.tcx(), uv.args)
}
// FIXME(generic_const_exprs): See comment in `fulfill.rs`
ty::ConstKind::Expr(_) => return Ok(EvaluatedToOk),
ty::ConstKind::Placeholder(_) => {
bug!("placeholder const {:?} in old solver", ct)
}
ty::ConstKind::Bound(_, _) => bug!("escaping bound vars in {:?}", ct),
ty::ConstKind::Param(param_ct) => {
param_ct.find_const_ty_from_env(obligation.param_env)
}
};
match self.infcx.at(&obligation.cause, obligation.param_env).eq(
// Only really exercised by generic_const_exprs
DefineOpaqueTypes::Yes,
ct_ty,
ty,
) {
Ok(inf_ok) => self.evaluate_predicates_recursively(
previous_stack,
inf_ok.into_obligations(),
),
Err(_) => Ok(EvaluatedToErr),
}
}
}
})
}
#[instrument(skip(self, previous_stack), level = "debug", ret)]
fn evaluate_trait_predicate_recursively<'o>(
&mut self,
previous_stack: TraitObligationStackList<'o, 'tcx>,
mut obligation: PolyTraitObligation<'tcx>,
) -> Result<EvaluationResult, OverflowError> {
if !matches!(self.infcx.typing_mode(), TypingMode::Coherence)
&& obligation.is_global()
&& obligation.param_env.caller_bounds().iter().all(|bound| bound.has_param())
{
// 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!("in global");
obligation.param_env = ty::ParamEnv::empty();
}
let stack = self.push_stack(previous_stack, &obligation);
let fresh_trait_pred = stack.fresh_trait_pred;
let param_env = obligation.param_env;
debug!(?fresh_trait_pred);
// If a trait predicate is in the (local or global) evaluation cache,
// then we know it holds without cycles.
if let Some(result) = self.check_evaluation_cache(param_env, fresh_trait_pred) {
debug!("CACHE HIT");
return Ok(result);
}
if let Some(result) = stack.cache().get_provisional(fresh_trait_pred) {
debug!("PROVISIONAL CACHE HIT");
stack.update_reached_depth(result.reached_depth);
return Ok(result.result);
}
// Check if this is a match for something already on the
// stack. If so, we don't want to insert the result into the
// main cache (it is cycle dependent) nor the provisional
// cache (which is meant for things that have completed but
// for a "backedge" -- this result *is* the backedge).
if let Some(cycle_result) = self.check_evaluation_cycle(&stack) {
return Ok(cycle_result);
}
let (result, dep_node) = self.in_task(|this| {
let mut result = this.evaluate_stack(&stack)?;
// fix issue #103563, we don't normalize
// nested obligations which produced by `TraitDef` candidate
// (i.e. using bounds on assoc items as assumptions).
// because we don't have enough information to
// normalize these obligations before evaluating.
// so we will try to normalize the obligation and evaluate again.
// we will replace it with new solver in the future.
if EvaluationResult::EvaluatedToErr == result
&& fresh_trait_pred.has_aliases()
&& fresh_trait_pred.is_global()
{
let mut nested_obligations = PredicateObligations::new();
let predicate = normalize_with_depth_to(
this,
param_env,
obligation.cause.clone(),
obligation.recursion_depth + 1,
obligation.predicate,
&mut nested_obligations,
);
if predicate != obligation.predicate {
let mut nested_result = EvaluationResult::EvaluatedToOk;
for obligation in nested_obligations {
nested_result = cmp::max(
this.evaluate_predicate_recursively(previous_stack, obligation)?,
nested_result,
);
}
if nested_result.must_apply_modulo_regions() {
let obligation = obligation.with(this.tcx(), predicate);
result = cmp::max(
nested_result,
this.evaluate_trait_predicate_recursively(previous_stack, obligation)?,
);
}
}
}
Ok::<_, OverflowError>(result)
});
let result = result?;
if !result.must_apply_modulo_regions() {
stack.cache().on_failure(stack.dfn);
}
let reached_depth = stack.reached_depth.get();
if reached_depth >= stack.depth {
debug!("CACHE MISS");
self.insert_evaluation_cache(param_env, fresh_trait_pred, dep_node, result);
stack.cache().on_completion(stack.dfn);
} else {
debug!("PROVISIONAL");
debug!(
"caching provisionally because {:?} \
is a cycle participant (at depth {}, reached depth {})",
fresh_trait_pred, stack.depth, reached_depth,
);
stack.cache().insert_provisional(stack.dfn, reached_depth, fresh_trait_pred, result);
}
Ok(result)
}
/// 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.
fn check_evaluation_cycle(
&mut self,
stack: &TraitObligationStack<'_, 'tcx>,
) -> Option<EvaluationResult> {
if let Some(cycle_depth) = stack
.iter()
.skip(1) // Skip top-most frame.
.find(|prev| {
stack.obligation.param_env == prev.obligation.param_env
&& stack.fresh_trait_pred == prev.fresh_trait_pred
})
.map(|stack| stack.depth)
{
debug!("evaluate_stack --> recursive at depth {}", cycle_depth);
// If we have a stack like `A B C D E A`, where the top of
// the stack is the final `A`, then this will iterate over
// `A, E, D, C, B` -- i.e., all the participants apart
// from the cycle head. We mark them as participating in a
// cycle. This suppresses caching for those nodes. See
// `in_cycle` field for more details.
stack.update_reached_depth(cycle_depth);
// 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_while(|s| s.depth >= cycle_depth);
let tcx = self.tcx();
let cycle = cycle.map(|stack| stack.obligation.predicate.upcast(tcx));
if self.coinductive_match(cycle) {
debug!("evaluate_stack --> recursive, coinductive");
Some(EvaluatedToOk)
} else {
debug!("evaluate_stack --> recursive, inductive");
Some(EvaluatedToAmbigStackDependent)
}
} else {
None
}
}
fn evaluate_stack<'o>(
&mut self,
stack: &TraitObligationStack<'o, 'tcx>,
) -> Result<EvaluationResult, OverflowError> {
debug_assert!(!self.infcx.next_trait_solver());
// In intercrate mode, whenever any of the generics 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_pred.skip_binder().trait_ref.args.types().any(|ty| ty.is_fresh());
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_pred, prev.fresh_trait_pred)
})
{
debug!("evaluate_stack --> unbound argument, recursive --> giving up",);
return Ok(EvaluatedToAmbigStackDependent);
}
match self.candidate_from_obligation(stack) {
Ok(Some(c)) => self.evaluate_candidate(stack, &c),
Ok(None) => Ok(EvaluatedToAmbig),
Err(SelectionError::Overflow(OverflowError::Canonical)) => {
Err(OverflowError::Canonical)
}
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 coinductive trait: an auto-trait or `Sized`,
/// - it also appears in the backtrace at some position `X`,
/// - all the predicates at positions `X..` between `X` and the top are
/// also coinductive traits.
pub(crate) fn coinductive_match<I>(&mut self, mut cycle: I) -> bool
where
I: Iterator<Item = ty::Predicate<'tcx>>,
{
cycle.all(|p| match p.kind().skip_binder() {
ty::PredicateKind::Clause(ty::ClauseKind::Trait(data)) => {
self.infcx.tcx.trait_is_coinductive(data.def_id())
}
ty::PredicateKind::Clause(ty::ClauseKind::WellFormed(_)) => {
// FIXME(generic_const_exprs): GCE needs well-formedness predicates to be
// coinductive, but GCE is on the way out anyways, so this should eventually
// be replaced with `false`.
self.infcx.tcx.features().generic_const_exprs()
}
_ => false,
})
}
/// Further evaluates `candidate` to decide whether all type parameters match and whether nested
/// obligations are met. Returns whether `candidate` remains viable after this further
/// scrutiny.
#[instrument(
level = "debug",
skip(self, stack),
fields(depth = stack.obligation.recursion_depth),
ret
)]
fn evaluate_candidate<'o>(
&mut self,
stack: &TraitObligationStack<'o, 'tcx>,
candidate: &SelectionCandidate<'tcx>,
) -> Result<EvaluationResult, OverflowError> {
let mut result = self.evaluation_probe(|this| {
match this.confirm_candidate(stack.obligation, candidate.clone()) {
Ok(selection) => {
debug!(?selection);
this.evaluate_predicates_recursively(
stack.list(),
selection.nested_obligations().into_iter(),
)
}
Err(..) => Ok(EvaluatedToErr),
}
})?;
// If we erased any lifetimes, then we want to use
// `EvaluatedToOkModuloRegions` instead of `EvaluatedToOk`
// as your final result. The result will be cached using
// the freshened trait predicate as a key, so we need
// our result to be correct by *any* choice of original lifetimes,
// not just the lifetime choice for this particular (non-erased)
// predicate.
