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fuchsia / third_party / rust / 57e8fc56852e7728d7160242bf13c3ab6e066bd8 / . / compiler / rustc_trait_selection / src / traits / select / mod.rs
blob: 57f1fedacbe5a38d6fc12633990c28f46e405414 [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 self::EvaluationResult::*;
use self::SelectionCandidate::*;
use super::coherence::{self, Conflict};
use super::const_evaluatable;
use super::project;
use super::project::normalize_with_depth_to;
use super::util;
use super::util::{closure_trait_ref_and_return_type, predicate_for_trait_def};
use super::wf;
use super::DerivedObligationCause;
use super::Obligation;
use super::ObligationCauseCode;
use super::Selection;
use super::SelectionResult;
use super::TraitQueryMode;
use super::{Normalized, ProjectionCacheKey};
use super::{ObligationCause, PredicateObligation, TraitObligation};
use super::{Overflow, SelectionError, Unimplemented};
use crate::infer::{InferCtxt, InferOk, TypeFreshener};
use crate::traits::error_reporting::InferCtxtExt;
use crate::traits::project::ProjectionCacheKeyExt;
use rustc_data_structures::fx::{FxHashMap, FxHashSet};
use rustc_data_structures::stack::ensure_sufficient_stack;
use rustc_errors::ErrorReported;
use rustc_hir as hir;
use rustc_hir::def_id::DefId;
use rustc_middle::dep_graph::{DepKind, DepNodeIndex};
use rustc_middle::mir::interpret::ErrorHandled;
use rustc_middle::ty::fast_reject;
use rustc_middle::ty::print::with_no_trimmed_paths;
use rustc_middle::ty::relate::TypeRelation;
use rustc_middle::ty::subst::{GenericArgKind, Subst, SubstsRef};
use rustc_middle::ty::{
self, ToPolyTraitRef, ToPredicate, Ty, TyCtxt, TypeFoldable, WithConstness,
};
use rustc_span::symbol::sym;
use std::cell::{Cell, RefCell};
use std::cmp;
use std::fmt::{self, Display};
use std::iter;
use std::rc::Rc;
pub use rustc_middle::traits::select::*;
mod candidate_assembly;
mod confirmation;
#[derive(Clone, Debug)]
pub enum IntercrateAmbiguityCause {
DownstreamCrate { trait_desc: String, self_desc: Option<String> },
UpstreamCrateUpdate { trait_desc: String, self_desc: Option<String> },
ReservationImpl { message: String },
}
impl IntercrateAmbiguityCause {
/// Emits notes when the overlap is caused by complex intercrate ambiguities.
/// See #23980 for details.
pub fn add_intercrate_ambiguity_hint(&self, err: &mut rustc_errors::DiagnosticBuilder<'_>) {
err.note(&self.intercrate_ambiguity_hint());
}
pub fn intercrate_ambiguity_hint(&self) -> String {
match self {
&IntercrateAmbiguityCause::DownstreamCrate { ref trait_desc, ref self_desc } => {
let self_desc = if let &Some(ref ty) = self_desc {
format!(" for type `{}`", ty)
} else {
String::new()
};
format!("downstream crates may implement trait `{}`{}", trait_desc, self_desc)
}
&IntercrateAmbiguityCause::UpstreamCrateUpdate { ref trait_desc, ref self_desc } => {
let self_desc = if let &Some(ref ty) = self_desc {
format!(" for type `{}`", ty)
} else {
String::new()
};
format!(
"upstream crates may add a new impl of trait `{}`{} \
in future versions",
trait_desc, self_desc
)
}
&IntercrateAmbiguityCause::ReservationImpl { ref message } => message.clone(),
}
}
}
pub struct SelectionContext<'cx, 'tcx> {
infcx: &'cx InferCtxt<'cx, '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 `true`, indicates that the evaluation should be conservative
/// and consider the possibility of types outside this crate.
/// This comes up primarily when resolving ambiguity. Imagine
/// there is some trait reference `$0: Bar` where `$0` is an
/// inference variable. If `intercrate` is true, then we can never
/// say for sure that this reference is not implemented, even if
/// there are *no impls at all for `Bar`*, because `$0` could be
/// bound to some type that in a downstream crate that implements
/// `Bar`. This is the suitable mode for coherence. Elsewhere,
/// though, we set this to false, because we are only interested
/// in types that the user could actually have written --- in
/// other words, we consider `$0: Bar` to be unimplemented if
/// there is no type that the user could *actually name* that
/// would satisfy it. This avoids crippling inference, basically.
intercrate: bool,
intercrate_ambiguity_causes: Option<Vec<IntercrateAmbiguityCause>>,
/// Controls whether or not to filter out negative impls when selecting.