// See issue #80691
if stack.fresh_trait_pred.has_erased_regions() {
result = result.max(EvaluatedToOkModuloRegions);
}
Ok(result)
}
fn check_evaluation_cache(
&self,
param_env: ty::ParamEnv<'tcx>,
trait_pred: ty::PolyTraitPredicate<'tcx>,
) -> Option<EvaluationResult> {
let infcx = self.infcx;
let tcx = infcx.tcx;
if self.can_use_global_caches(param_env, trait_pred) {
let key = (infcx.typing_env(param_env), trait_pred);
if let Some(res) = tcx.evaluation_cache.get(&key, tcx) {
Some(res)
} else {
debug_assert_eq!(infcx.evaluation_cache.get(&(param_env, trait_pred), tcx), None);
None
}
} else {
self.infcx.evaluation_cache.get(&(param_env, trait_pred), tcx)
}
}
fn insert_evaluation_cache(
&mut self,
param_env: ty::ParamEnv<'tcx>,
trait_pred: ty::PolyTraitPredicate<'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;
}
let infcx = self.infcx;
let tcx = infcx.tcx;
if self.can_use_global_caches(param_env, trait_pred) {
debug!(?trait_pred, ?result, "insert_evaluation_cache global");
// This may overwrite the cache with the same value
tcx.evaluation_cache.insert(
(infcx.typing_env(param_env), trait_pred),
dep_node,
result,
);
return;
} else {
debug!(?trait_pred, ?result, "insert_evaluation_cache local");
self.infcx.evaluation_cache.insert((param_env, trait_pred), dep_node, result);
}
}
fn check_recursion_depth<T>(
&self,
depth: usize,
error_obligation: &Obligation<'tcx, T>,
) -> Result<(), OverflowError>
where
T: Upcast<TyCtxt<'tcx>, ty::Predicate<'tcx>> + Clone,
{
if !self.infcx.tcx.recursion_limit().value_within_limit(depth) {
match self.query_mode {
TraitQueryMode::Standard => {
if let Some(e) = self.infcx.tainted_by_errors() {
return Err(OverflowError::Error(e));
}
self.infcx.err_ctxt().report_overflow_obligation(error_obligation, true);
}
TraitQueryMode::Canonical => {
return Err(OverflowError::Canonical);
}
}
}
Ok(())
}
/// Checks that the recursion limit has not been exceeded.
///
/// The weird return type of this function allows it to be used with the `try` (`?`)
/// operator within certain functions.
#[inline(always)]
fn check_recursion_limit<T: Display + TypeFoldable<TyCtxt<'tcx>>, V>(
&self,
obligation: &Obligation<'tcx, T>,
error_obligation: &Obligation<'tcx, V>,
) -> Result<(), OverflowError>
where
V: Upcast<TyCtxt<'tcx>, ty::Predicate<'tcx>> + Clone,
{
self.check_recursion_depth(obligation.recursion_depth, error_obligation)
}
fn in_task<OP, R>(&mut self, op: OP) -> (R, DepNodeIndex)
where
OP: FnOnce(&mut Self) -> R,
{
self.tcx().dep_graph.with_anon_task(self.tcx(), dep_kinds::TraitSelect, || op(self))
}
/// filter_impls filters candidates that have a positive impl for a negative
/// goal and a negative impl for a positive goal
#[instrument(level = "debug", skip(self, candidates))]
fn filter_impls(
&mut self,
candidates: Vec<SelectionCandidate<'tcx>>,
obligation: &PolyTraitObligation<'tcx>,
) -> Vec<SelectionCandidate<'tcx>> {
trace!("{candidates:#?}");
let tcx = self.tcx();
let mut result = Vec::with_capacity(candidates.len());
for candidate in candidates {
if let ImplCandidate(def_id) = candidate {
match (tcx.impl_polarity(def_id), obligation.polarity()) {
(ty::ImplPolarity::Reservation, _)
| (ty::ImplPolarity::Positive, ty::PredicatePolarity::Positive)
| (ty::ImplPolarity::Negative, ty::PredicatePolarity::Negative) => {
result.push(candidate);
}
_ => {}
}
} else {
result.push(candidate);
}
}
trace!("{result:#?}");
result
}
/// filter_reservation_impls filter reservation impl for any goal as ambiguous
#[instrument(level = "debug", skip(self))]
fn filter_reservation_impls(
&mut self,
candidate: SelectionCandidate<'tcx>,
) -> SelectionResult<'tcx, SelectionCandidate<'tcx>> {
let tcx = self.tcx();
// Treat reservation impls as ambiguity.
if let ImplCandidate(def_id) = candidate {
if let ty::ImplPolarity::Reservation = tcx.impl_polarity(def_id) {
if let Some(intercrate_ambiguity_clauses) = &mut self.intercrate_ambiguity_causes {
let message = tcx
.get_attr(def_id, sym::rustc_reservation_impl)
.and_then(|a| a.value_str());
if let Some(message) = message {
debug!(
"filter_reservation_impls: \
reservation impl ambiguity on {:?}",
def_id
);
intercrate_ambiguity_clauses
.insert(IntercrateAmbiguityCause::ReservationImpl { message });
}
}
return Ok(None);
}
}
Ok(Some(candidate))
}
fn is_knowable<'o>(&mut self, stack: &TraitObligationStack<'o, 'tcx>) -> Result<(), Conflict> {
let obligation = &stack.obligation;
match self.infcx.typing_mode() {
TypingMode::Coherence => {}
TypingMode::Analysis { .. }
| TypingMode::Borrowck { .. }
| TypingMode::PostBorrowckAnalysis { .. }
| TypingMode::PostAnalysis => return Ok(()),
}
debug!("is_knowable()");
let predicate = self.infcx.resolve_vars_if_possible(obligation.predicate);
// Okay 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;
coherence::trait_ref_is_knowable(self.infcx, trait_ref, |ty| Ok::<_, !>(ty)).into_ok()
}
/// Returns `true` if the global caches can be used.
fn can_use_global_caches(
&self,
param_env: ty::ParamEnv<'tcx>,
pred: ty::PolyTraitPredicate<'tcx>,
) -> bool {
// If there are any inference variables in the `ParamEnv`, then we
// always use a cache local to this particular scope. Otherwise, we
// switch to a global cache.
if param_env.has_infer() || pred.has_infer() {
return false;
}
match self.infcx.typing_mode() {
// Avoid using the global cache during coherence and just rely
// on the local cache. 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.
TypingMode::Coherence => false,
// Avoid using the global cache when we're defining opaque types
// as their hidden type may impact the result of candidate selection.
//
// HACK: This is still theoretically unsound. Goals can indirectly rely
// on opaques in the defining scope, and it's easier to do so with TAIT.
// However, if we disqualify *all* goals from being cached, perf suffers.
// This is likely fixed by better caching in general in the new solver.
// See: <https://github.com/rust-lang/rust/issues/132064>.
TypingMode::Analysis {
defining_opaque_types_and_generators: defining_opaque_types,
}
| TypingMode::Borrowck { defining_opaque_types } => {
defining_opaque_types.is_empty()
|| (!pred.has_opaque_types() && !pred.has_coroutines())
}
// The hidden types of `defined_opaque_types` is not local to the current
// inference context, so we can freely move this to the global cache.
TypingMode::PostBorrowckAnalysis { .. } => true,
// The global cache is only used if there are no opaque types in
// the defining scope or we're outside of analysis.
//
// FIXME(#132279): This is still incorrect as we treat opaque types
// and default associated items differently between these two modes.
TypingMode::PostAnalysis => 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 infcx = self.infcx;
let tcx = infcx.tcx;
let pred = cache_fresh_trait_pred.skip_binder();
if self.can_use_global_caches(param_env, cache_fresh_trait_pred) {
if let Some(res) = tcx.selection_cache.get(&(infcx.typing_env(param_env), pred), tcx) {
return Some(res);
} else if cfg!(debug_assertions) {
match infcx.selection_cache.get(&(param_env, pred), tcx) {
None | Some(Err(SelectionError::Overflow(OverflowError::Canonical))) => {}
res => bug!("unexpected local cache result: {res:?}"),
}
}
}
// Subtle: we need to check the local cache even if we're able to use the
// global cache as we don't cache overflow in the global cache but need to
// cache it as otherwise rustdoc hangs when compiling diesel.
infcx.selection_cache.get(&(param_env, pred), tcx)
}
/// Determines whether can we safely cache the result
/// of selecting an obligation. This is almost always `true`,
/// except when dealing with certain `ParamCandidate`s.
///
/// 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.has_infer(),
_ => true,
}
}
#[instrument(skip(self, param_env, cache_fresh_trait_pred, dep_node), level = "debug")]
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 infcx = self.infcx;
let tcx = infcx.tcx;
let pred = cache_fresh_trait_pred.skip_binder();
if !self.can_cache_candidate(&candidate) {
debug!(?pred, ?candidate, "insert_candidate_cache - candidate is not cacheable");
return;
}
if self.can_use_global_caches(param_env, cache_fresh_trait_pred) {
if let Err(SelectionError::Overflow(OverflowError::Canonical)) = candidate {
// Don't cache overflow globally; we only produce this in certain modes.
} else {
debug!(?pred, ?candidate, "insert_candidate_cache global");
debug_assert!(!candidate.has_infer());
// This may overwrite the cache with the same value.
tcx.selection_cache.insert(
(infcx.typing_env(param_env), pred),
dep_node,
candidate,
);
return;
}
}
debug!(?pred, ?candidate, "insert_candidate_cache local");
self.infcx.selection_cache.insert((param_env, pred), dep_node, candidate);
}
/// Looks at the item bounds of the projection or opaque type.
/// If this is a nested rigid projection, such as
/// `<<T as Tr1>::Assoc as Tr2>::Assoc`, consider the item bounds
/// on both `Tr1::Assoc` and `Tr2::Assoc`, since we may encounter
/// relative bounds on both via the `associated_type_bounds` feature.
pub(super) fn for_each_item_bound<T>(
&mut self,
mut self_ty: Ty<'tcx>,
mut for_each: impl FnMut(&mut Self, ty::Clause<'tcx>, usize) -> ControlFlow<T, ()>,
on_ambiguity: impl FnOnce(),
) -> ControlFlow<T, ()> {
let mut idx = 0;
let mut in_parent_alias_type = false;
loop {
let (kind, alias_ty) = match *self_ty.kind() {
ty::Alias(kind @ (ty::Projection | ty::Opaque), alias_ty) => (kind, alias_ty),
ty::Infer(ty::TyVar(_)) => {
on_ambiguity();
return ControlFlow::Continue(());
}
_ => return ControlFlow::Continue(()),
};
// HACK: On subsequent recursions, we only care about bounds that don't
// share the same type as `self_ty`. This is because for truly rigid
// projections, we will never be able to equate, e.g. `<T as Tr>::A`
// with `<<T as Tr>::A as Tr>::A`.
let relevant_bounds = if in_parent_alias_type {
self.tcx().item_non_self_bounds(alias_ty.def_id)
} else {
self.tcx().item_self_bounds(alias_ty.def_id)
};
for bound in relevant_bounds.instantiate(self.tcx(), alias_ty.args) {
for_each(self, bound, idx)?;
idx += 1;
}
if kind == ty::Projection {
self_ty = alias_ty.self_ty();
} else {
return ControlFlow::Continue(());
}
in_parent_alias_type = true;
}
}
/// Equates the trait in `obligation` with trait bound. If the two traits
/// can be equated and the normalized trait bound doesn't contain inference
/// variables or placeholders, the normalized bound is returned.
fn match_normalize_trait_ref(
&mut self,
obligation: &PolyTraitObligation<'tcx>,
placeholder_trait_ref: ty::TraitRef<'tcx>,
trait_bound: ty::PolyTraitRef<'tcx>,
) -> Result<Option<ty::TraitRef<'tcx>>, ()> {
debug_assert!(!placeholder_trait_ref.has_escaping_bound_vars());
if placeholder_trait_ref.def_id != trait_bound.def_id() {
// Avoid unnecessary normalization
return Err(());
}
let drcx = DeepRejectCtxt::relate_rigid_rigid(self.infcx.tcx);
let obligation_args = obligation.predicate.skip_binder().trait_ref.args;
if !drcx.args_may_unify(obligation_args, trait_bound.skip_binder().args) {
return Err(());
}
let trait_bound = self.infcx.instantiate_binder_with_fresh_vars(
obligation.cause.span,
HigherRankedType,
trait_bound,
);
let Normalized { value: trait_bound, obligations: _ } = ensure_sufficient_stack(|| {
normalize_with_depth(
self,
obligation.param_env,
obligation.cause.clone(),
obligation.recursion_depth + 1,
trait_bound,
)
});
self.infcx
.at(&obligation.cause, obligation.param_env)
.eq(DefineOpaqueTypes::No, placeholder_trait_ref, trait_bound)
.map(|InferOk { obligations: _, value: () }| {
// This method is called within a probe, so we can't have
// inference variables and placeholders escape.
if !trait_bound.has_infer() && !trait_bound.has_placeholders() {
Some(trait_bound)
} else {
None
}
})
.map_err(|_| ())
}
fn where_clause_may_apply<'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),
Err(()) => Ok(EvaluatedToErr),
}
})
}
/// Return `Yes` if the obligation's predicate type applies to the env_predicate, and
/// `No` if it does not. Return `Ambiguous` in the case that the projection type is a GAT,
/// and applying this env_predicate constrains any of the obligation's GAT parameters.
///
/// This behavior is a somewhat of a hack to prevent over-constraining inference variables
/// in cases like #91762.
pub(super) fn match_projection_projections(
&mut self,
obligation: &ProjectionTermObligation<'tcx>,
env_predicate: PolyProjectionPredicate<'tcx>,
potentially_unnormalized_candidates: bool,
) -> ProjectionMatchesProjection {
debug_assert_eq!(obligation.predicate.def_id, env_predicate.item_def_id());
let mut nested_obligations = PredicateObligations::new();
let infer_predicate = self.infcx.instantiate_binder_with_fresh_vars(
obligation.cause.span,
BoundRegionConversionTime::HigherRankedType,
env_predicate,
);
let infer_projection = if potentially_unnormalized_candidates {
ensure_sufficient_stack(|| {
normalize_with_depth_to(
self,
obligation.param_env,
obligation.cause.clone(),
obligation.recursion_depth + 1,
infer_predicate.projection_term,
&mut nested_obligations,
)
})
} else {
infer_predicate.projection_term
};
let is_match = self
.infcx
.at(&obligation.cause, obligation.param_env)
.eq(DefineOpaqueTypes::No, obligation.predicate, infer_projection)
.is_ok_and(|InferOk { obligations, value: () }| {
self.evaluate_predicates_recursively(
TraitObligationStackList::empty(&ProvisionalEvaluationCache::default()),
nested_obligations.into_iter().chain(obligations),
)
.is_ok_and(|res| res.may_apply())
});
if is_match {
let generics = self.tcx().generics_of(obligation.predicate.def_id);
// FIXME(generic_associated_types): Addresses aggressive inference in #92917.
// If this type is a GAT, and of the GAT args resolve to something new,
// that means that we must have newly inferred something about the GAT.
// We should give up in that case.
//
// This only detects one layer of inference, which is probably not what we actually
// want, but fixing it causes some ambiguity:
// <https://github.com/rust-lang/rust/issues/125196>.
if !generics.is_own_empty()
&& obligation.predicate.args[generics.parent_count..].iter().any(|&p| {
p.has_non_region_infer()
&& match p.kind() {
ty::GenericArgKind::Const(ct) => {
self.infcx.shallow_resolve_const(ct) != ct
}
ty::GenericArgKind::Type(ty) => self.infcx.shallow_resolve(ty) != ty,
ty::GenericArgKind::Lifetime(_) => false,
}
})
{
ProjectionMatchesProjection::Ambiguous
} else {
ProjectionMatchesProjection::Yes
}
} else {
ProjectionMatchesProjection::No
}
}
}
/// ## Winnowing
///
/// 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.
impl<'tcx> SelectionContext<'_, 'tcx> {
/// If there are multiple ways to prove a trait goal, we make some
/// *fairly arbitrary* choices about which candidate is actually used.
///
/// For more details, look at the implementation of this method :)
#[instrument(level = "debug", skip(self), ret)]
fn winnow_candidates(
&mut self,
has_non_region_infer: bool,
mut candidates: Vec<EvaluatedCandidate<'tcx>>,
) -> Option<SelectionCandidate<'tcx>> {
if candidates.len() == 1 {
return Some(candidates.pop().unwrap().candidate);
}
// We prefer `Sized` candidates over everything.
let mut sized_candidates =
candidates.iter().filter(|c| matches!(c.candidate, SizedCandidate));
if let Some(sized_candidate) = sized_candidates.next() {
// There should only ever be a single sized candidate
// as they would otherwise overlap.
debug_assert_eq!(sized_candidates.next(), None);
// Only prefer the built-in `Sized` candidate if its nested goals are certain.
// Otherwise, we may encounter failure later on if inference causes this candidate
// to not hold, but a where clause would've applied instead.
if sized_candidate.evaluation.must_apply_modulo_regions() {
return Some(sized_candidate.candidate.clone());
} else {
return None;
}
}
// Before we consider where-bounds, we have to deduplicate them here and also
// drop where-bounds in case the same where-bound exists without bound vars.
// This is necessary as elaborating super-trait bounds may result in duplicates.