/// This is used in librustdoc to distinguish between the lack of an impl
/// and a negative impl
allow_negative_impls: bool,
/// The mode that trait queries run in, which informs our error handling
/// policy. In essence, canonicalized queries need their errors propagated
/// rather than immediately reported because we do not have accurate spans.
query_mode: TraitQueryMode,
}
// A stack that walks back up the stack frame.
struct TraitObligationStack<'prev, 'tcx> {
obligation: &'prev TraitObligation<'tcx>,
/// The trait ref from `obligation` but "freshened" with the
/// selection-context's freshener. Used to check for recursion.
fresh_trait_ref: ty::PolyTraitRef<'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,
}
/// When does the builtin impl for `T: Trait` apply?
enum BuiltinImplConditions<'tcx> {
/// The impl is conditional on `T1, T2, ...: Trait`.
Where(ty::Binder<Vec<Ty<'tcx>>>),
/// There is no built-in impl. There may be some other
/// candidate (a where-clause or user-defined impl).
None,
/// It is unknown whether there is an impl.
Ambiguous,
}
impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
pub fn new(infcx: &'cx InferCtxt<'cx, 'tcx>) -> SelectionContext<'cx, 'tcx> {
SelectionContext {
infcx,
freshener: infcx.freshener(),
intercrate: false,
intercrate_ambiguity_causes: None,
allow_negative_impls: false,
query_mode: TraitQueryMode::Standard,
}
}
pub fn intercrate(infcx: &'cx InferCtxt<'cx, 'tcx>) -> SelectionContext<'cx, 'tcx> {
SelectionContext {
infcx,
freshener: infcx.freshener(),
intercrate: true,
intercrate_ambiguity_causes: None,
allow_negative_impls: false,
query_mode: TraitQueryMode::Standard,
}
}
pub fn with_negative(
infcx: &'cx InferCtxt<'cx, 'tcx>,
allow_negative_impls: bool,
) -> SelectionContext<'cx, 'tcx> {
debug!("with_negative({:?})", allow_negative_impls);
SelectionContext {
infcx,
freshener: infcx.freshener(),
intercrate: false,
intercrate_ambiguity_causes: None,
allow_negative_impls,
query_mode: TraitQueryMode::Standard,
}
}
pub fn with_query_mode(
infcx: &'cx InferCtxt<'cx, 'tcx>,
query_mode: TraitQueryMode,
) -> SelectionContext<'cx, 'tcx> {
debug!("with_query_mode({:?})", query_mode);
SelectionContext {
infcx,
freshener: infcx.freshener(),
intercrate: false,
intercrate_ambiguity_causes: None,
allow_negative_impls: false,
query_mode,
}
}
/// Enables tracking of intercrate ambiguity causes. These are
/// used in coherence to give improved diagnostics. We don't do
/// this until we detect a coherence error because it can lead to
/// false overflow results (#47139) and because it costs
/// computation time.
pub fn enable_tracking_intercrate_ambiguity_causes(&mut self) {
assert!(self.intercrate);
assert!(self.intercrate_ambiguity_causes.is_none());
self.intercrate_ambiguity_causes = Some(vec![]);
debug!("selcx: enable_tracking_intercrate_ambiguity_causes");
}
/// Gets the intercrate ambiguity causes collected since tracking
/// was enabled and disables tracking at the same time. If
/// tracking is not enabled, just returns an empty vector.
pub fn take_intercrate_ambiguity_causes(&mut self) -> Vec<IntercrateAmbiguityCause> {
assert!(self.intercrate);
self.intercrate_ambiguity_causes.take().unwrap_or(vec![])
}
pub fn infcx(&self) -> &'cx InferCtxt<'cx, 'tcx> {
self.infcx
}
pub fn tcx(&self) -> TyCtxt<'tcx> {
self.infcx.tcx
}
pub fn closure_typer(&self) -> &'cx InferCtxt<'cx, 'tcx> {
self.infcx
}
///////////////////////////////////////////////////////////////////////////
// Selection
//
// The selection phase tries to identify *how* an obligation will
// be resolved. For example, it will identify which impl or
// parameter bound is to be used. The process can be inconclusive
// if the self type in the obligation is not fully inferred. Selection
// can result in an error in one of two ways:
//
// 1. If no applicable impl or parameter bound can be found.