'search_victim: loop {
for (i, this) in candidates.iter().enumerate() {
let ParamCandidate(this) = this.candidate else { continue };
for (j, other) in candidates.iter().enumerate() {
if i == j {
continue;
}
let ParamCandidate(other) = other.candidate else { continue };
if this == other {
candidates.remove(j);
continue 'search_victim;
}
if this.skip_binder().trait_ref == other.skip_binder().trait_ref
&& this.skip_binder().polarity == other.skip_binder().polarity
&& !this.skip_binder().trait_ref.has_escaping_bound_vars()
{
candidates.remove(j);
continue 'search_victim;
}
}
}
break;
}
// The next highest priority is for non-global where-bounds. However, while we don't
// prefer global where-clauses here, we do bail with ambiguity when encountering both
// a global and a non-global where-clause.
//
// Our handling of where-bounds is generally fairly messy but necessary for backwards
// compatibility, see #50825 for why we need to handle global where-bounds like this.
let is_global = |c: ty::PolyTraitPredicate<'tcx>| c.is_global() && !c.has_bound_vars();
let param_candidates = candidates
.iter()
.filter_map(|c| if let ParamCandidate(p) = c.candidate { Some(p) } else { None });
let mut has_global_bounds = false;
let mut param_candidate = None;
for c in param_candidates {
if is_global(c) {
has_global_bounds = true;
} else if param_candidate.replace(c).is_some() {
// Ambiguity, two potentially different where-clauses
return None;
}
}
if let Some(predicate) = param_candidate {
// Ambiguity, a global and a non-global where-bound.
if has_global_bounds {
return None;
} else {
return Some(ParamCandidate(predicate));
}
}
// Prefer alias-bounds over blanket impls for rigid associated types. This is
// fairly arbitrary but once again necessary for backwards compatibility.
// If there are multiple applicable candidates which don't affect type inference,
// choose the one with the lowest index.
let alias_bound = candidates
.iter()
.filter_map(|c| if let ProjectionCandidate(i) = c.candidate { Some(i) } else { None })
.try_reduce(|c1, c2| if has_non_region_infer { None } else { Some(c1.min(c2)) });
match alias_bound {
Some(Some(index)) => return Some(ProjectionCandidate(index)),
Some(None) => {}
None => return None,
}
// Need to prioritize builtin trait object impls as `<dyn Any as Any>::type_id`
// should use the vtable method and not the method provided by the user-defined
// impl `impl<T: ?Sized> Any for T { .. }`. This really shouldn't exist but is
// necessary due to #57893. We again arbitrarily prefer the applicable candidate
// with the lowest index.
//
// We do not want to use these impls to guide inference in case a user-written impl
// may also apply.
let object_bound = candidates
.iter()
.filter_map(|c| if let ObjectCandidate(i) = c.candidate { Some(i) } else { None })
.try_reduce(|c1, c2| if has_non_region_infer { None } else { Some(c1.min(c2)) });
match object_bound {
Some(Some(index)) => {
return if has_non_region_infer
&& candidates.iter().any(|c| matches!(c.candidate, ImplCandidate(_)))
{
None
} else {
Some(ObjectCandidate(index))
};
}
Some(None) => {}
None => return None,
}
// Same for upcasting.
let upcast_bound = candidates
.iter()
.filter_map(|c| {
if let TraitUpcastingUnsizeCandidate(i) = c.candidate { Some(i) } else { None }
})
.try_reduce(|c1, c2| if has_non_region_infer { None } else { Some(c1.min(c2)) });
match upcast_bound {
Some(Some(index)) => return Some(TraitUpcastingUnsizeCandidate(index)),
Some(None) => {}
None => return None,
}
// Finally, handle overlapping user-written impls.
let impls = candidates.iter().filter_map(|c| {
if let ImplCandidate(def_id) = c.candidate {
Some((def_id, c.evaluation))
} else {
None
}
});
let mut impl_candidate = None;
for c in impls {
if let Some(prev) = impl_candidate.replace(c) {
if self.prefer_lhs_over_victim(has_non_region_infer, c, prev.0) {
// Ok, prefer `c` over the previous entry
} else if self.prefer_lhs_over_victim(has_non_region_infer, prev, c.0) {
// Ok, keep `prev` instead of the new entry
impl_candidate = Some(prev);
} else {
// Ambiguity, two potentially different where-clauses
return None;
}
}
}
if let Some((def_id, _evaluation)) = impl_candidate {
// Don't use impl candidates which overlap with other candidates.
// This should pretty much only ever happen with malformed impls.
if candidates.iter().all(|c| match c.candidate {
SizedCandidate
| BuiltinCandidate
| TransmutabilityCandidate
| AutoImplCandidate
| ClosureCandidate { .. }
| AsyncClosureCandidate
| AsyncFnKindHelperCandidate
| CoroutineCandidate
| FutureCandidate
| IteratorCandidate
| AsyncIteratorCandidate
| FnPointerCandidate
| TraitAliasCandidate
| TraitUpcastingUnsizeCandidate(_)
| BuiltinObjectCandidate
| BuiltinUnsizeCandidate
| BikeshedGuaranteedNoDropCandidate => false,
// Non-global param candidates have already been handled, global
// where-bounds get ignored.
ParamCandidate(_) | ImplCandidate(_) => true,
ProjectionCandidate(_) | ObjectCandidate(_) => unreachable!(),
}) {
return Some(ImplCandidate(def_id));
} else {
return None;
}
}
if candidates.len() == 1 {
Some(candidates.pop().unwrap().candidate)
} else {
// Also try ignoring all global where-bounds and check whether we end
// with a unique candidate in this case.
let mut not_a_global_where_bound = candidates
.into_iter()
.filter(|c| !matches!(c.candidate, ParamCandidate(p) if is_global(p)));
not_a_global_where_bound
.next()
.map(|c| c.candidate)
.filter(|_| not_a_global_where_bound.next().is_none())
}
}
fn prefer_lhs_over_victim(
&self,
has_non_region_infer: bool,
(lhs, lhs_evaluation): (DefId, EvaluationResult),
victim: DefId,
) -> bool {
let tcx = self.tcx();
// See if we can toss out `victim` based on specialization.
//
// While this requires us to know *for sure* that the `lhs` impl applies
// we still use modulo regions here. This is fine as specialization currently
// assumes that specializing impls have to be always applicable, meaning that
// the only allowed region constraints may be constraints also present on the default impl.
if lhs_evaluation.must_apply_modulo_regions() {
if tcx.specializes((lhs, victim)) {
return true;
}
}
match tcx.impls_are_allowed_to_overlap(lhs, victim) {
// For candidates which already reference errors it doesn't really
// matter what we do 🤷
Some(ty::ImplOverlapKind::Permitted { marker: false }) => {
lhs_evaluation.must_apply_considering_regions()
}
Some(ty::ImplOverlapKind::Permitted { marker: true }) => {
// Subtle: If the predicate we are evaluating has inference
// variables, do *not* allow discarding candidates due to
// marker trait impls.
//
// Without this restriction, we could end up accidentally
// constraining inference variables based on an arbitrarily
// chosen trait impl.
//
// Imagine we have the following code:
//
// ```rust
// #[marker] trait MyTrait {}
// impl MyTrait for u8 {}
// impl MyTrait for bool {}
// ```
//
// And we are evaluating the predicate `<_#0t as MyTrait>`.
//
// During selection, we will end up with one candidate for each
// impl of `MyTrait`. If we were to discard one impl in favor
// of the other, we would be left with one candidate, causing
// us to "successfully" select the predicate, unifying
// _#0t with (for example) `u8`.
//
// However, we have no reason to believe that this unification
// is correct - we've essentially just picked an arbitrary
// *possibility* for _#0t, and required that this be the *only*
// possibility.
//
// Eventually, we will either:
// 1) Unify all inference variables in the predicate through
// some other means (e.g. type-checking of a function). We will
// then be in a position to drop marker trait candidates
// without constraining inference variables (since there are
// none left to constrain)
// 2) Be left with some unconstrained inference variables. We
// will then correctly report an inference error, since the
// existence of multiple marker trait impls tells us nothing
// about which one should actually apply.
!has_non_region_infer && lhs_evaluation.must_apply_considering_regions()
}
None => false,
}
}
}
impl<'tcx> SelectionContext<'_, 'tcx> {
fn sizedness_conditions(
&mut self,
self_ty: Ty<'tcx>,
sizedness: SizedTraitKind,
) -> ty::Binder<'tcx, Vec<Ty<'tcx>>> {
match self_ty.kind() {
ty::Infer(ty::IntVar(_) | ty::FloatVar(_))
| ty::Uint(_)
| ty::Int(_)
| ty::Bool
| ty::Float(_)
| ty::FnDef(..)
| ty::FnPtr(..)
| ty::RawPtr(..)
| ty::Char
| ty::Ref(..)
| ty::Coroutine(..)
| ty::CoroutineWitness(..)
| ty::Array(..)
| ty::Closure(..)
| ty::CoroutineClosure(..)
| ty::Never
| ty::Error(_) => ty::Binder::dummy(vec![]),
ty::Str | ty::Slice(_) | ty::Dynamic(..) => match sizedness {
SizedTraitKind::Sized => unreachable!("tried to assemble `Sized` for unsized type"),
SizedTraitKind::MetaSized => ty::Binder::dummy(vec![]),
},
ty::Foreign(..) => unreachable!("tried to assemble `Sized` for unsized type"),
ty::Tuple(tys) => {
ty::Binder::dummy(tys.last().map_or_else(Vec::new, |&last| vec![last]))
}
ty::Pat(ty, _) => ty::Binder::dummy(vec![*ty]),
ty::Adt(def, args) => {
if let Some(crit) = def.sizedness_constraint(self.tcx(), sizedness) {
ty::Binder::dummy(vec![crit.instantiate(self.tcx(), args)])
} else {
ty::Binder::dummy(vec![])
}
}
ty::UnsafeBinder(binder_ty) => binder_ty.map_bound(|ty| vec![ty]),
ty::Alias(..)
| ty::Param(_)
| ty::Placeholder(..)
| ty::Infer(ty::TyVar(_) | ty::FreshTy(_) | ty::FreshIntTy(_) | ty::FreshFloatTy(_))
| ty::Bound(..) => {
bug!("asked to assemble `Sized` of unexpected type: {:?}", self_ty);
}
}
}
fn copy_clone_conditions(&mut self, self_ty: Ty<'tcx>) -> ty::Binder<'tcx, Vec<Ty<'tcx>>> {
match *self_ty.kind() {
ty::FnDef(..) | ty::FnPtr(..) | ty::Error(_) => ty::Binder::dummy(vec![]),
ty::Uint(_)
| ty::Int(_)
| ty::Infer(ty::IntVar(_) | ty::FloatVar(_))
| ty::Bool
| ty::Float(_)
| ty::Char
| ty::RawPtr(..)
| ty::Never
| ty::Ref(_, _, hir::Mutability::Not)
| ty::Array(..) => {
unreachable!("tried to assemble `Sized` for type with libcore-provided impl")
}
// FIXME(unsafe_binder): Should we conditionally
// (i.e. universally) implement copy/clone?
ty::UnsafeBinder(_) => unreachable!("tried to assemble `Sized` for unsafe binder"),
ty::Tuple(tys) => {
// (*) binder moved here
ty::Binder::dummy(tys.iter().collect())
}
ty::Pat(ty, _) => {
// (*) binder moved here
ty::Binder::dummy(vec![ty])
}
ty::Coroutine(coroutine_def_id, args) => {
match self.tcx().coroutine_movability(coroutine_def_id) {
hir::Movability::Static => {
unreachable!("tried to assemble `Sized` for static coroutine")
}
hir::Movability::Movable => {
if self.tcx().features().coroutine_clone() {
ty::Binder::dummy(
args.as_coroutine()
.upvar_tys()
.iter()
.chain([args.as_coroutine().witness()])
.collect::<Vec<_>>(),
)
} else {
unreachable!(
"tried to assemble `Sized` for coroutine without enabled feature"
)
}
}
}
}
ty::CoroutineWitness(def_id, args) => self
.infcx
.tcx
.coroutine_hidden_types(def_id)
.instantiate(self.infcx.tcx, args)
.map_bound(|witness| witness.types.to_vec()),
ty::Closure(_, args) => ty::Binder::dummy(args.as_closure().upvar_tys().to_vec()),
ty::CoroutineClosure(_, args) => {
ty::Binder::dummy(args.as_coroutine_closure().upvar_tys().to_vec())
}
ty::Foreign(..)
| ty::Str
| ty::Slice(_)
| ty::Dynamic(..)
| ty::Adt(..)
| ty::Alias(..)
| ty::Param(..)
| ty::Placeholder(..)
| ty::Bound(..)
| ty::Ref(_, _, ty::Mutability::Mut)
| ty::Infer(ty::TyVar(_) | ty::FreshTy(_) | ty::FreshIntTy(_) | ty::FreshFloatTy(_)) => {
bug!("asked to assemble builtin bounds of unexpected type: {:?}", self_ty);
}
}
}
fn coroutine_is_gen(&mut self, self_ty: Ty<'tcx>) -> bool {
matches!(*self_ty.kind(), ty::Coroutine(did, ..)
if self.tcx().coroutine_is_gen(did))
}
/// 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:
///
/// ```ignore (illustrative)
/// (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]
/// ```
#[instrument(level = "debug", skip(self), ret)]
fn constituent_types_for_auto_trait(
&self,
t: Ty<'tcx>,
) -> Result<ty::Binder<'tcx, AutoImplConstituents<'tcx>>, SelectionError<'tcx>> {
Ok(match *t.kind() {
ty::Uint(_)
| ty::Int(_)
| ty::Bool
| ty::Float(_)
| ty::FnDef(..)
| ty::FnPtr(..)
| ty::Error(_)
| ty::Infer(ty::IntVar(_) | ty::FloatVar(_))
| ty::Never
| ty::Char => {
ty::Binder::dummy(AutoImplConstituents { types: vec![], assumptions: vec![] })
}
// This branch is only for `experimental_default_bounds`.
// Other foreign types were rejected earlier in
// `assemble_candidates_from_auto_impls`.
ty::Foreign(..) => {
ty::Binder::dummy(AutoImplConstituents { types: vec![], assumptions: vec![] })
}
ty::UnsafeBinder(ty) => {
ty.map_bound(|ty| AutoImplConstituents { types: vec![ty], assumptions: vec![] })
}
// Treat this like `struct str([u8]);`
ty::Str => ty::Binder::dummy(AutoImplConstituents {
types: vec![Ty::new_slice(self.tcx(), self.tcx().types.u8)],
assumptions: vec![],
}),
ty::Placeholder(..)
| ty::Dynamic(..)
| ty::Param(..)
| ty::Alias(ty::Projection | ty::Inherent | ty::Free, ..)
| ty::Bound(..)
| ty::Infer(ty::TyVar(_) | ty::FreshTy(_) | ty::FreshIntTy(_) | ty::FreshFloatTy(_)) => {
bug!("asked to assemble constituent types of unexpected type: {:?}", t);
}
ty::RawPtr(element_ty, _) | ty::Ref(_, element_ty, _) => {
ty::Binder::dummy(AutoImplConstituents {
types: vec![element_ty],
assumptions: vec![],
})
}
ty::Pat(ty, _) | ty::Array(ty, _) | ty::Slice(ty) => {
ty::Binder::dummy(AutoImplConstituents { types: vec![ty], assumptions: vec![] })
}
ty::Tuple(tys) => {
// (T1, ..., Tn) -- meets any bound that all of T1...Tn meet
ty::Binder::dummy(AutoImplConstituents {
types: tys.iter().collect(),
assumptions: vec![],
})
}
ty::Closure(_, args) => {
let ty = self.infcx.shallow_resolve(args.as_closure().tupled_upvars_ty());
ty::Binder::dummy(AutoImplConstituents { types: vec![ty], assumptions: vec![] })
}
ty::CoroutineClosure(_, args) => {
let ty = self.infcx.shallow_resolve(args.as_coroutine_closure().tupled_upvars_ty());
ty::Binder::dummy(AutoImplConstituents { types: vec![ty], assumptions: vec![] })
}
ty::Coroutine(_, args) => {
let ty = self.infcx.shallow_resolve(args.as_coroutine().tupled_upvars_ty());
let witness = args.as_coroutine().witness();
ty::Binder::dummy(AutoImplConstituents {
types: [ty].into_iter().chain(iter::once(witness)).collect(),
assumptions: vec![],
})
}
ty::CoroutineWitness(def_id, args) => self
.infcx
.tcx
.coroutine_hidden_types(def_id)
.instantiate(self.infcx.tcx, args)
.map_bound(|witness| AutoImplConstituents {
types: witness.types.to_vec(),
assumptions: witness.assumptions.to_vec(),
}),
// For `PhantomData<T>`, we pass `T`.
ty::Adt(def, args) if def.is_phantom_data() => {
ty::Binder::dummy(AutoImplConstituents {
types: args.types().collect(),
assumptions: vec![],
})
}
ty::Adt(def, args) => ty::Binder::dummy(AutoImplConstituents {
types: def.all_fields().map(|f| f.ty(self.tcx(), args)).collect(),
assumptions: vec![],
}),
ty::Alias(ty::Opaque, ty::AliasTy { def_id, args, .. }) => {
if self.infcx.can_define_opaque_ty(def_id) {
unreachable!()
} else {
// We can resolve the `impl Trait` to its concrete type,
// which enforces a DAG between the functions requiring
// the auto trait bounds in question.
match self.tcx().type_of_opaque(def_id) {
Ok(ty) => ty::Binder::dummy(AutoImplConstituents {
types: vec![ty.instantiate(self.tcx(), args)],
assumptions: vec![],
}),
Err(_) => {
return Err(SelectionError::OpaqueTypeAutoTraitLeakageUnknown(def_id));
}
}
}
}
})
}
fn collect_predicates_for_types(
&mut self,
param_env: ty::ParamEnv<'tcx>,
cause: ObligationCause<'tcx>,
recursion_depth: usize,
trait_def_id: DefId,
types: Vec<Ty<'tcx>>,
) -> PredicateObligations<'tcx> {
// Because the types were potentially derived from
// higher-ranked obligations they may reference late-bound
// regions. For example, `for<'a> Foo<&'a i32> : Copy` would
// yield a type like `for<'a> &'a i32`. 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 i32` becomes `&0 i32`.