// 2. If the output type parameters in the obligation do not match
// those specified by the impl/bound. For example, if the obligation
// is `Vec<Foo>: Iterable<Bar>`, but the impl specifies
// `impl<T> Iterable<T> for Vec<T>`, than an error would result.
/// Attempts to satisfy the obligation. If successful, this will affect the surrounding
/// type environment by performing unification.
pub fn select(
&mut self,
obligation: &TraitObligation<'tcx>,
) -> SelectionResult<'tcx, Selection<'tcx>> {
debug!("select({:?})", obligation);
debug_assert!(!obligation.predicate.has_escaping_bound_vars());
let pec = &ProvisionalEvaluationCache::default();
let stack = self.push_stack(TraitObligationStackList::empty(pec), obligation);
let candidate = match self.candidate_from_obligation(&stack) {
Err(SelectionError::Overflow) => {
// In standard mode, overflow must have been caught and reported
// earlier.
assert!(self.query_mode == TraitQueryMode::Canonical);
return Err(SelectionError::Overflow);
}
Err(e) => {
return Err(e);
}
Ok(None) => {
return Ok(None);
}
Ok(Some(candidate)) => candidate,
};
match self.confirm_candidate(obligation, candidate) {
Err(SelectionError::Overflow) => {
assert!(self.query_mode == TraitQueryMode::Canonical);
Err(SelectionError::Overflow)
}
Err(e) => Err(e),
Ok(candidate) => Ok(Some(candidate)),
}
}
///////////////////////////////////////////////////////////////////////////
// EVALUATION
//
// Tests whether an obligation can be selected or whether an impl
// can be applied to particular types. It skips the "confirmation"
// step and hence completely ignores output type parameters.
//
// The result is "true" if the obligation *may* hold and "false" if
// we can be sure it does not.
/// Evaluates whether the obligation `obligation` can be satisfied (by any means).
pub fn predicate_may_hold_fatal(&mut self, obligation: &PredicateObligation<'tcx>) -> bool {
debug!("predicate_may_hold_fatal({:?})", obligation);
// This fatal query is a stopgap that should only be used in standard mode,
// where we do not expect overflow to be propagated.
assert!(self.query_mode == TraitQueryMode::Standard);
self.evaluate_root_obligation(obligation)
.expect("Overflow should be caught earlier in standard query mode")
.may_apply()
}
/// Evaluates whether the obligation `obligation` can be satisfied
/// and returns an `EvaluationResult`. This is meant for the
/// *initial* call.
pub fn evaluate_root_obligation(
&mut self,
obligation: &PredicateObligation<'tcx>,
) -> Result<EvaluationResult, OverflowError> {
self.evaluation_probe(|this| {
this.evaluate_predicate_recursively(
TraitObligationStackList::empty(&ProvisionalEvaluationCache::default()),
obligation.clone(),
)
})
}
fn evaluation_probe(
&mut self,
op: impl FnOnce(&mut Self) -> Result<EvaluationResult, OverflowError>,
) -> Result<EvaluationResult, OverflowError> {
self.infcx.probe(|snapshot| -> Result<EvaluationResult, OverflowError> {
let result = op(self)?;
match self.infcx.leak_check(true, snapshot) {
Ok(()) => {}
Err(_) => return Ok(EvaluatedToErr),
}
match self.infcx.region_constraints_added_in_snapshot(snapshot) {
None => Ok(result),
Some(_) => Ok(result.max(EvaluatedToOkModuloRegions)),
}
})
}
/// Evaluates the predicates in `predicates` recursively. Note that
/// this applies projections in the predicates, and therefore
/// is run within an inference probe.
fn evaluate_predicates_recursively<'o, I>(
&mut self,
stack: TraitObligationStackList<'o, 'tcx>,
predicates: I,
) -> Result<EvaluationResult, OverflowError>
where
I: IntoIterator<Item = PredicateObligation<'tcx>>,
{
let mut result = EvaluatedToOk;
for obligation in predicates {
let eval = self.evaluate_predicate_recursively(stack, obligation.clone())?;
debug!("evaluate_predicate_recursively({:?}) = {:?}", obligation, eval);
if let EvaluatedToErr = eval {
// fast-path - EvaluatedToErr is the top of the lattice,
// so we don't need to look on the other predicates.