// 2. Produce something like `&'0 i32 : Copy`
// 3. Re-bind the regions back to `for<'a> &'a i32 : Copy`
types
.into_iter()
.flat_map(|placeholder_ty| {
let Normalized { value: normalized_ty, mut obligations } =
ensure_sufficient_stack(|| {
normalize_with_depth(
self,
param_env,
cause.clone(),
recursion_depth,
placeholder_ty,
)
});
let tcx = self.tcx();
let trait_ref = if tcx.generics_of(trait_def_id).own_params.len() == 1 {
ty::TraitRef::new(tcx, trait_def_id, [normalized_ty])
} else {
// If this is an ill-formed auto/built-in trait, then synthesize
// new error args for the missing generics.
let err_args = ty::GenericArgs::extend_with_error(
tcx,
trait_def_id,
&[normalized_ty.into()],
);
ty::TraitRef::new_from_args(tcx, trait_def_id, err_args)
};
let obligation = Obligation::new(self.tcx(), cause.clone(), param_env, trait_ref);
obligations.push(obligation);
obligations
})
.collect()
}
///////////////////////////////////////////////////////////////////////////
// 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: &PolyTraitObligation<'tcx>,
) -> Normalized<'tcx, GenericArgsRef<'tcx>> {
let impl_trait_header = self.tcx().impl_trait_header(impl_def_id).unwrap();
match self.match_impl(impl_def_id, impl_trait_header, obligation) {
Ok(args) => args,
Err(()) => {
let predicate = self.infcx.resolve_vars_if_possible(obligation.predicate);
bug!("impl {impl_def_id:?} was matchable against {predicate:?} but now is not")
}
}
}
#[instrument(level = "debug", skip(self), ret)]
fn match_impl(
&mut self,
impl_def_id: DefId,
impl_trait_header: ty::ImplTraitHeader<'tcx>,
obligation: &PolyTraitObligation<'tcx>,
) -> Result<Normalized<'tcx, GenericArgsRef<'tcx>>, ()> {
let placeholder_obligation =
self.infcx.enter_forall_and_leak_universe(obligation.predicate);
let placeholder_obligation_trait_ref = placeholder_obligation.trait_ref;
let impl_args = self.infcx.fresh_args_for_item(obligation.cause.span, impl_def_id);
let trait_ref = impl_trait_header.trait_ref.instantiate(self.tcx(), impl_args);
debug!(?impl_trait_header);
let Normalized { value: impl_trait_ref, obligations: mut nested_obligations } =
ensure_sufficient_stack(|| {
normalize_with_depth(
self,
obligation.param_env,
obligation.cause.clone(),
obligation.recursion_depth + 1,
trait_ref,
)
});
debug!(?impl_trait_ref, ?placeholder_obligation_trait_ref);
let cause = ObligationCause::new(
obligation.cause.span,
obligation.cause.body_id,
ObligationCauseCode::MatchImpl(obligation.cause.clone(), impl_def_id),
);
let InferOk { obligations, .. } = self
.infcx
.at(&cause, obligation.param_env)
.eq(DefineOpaqueTypes::No, placeholder_obligation_trait_ref, impl_trait_ref)
.map_err(|e| {
debug!("match_impl: failed eq_trait_refs due to `{}`", e.to_string(self.tcx()))
})?;
nested_obligations.extend(obligations);
if impl_trait_header.polarity == ty::ImplPolarity::Reservation
&& !matches!(self.infcx.typing_mode(), TypingMode::Coherence)
{
debug!("reservation impls only apply in intercrate mode");
return Err(());
}
Ok(Normalized { value: impl_args, obligations: nested_obligations })
}
fn match_upcast_principal(
&mut self,
obligation: &PolyTraitObligation<'tcx>,
unnormalized_upcast_principal: ty::PolyTraitRef<'tcx>,
a_data: &'tcx ty::List<ty::PolyExistentialPredicate<'tcx>>,
b_data: &'tcx ty::List<ty::PolyExistentialPredicate<'tcx>>,
a_region: ty::Region<'tcx>,
b_region: ty::Region<'tcx>,
) -> SelectionResult<'tcx, PredicateObligations<'tcx>> {
let tcx = self.tcx();
let mut nested = PredicateObligations::new();
// We may upcast to auto traits that are either explicitly listed in
// the object type's bounds, or implied by the principal trait ref's
// supertraits.
let a_auto_traits: FxIndexSet<DefId> = a_data
.auto_traits()
.chain(a_data.principal_def_id().into_iter().flat_map(|principal_def_id| {
elaborate::supertrait_def_ids(tcx, principal_def_id)
.filter(|def_id| tcx.trait_is_auto(*def_id))
}))
.collect();
let upcast_principal = normalize_with_depth_to(
self,
obligation.param_env,
obligation.cause.clone(),
obligation.recursion_depth + 1,
unnormalized_upcast_principal,
&mut nested,
);
for bound in b_data {
match bound.skip_binder() {
// Check that a_ty's supertrait (upcast_principal) is compatible
// with the target (b_ty).
ty::ExistentialPredicate::Trait(target_principal) => {
let hr_source_principal = upcast_principal.map_bound(|trait_ref| {
ty::ExistentialTraitRef::erase_self_ty(tcx, trait_ref)
});
let hr_target_principal = bound.rebind(target_principal);
nested.extend(
self.infcx
.enter_forall(hr_target_principal, |target_principal| {
let source_principal =
self.infcx.instantiate_binder_with_fresh_vars(
obligation.cause.span,
HigherRankedType,
hr_source_principal,
);
self.infcx.at(&obligation.cause, obligation.param_env).eq_trace(
DefineOpaqueTypes::Yes,
ToTrace::to_trace(
&obligation.cause,
hr_target_principal,
hr_source_principal,
),
target_principal,
source_principal,
)
})
.map_err(|_| SelectionError::Unimplemented)?
.into_obligations(),
);
}
// Check that b_ty's projection is satisfied by exactly one of
// a_ty's projections. First, we look through the list to see if
// any match. If not, error. Then, if *more* than one matches, we
// return ambiguity. Otherwise, if exactly one matches, equate
// it with b_ty's projection.
ty::ExistentialPredicate::Projection(target_projection) => {
let hr_target_projection = bound.rebind(target_projection);
let mut matching_projections =
a_data.projection_bounds().filter(|&hr_source_projection| {
// Eager normalization means that we can just use can_eq
// here instead of equating and processing obligations.
hr_source_projection.item_def_id() == hr_target_projection.item_def_id()
&& self.infcx.probe(|_| {
self.infcx
.enter_forall(hr_target_projection, |target_projection| {
let source_projection =
self.infcx.instantiate_binder_with_fresh_vars(
obligation.cause.span,
HigherRankedType,
hr_source_projection,
);
self.infcx
.at(&obligation.cause, obligation.param_env)
.eq_trace(
DefineOpaqueTypes::Yes,
ToTrace::to_trace(
&obligation.cause,
hr_target_projection,
hr_source_projection,
),
target_projection,
source_projection,
)
})
.is_ok()
})
});
let Some(hr_source_projection) = matching_projections.next() else {
return Err(SelectionError::Unimplemented);
};
if matching_projections.next().is_some() {
return Ok(None);
}
nested.extend(
self.infcx
.enter_forall(hr_target_projection, |target_projection| {
let source_projection =
self.infcx.instantiate_binder_with_fresh_vars(
obligation.cause.span,
HigherRankedType,
hr_source_projection,
);
self.infcx.at(&obligation.cause, obligation.param_env).eq_trace(
DefineOpaqueTypes::Yes,
ToTrace::to_trace(
&obligation.cause,
hr_target_projection,
hr_source_projection,
),
target_projection,
source_projection,
)
})
.map_err(|_| SelectionError::Unimplemented)?