return Ok(EvaluatedToErr);
} else {
result = cmp::max(result, eval);
}
}
Ok(result)
}
fn evaluate_predicate_recursively<'o>(
&mut self,
previous_stack: TraitObligationStackList<'o, 'tcx>,
obligation: PredicateObligation<'tcx>,
) -> Result<EvaluationResult, OverflowError> {
debug!(
"evaluate_predicate_recursively(previous_stack={:?}, obligation={:?})",
previous_stack.head(),
obligation
);
// `previous_stack` stores a `TraitObligation`, 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)?,
}
ensure_sufficient_stack(|| {
match obligation.predicate.skip_binders() {
ty::PredicateAtom::Trait(t, _) => {
let t = ty::Binder::bind(t);
debug_assert!(!t.has_escaping_bound_vars());
let obligation = obligation.with(t);
self.evaluate_trait_predicate_recursively(previous_stack, obligation)
}
ty::PredicateAtom::Subtype(p) => {
let p = ty::Binder::bind(p);
// Does this code ever run?
match self.infcx.subtype_predicate(&obligation.cause, obligation.param_env, p) {
Some(Ok(InferOk { mut obligations, .. })) => {
self.add_depth(obligations.iter_mut(), obligation.recursion_depth);
self.evaluate_predicates_recursively(
previous_stack,
obligations.into_iter(),
)
}
Some(Err(_)) => Ok(EvaluatedToErr),
None => Ok(EvaluatedToAmbig),
}
}
ty::PredicateAtom::WellFormed(arg) => match wf::obligations(
self.infcx,
obligation.param_env,
obligation.cause.body_id,
arg,
obligation.cause.span,
) {
Some(mut obligations) => {
self.add_depth(obligations.iter_mut(), obligation.recursion_depth);
self.evaluate_predicates_recursively(
previous_stack,
obligations.into_iter(),
)
}
None => Ok(EvaluatedToAmbig),
},
ty::PredicateAtom::TypeOutlives(..) | ty::PredicateAtom::RegionOutlives(..) => {
// We do not consider region relationships when evaluating trait matches.
Ok(EvaluatedToOkModuloRegions)
}
ty::PredicateAtom::ObjectSafe(trait_def_id) => {
if self.tcx().is_object_safe(trait_def_id) {
Ok(EvaluatedToOk)
} else {
Ok(EvaluatedToErr)
}
}
ty::PredicateAtom::Projection(data) => {
let data = ty::Binder::bind(data);
let project_obligation = obligation.with(data);
match project::poly_project_and_unify_type(self, &project_obligation) {
Ok(Ok(Some(mut subobligations))) => {
self.add_depth(subobligations.iter_mut(), obligation.recursion_depth);
let result = self.evaluate_predicates_recursively(
previous_stack,
subobligations.into_iter(),
);
if let Some(key) =
ProjectionCacheKey::from_poly_projection_predicate(self, data)
{
self.infcx.inner.borrow_mut().projection_cache().complete(key);
}
result
}
Ok(Ok(None)) => Ok(EvaluatedToAmbig),
// EvaluatedToRecur might also be acceptable here, but use
// Unknown for now because it means that we won't dismiss a
// selection candidate solely because it has a projection
// cycle. This is closest to the previous behavior of
// immediately erroring.