.into_obligations(),
);
}
// Check that b_ty's auto traits are present in a_ty's bounds.
ty::ExistentialPredicate::AutoTrait(def_id) => {
if !a_auto_traits.contains(&def_id) {
return Err(SelectionError::Unimplemented);
}
}
}
}
nested.push(Obligation::with_depth(
tcx,
obligation.cause.clone(),
obligation.recursion_depth + 1,
obligation.param_env,
ty::Binder::dummy(ty::OutlivesPredicate(a_region, b_region)),
));
Ok(Some(nested))
}
/// Normalize `where_clause_trait_ref` and try to match it against
/// `obligation`. If successful, return any predicates that
/// result from the normalization.
fn match_where_clause_trait_ref(
&mut self,
obligation: &PolyTraitObligation<'tcx>,
where_clause_trait_ref: ty::PolyTraitRef<'tcx>,
) -> Result<PredicateObligations<'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.
#[instrument(skip(self), level = "debug")]
fn match_poly_trait_ref(
&mut self,
obligation: &PolyTraitObligation<'tcx>,
poly_trait_ref: ty::PolyTraitRef<'tcx>,
) -> Result<PredicateObligations<'tcx>, ()> {
let predicate = self.infcx.enter_forall_and_leak_universe(obligation.predicate);
let trait_ref = self.infcx.instantiate_binder_with_fresh_vars(
obligation.cause.span,
HigherRankedType,
poly_trait_ref,
);
self.infcx
.at(&obligation.cause, obligation.param_env)
.eq(DefineOpaqueTypes::No, predicate.trait_ref, trait_ref)
.map(|InferOk { obligations, .. }| obligations)
.map_err(|_| ())
}
///////////////////////////////////////////////////////////////////////////
// Miscellany
fn match_fresh_trait_refs(
&self,
previous: ty::PolyTraitPredicate<'tcx>,
current: ty::PolyTraitPredicate<'tcx>,
) -> bool {
let mut matcher = _match::MatchAgainstFreshVars::new(self.tcx());
matcher.relate(previous, current).is_ok()
}
fn push_stack<'o>(
&mut self,
previous_stack: TraitObligationStackList<'o, 'tcx>,
obligation: &'o PolyTraitObligation<'tcx>,
) -> TraitObligationStack<'o, 'tcx> {
let fresh_trait_pred = obligation.predicate.fold_with(&mut self.freshener);
let dfn = previous_stack.cache.next_dfn();
let depth = previous_stack.depth() + 1;
TraitObligationStack {
obligation,
fresh_trait_pred,
reached_depth: Cell::new(depth),
previous: previous_stack,
dfn,
depth,
}
}
#[instrument(skip(self), level = "debug")]
fn closure_trait_ref_unnormalized(
&mut self,
self_ty: Ty<'tcx>,
fn_trait_def_id: DefId,
) -> ty::PolyTraitRef<'tcx> {
let ty::Closure(_, args) = *self_ty.kind() else {
bug!("expected closure, found {self_ty}");
};
let closure_sig = args.as_closure().sig();
closure_trait_ref_and_return_type(
self.tcx(),
fn_trait_def_id,
self_ty,
closure_sig,
util::TupleArgumentsFlag::No,
)
.map_bound(|(trait_ref, _)| trait_ref)
}
/// Returns the obligations that are implied by instantiating an
/// impl or trait. The obligations are instantiated and fully
/// normalized. This is used when confirming an impl or default
/// impl.
#[instrument(level = "debug", skip(self, cause, param_env))]
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
args: GenericArgsRef<'tcx>, // for impl or trait
parent_trait_pred: ty::Binder<'tcx, ty::TraitPredicate<'tcx>>,
) -> PredicateObligations<'tcx> {
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<Item: Copy>, V: Iterator<Item = U>> Foo for V
// 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 instantiate, 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 predicates = predicates.instantiate_own(tcx, args);
let mut obligations = PredicateObligations::with_capacity(predicates.len());
for (index, (predicate, span)) in predicates.into_iter().enumerate() {
let cause = if tcx.is_lang_item(parent_trait_pred.def_id(), LangItem::CoerceUnsized) {
cause.clone()
} else {
cause.clone().derived_cause(parent_trait_pred, |derived| {
ObligationCauseCode::ImplDerived(Box::new(ImplDerivedCause {
derived,
impl_or_alias_def_id: def_id,
impl_def_predicate_index: Some(index),
span,
}))
})
};
let clause = normalize_with_depth_to(
self,
param_env,
cause.clone(),
recursion_depth,
predicate,
&mut obligations,
);
obligations.push(Obligation {
cause,
recursion_depth,
param_env,
predicate: clause.as_predicate(),
});
}
// Register any outlives obligations from the trait here, cc #124336.
if matches!(tcx.def_kind(def_id), DefKind::Impl { of_trait: true }) {
for clause in tcx.impl_super_outlives(def_id).iter_instantiated(tcx, args) {
let clause = normalize_with_depth_to(
self,
param_env,
cause.clone(),
recursion_depth,
clause,
&mut obligations,
);
obligations.push(Obligation {
cause: cause.clone(),
recursion_depth,
param_env,
predicate: clause.as_predicate(),
});
}
}
obligations
}
fn should_stall_coroutine_witness(&self, def_id: DefId) -> bool {
match self.infcx.typing_mode() {
TypingMode::Analysis { defining_opaque_types_and_generators: stalled_generators } => {
def_id.as_local().is_some_and(|def_id| stalled_generators.contains(&def_id))
}
TypingMode::Coherence
| TypingMode::PostAnalysis
| TypingMode::Borrowck { defining_opaque_types: _ }
| TypingMode::PostBorrowckAnalysis { defined_opaque_types: _ } => false,
}
}
}
impl<'o, 'tcx> TraitObligationStack<'o, 'tcx> {
fn list(&'o self) -> TraitObligationStackList<'o, 'tcx> {
TraitObligationStackList::with(self)
}
fn cache(&self) -> &'o ProvisionalEvaluationCache<'tcx> {
self.previous.cache
}
fn iter(&'o self) -> TraitObligationStackList<'o, 'tcx> {
self.list()
}
/// Indicates that attempting to evaluate this stack entry
/// required accessing something from the stack at depth `reached_depth`.
fn update_reached_depth(&self, reached_depth: usize) {
assert!(
self.depth >= reached_depth,
"invoked `update_reached_depth` with something under this stack: \
self.depth={} reached_depth={}",
self.depth,
reached_depth,
);
debug!(reached_depth, "update_reached_depth");
let mut p = self;
while reached_depth < p.depth {
debug!(?p.fresh_trait_pred, "update_reached_depth: marking as cycle participant");
p.reached_depth.set(p.reached_depth.get().min(reached_depth));
p = p.previous.head.unwrap();
}
}
}
/// The "provisional evaluation cache" is used to store intermediate cache results
/// when solving auto traits. Auto traits are unusual in that they can support
/// cycles. So, for example, a "proof tree" like this would be ok:
///
/// - `Foo<T>: Send` :-
/// - `Bar<T>: Send` :-
/// - `Foo<T>: Send` -- cycle, but ok
/// - `Baz<T>: Send`
///
/// Here, to prove `Foo<T>: Send`, we have to prove `Bar<T>: Send` and
/// `Baz<T>: Send`. Proving `Bar<T>: Send` in turn required `Foo<T>: Send`.
/// For non-auto traits, this cycle would be an error, but for auto traits (because
/// they are coinductive) it is considered ok.
///
/// However, there is a complication: at the point where we have
/// "proven" `Bar<T>: Send`, we have in fact only proven it
/// *provisionally*. In particular, we proved that `Bar<T>: Send`
/// *under the assumption* that `Foo<T>: Send`. But what if we later
/// find out this assumption is wrong? Specifically, we could
/// encounter some kind of error proving `Baz<T>: Send`. In that case,
/// `Bar<T>: Send` didn't turn out to be true.
///
/// In Issue #60010, we found a bug in rustc where it would cache
/// these intermediate results. This was fixed in #60444 by disabling
/// *all* caching for things involved in a cycle -- in our example,
/// that would mean we don't cache that `Bar<T>: Send`. But this led
/// to large slowdowns.
///
/// Specifically, imagine this scenario, where proving `Baz<T>: Send`
/// first requires proving `Bar<T>: Send` (which is true:
///
/// - `Foo<T>: Send` :-
/// - `Bar<T>: Send` :-
/// - `Foo<T>: Send` -- cycle, but ok
/// - `Baz<T>: Send`
/// - `Bar<T>: Send` -- would be nice for this to be a cache hit!
/// - `*const T: Send` -- but what if we later encounter an error?
///
/// The *provisional evaluation cache* resolves this issue. It stores
/// cache results that we've proven but which were involved in a cycle
/// in some way. We track the minimal stack depth (i.e., the
/// farthest from the top of the stack) that we are dependent on.
/// The idea is that the cache results within are all valid -- so long as
/// none of the nodes in between the current node and the node at that minimum
/// depth result in an error (in which case the cached results are just thrown away).
///
/// During evaluation, we consult this provisional cache and rely on
/// it. Accessing a cached value is considered equivalent to accessing
/// a result at `reached_depth`, so it marks the *current* solution as
/// provisional as well. If an error is encountered, we toss out any
/// provisional results added from the subtree that encountered the
/// error. When we pop the node at `reached_depth` from the stack, we
/// can commit all the things that remain in the provisional cache.
struct ProvisionalEvaluationCache<'tcx> {
/// next "depth first number" to issue -- just a counter
dfn: Cell<usize>,
/// Map from cache key to the provisionally evaluated thing.
/// The cache entries contain the result but also the DFN in which they
/// were added. The DFN is used to clear out values on failure.
///
/// Imagine we have a stack like:
///
/// - `A B C` and we add a cache for the result of C (DFN 2)
/// - Then we have a stack `A B D` where `D` has DFN 3
/// - We try to solve D by evaluating E: `A B D E` (DFN 4)
/// - `E` generates various cache entries which have cyclic dependencies on `B`
/// - `A B D E F` and so forth
/// - the DFN of `F` for example would be 5
/// - then we determine that `E` is in error -- we will then clear
/// all cache values whose DFN is >= 4 -- in this case, that
/// means the cached value for `F`.
map: RefCell<FxIndexMap<ty::PolyTraitPredicate<'tcx>, ProvisionalEvaluation>>,
/// The stack of terms that we assume to be well-formed because a `WF(term)` predicate
/// is on the stack above (and because of wellformedness is coinductive).
/// In an "ideal" world, this would share a stack with trait predicates in
/// `TraitObligationStack`. However, trait predicates are *much* hotter than
/// `WellFormed` predicates, and it's very likely that the additional matches
/// will have a perf effect. The value here is the well-formed `GenericArg`
/// and the depth of the trait predicate *above* that well-formed predicate.
wf_args: RefCell<Vec<(ty::Term<'tcx>, usize)>>,
}
/// A cache value for the provisional cache: contains the depth-first
/// number (DFN) and result.
#[derive(Copy, Clone, Debug)]
struct ProvisionalEvaluation {
from_dfn: usize,
reached_depth: usize,
result: EvaluationResult,
}
impl<'tcx> Default for ProvisionalEvaluationCache<'tcx> {
fn default() -> Self {
Self { dfn: Cell::new(0), map: Default::default(), wf_args: Default::default() }
}
}
impl<'tcx> ProvisionalEvaluationCache<'tcx> {
/// Get the next DFN in sequence (basically a counter).
fn next_dfn(&self) -> usize {
let result = self.dfn.get();
self.dfn.set(result + 1);
result
}
/// Check the provisional cache for any result for
/// `fresh_trait_ref`. If there is a hit, then you must consider
/// it an access to the stack slots at depth
/// `reached_depth` (from the returned value).
fn get_provisional(
&self,
fresh_trait_pred: ty::PolyTraitPredicate<'tcx>,
) -> Option<ProvisionalEvaluation> {
debug!(
?fresh_trait_pred,
"get_provisional = {:#?}",
self.map.borrow().get(&fresh_trait_pred),
);
Some(*self.map.borrow().get(&fresh_trait_pred)?)
}
/// Insert a provisional result into the cache. The result came
/// from the node with the given DFN. It accessed a minimum depth
/// of `reached_depth` to compute. It evaluated `fresh_trait_pred`
/// and resulted in `result`.
fn insert_provisional(
&self,
from_dfn: usize,
reached_depth: usize,
fresh_trait_pred: ty::PolyTraitPredicate<'tcx>,
result: EvaluationResult,
) {
debug!(?from_dfn, ?fresh_trait_pred, ?result, "insert_provisional");
let mut map = self.map.borrow_mut();
// Subtle: when we complete working on the DFN `from_dfn`, anything
// that remains in the provisional cache must be dependent on some older
// stack entry than `from_dfn`. We have to update their depth with our transitive
// depth in that case or else it would be referring to some popped note.
//
// Example:
// A (reached depth 0)
// ...
// B // depth 1 -- reached depth = 0
// C // depth 2 -- reached depth = 1 (should be 0)
// B
// A // depth 0
// D (reached depth 1)
// C (cache -- reached depth = 2)
for (_k, v) in &mut *map {
if v.from_dfn >= from_dfn {
v.reached_depth = reached_depth.min(v.reached_depth);
}
}
map.insert(fresh_trait_pred, ProvisionalEvaluation { from_dfn, reached_depth, result });
}
/// Invoked when the node with dfn `dfn` does not get a successful
/// result. This will clear out any provisional cache entries
/// that were added since `dfn` was created. This is because the
/// provisional entries are things which must assume that the
/// things on the stack at the time of their creation succeeded --
/// since the failing node is presently at the top of the stack,
/// these provisional entries must either depend on it or some
/// ancestor of it.
fn on_failure(&self, dfn: usize) {
debug!(?dfn, "on_failure");
self.map.borrow_mut().retain(|key, eval| {
if !eval.from_dfn >= dfn {
debug!("on_failure: removing {:?}", key);
false
} else {
true
}
});
}
/// Invoked when the node at depth `depth` completed without
/// depending on anything higher in the stack (if that completion
/// was a failure, then `on_failure` should have been invoked
/// already).
///
/// Note that we may still have provisional cache items remaining
/// in the cache when this is done. For example, if there is a
/// cycle:
///
/// * A depends on...
/// * B depends on A
/// * C depends on...
/// * D depends on C
/// * ...
///
/// Then as we complete the C node we will have a provisional cache
/// with results for A, B, C, and D. This method would clear out
/// the C and D results, but leave A and B provisional.
///
/// This is determined based on the DFN: we remove any provisional
/// results created since `dfn` started (e.g., in our example, dfn
/// would be 2, representing the C node, and hence we would
/// remove the result for D, which has DFN 3, but not the results for
/// A and B, which have DFNs 0 and 1 respectively).
///
/// Note that we *do not* attempt to cache these cycle participants
/// in the evaluation cache. Doing so would require carefully computing
/// the correct `DepNode` to store in the cache entry:
/// cycle participants may implicitly depend on query results
/// related to other participants in the cycle, due to our logic
/// which examines the evaluation stack.
///
/// We used to try to perform this caching,
/// but it lead to multiple incremental compilation ICEs
/// (see #92987 and #96319), and was very hard to understand.
/// Fortunately, removing the caching didn't seem to
/// have a performance impact in practice.
fn on_completion(&self, dfn: usize) {
debug!(?dfn, "on_completion");
self.map.borrow_mut().retain(|fresh_trait_pred, eval| {
if eval.from_dfn >= dfn {
debug!(?fresh_trait_pred, ?eval, "on_completion");
return false;
}
true
});
}
}
#[derive(Copy, Clone)]
struct TraitObligationStackList<'o, 'tcx> {
cache: &'o ProvisionalEvaluationCache<'tcx>,
head: Option<&'o TraitObligationStack<'o, 'tcx>>,
}
impl<'o, 'tcx> TraitObligationStackList<'o, 'tcx> {
fn empty(cache: &'o ProvisionalEvaluationCache<'tcx>) -> TraitObligationStackList<'o, 'tcx> {
TraitObligationStackList { cache, head: None }
}
fn with(r: &'o TraitObligationStack<'o, 'tcx>) -> TraitObligationStackList<'o, 'tcx> {
TraitObligationStackList { cache: r.cache(), head: Some(r) }
}
fn head(&self) -> Option<&'o TraitObligationStack<'o, 'tcx>> {
self.head
}
fn depth(&self) -> usize {
if let Some(head) = self.head { head.depth } else { 0 }
}
}
impl<'o, 'tcx> Iterator for TraitObligationStackList<'o, 'tcx> {
type Item = &'o TraitObligationStack<'o, 'tcx>;
fn next(&mut self) -> Option<&'o TraitObligationStack<'o, 'tcx>> {
let o = self.head?;
*self = o.previous;
Some(o)
}
}
impl<'o, 'tcx> fmt::Debug for TraitObligationStack<'o, 'tcx> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
write!(f, "TraitObligationStack({:?})", self.obligation)
}
}
pub(crate) enum ProjectionMatchesProjection {
Yes,
Ambiguous,
No,
}
#[derive(Clone, Debug, TypeFoldable, TypeVisitable)]
pub(crate) struct AutoImplConstituents<'tcx> {
pub types: Vec<Ty<'tcx>>,
pub assumptions: Vec<ty::ArgOutlivesPredicate<'tcx>>,
}