Ok(Err(project::InProgress)) => Ok(EvaluatedToUnknown),
Err(_) => Ok(EvaluatedToErr),
}
}
ty::PredicateAtom::ClosureKind(_, closure_substs, kind) => {
match self.infcx.closure_kind(closure_substs) {
Some(closure_kind) => {
if closure_kind.extends(kind) {
Ok(EvaluatedToOk)
} else {
Ok(EvaluatedToErr)
}
}
None => Ok(EvaluatedToAmbig),
}
}
ty::PredicateAtom::ConstEvaluatable(def_id, substs) => {
match const_evaluatable::is_const_evaluatable(
self.infcx,
def_id,
substs,
obligation.param_env,
obligation.cause.span,
) {
Ok(()) => Ok(EvaluatedToOk),
Err(ErrorHandled::TooGeneric) => Ok(EvaluatedToAmbig),
Err(_) => Ok(EvaluatedToErr),
}
}
ty::PredicateAtom::ConstEquate(c1, c2) => {
debug!(
"evaluate_predicate_recursively: equating consts c1={:?} c2={:?}",
c1, c2
);
let evaluate = |c: &'tcx ty::Const<'tcx>| {
if let ty::ConstKind::Unevaluated(def, substs, promoted) = c.val {
self.infcx
.const_eval_resolve(
obligation.param_env,
def,
substs,
promoted,
Some(obligation.cause.span),
)
.map(|val| ty::Const::from_value(self.tcx(), val, c.ty))
} else {
Ok(c)
}
};
match (evaluate(c1), evaluate(c2)) {
(Ok(c1), Ok(c2)) => {
match self
.infcx()
.at(&obligation.cause, obligation.param_env)
.eq(c1, c2)
{
Ok(_) => Ok(EvaluatedToOk),
Err(_) => Ok(EvaluatedToErr),
}
}
(Err(ErrorHandled::Reported(ErrorReported)), _)
| (_, Err(ErrorHandled::Reported(ErrorReported))) => Ok(EvaluatedToErr),
(Err(ErrorHandled::Linted), _) | (_, Err(ErrorHandled::Linted)) => {
span_bug!(
obligation.cause.span(self.tcx()),
"ConstEquate: const_eval_resolve returned an unexpected error"
)
}
(Err(ErrorHandled::TooGeneric), _) | (_, Err(ErrorHandled::TooGeneric)) => {
Ok(EvaluatedToAmbig)
}
}
}
ty::PredicateAtom::TypeWellFormedFromEnv(..) => {
bug!("TypeWellFormedFromEnv is only used for chalk")
}
}
})
}
fn evaluate_trait_predicate_recursively<'o>(
&mut self,
previous_stack: TraitObligationStackList<'o, 'tcx>,
mut obligation: TraitObligation<'tcx>,
) -> Result<EvaluationResult, OverflowError> {
debug!("evaluate_trait_predicate_recursively({:?})", obligation);
if !self.intercrate
&& obligation.is_global()
&& obligation.param_env.caller_bounds().iter().all(|bound| bound.needs_subst())
{
// If a param env has no global bounds, global obligations do not
// depend on its particular value in order to work, so we can clear
// out the param env and get better caching.
debug!("evaluate_trait_predicate_recursively({:?}) - in global", obligation);
obligation.param_env = obligation.param_env.without_caller_bounds();
}
let stack = self.push_stack(previous_stack, &obligation);
let fresh_trait_ref = stack.fresh_trait_ref;
if let Some(result) = self.check_evaluation_cache(obligation.param_env, fresh_trait_ref) {
debug!("CACHE HIT: EVAL({:?})={:?}", fresh_trait_ref, result);
return Ok(result);
}
if let Some(result) = stack.cache().get_provisional(fresh_trait_ref) {
debug!("PROVISIONAL CACHE HIT: EVAL({:?})={:?}", fresh_trait_ref, result);
stack.update_reached_depth(stack.cache().current_reached_depth());
return Ok(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| this.evaluate_stack(&stack));
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: EVAL({:?})={:?}", fresh_trait_ref, result);
self.insert_evaluation_cache(obligation.param_env, fresh_trait_ref, dep_node, result);
stack.cache().on_completion(stack.depth, |fresh_trait_ref, provisional_result| {
self.insert_evaluation_cache(
obligation.param_env,
fresh_trait_ref,
dep_node,
provisional_result.max(result),
);
});
} else {
debug!("PROVISIONAL: {:?}={:?}", fresh_trait_ref, result);
debug!(
"evaluate_trait_predicate_recursively: caching provisionally because {:?} \
is a cycle participant (at depth {}, reached depth {})",
fresh_trait_ref, stack.depth, reached_depth,
);
stack.cache().insert_provisional(stack.dfn, reached_depth, fresh_trait_ref, 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_ref == prev.fresh_trait_ref
})
.map(|stack| stack.depth)
{
debug!(
"evaluate_stack({:?}) --> recursive at depth {}",
stack.fresh_trait_ref, 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.without_const().to_predicate(tcx));
if self.coinductive_match(cycle) {
debug!("evaluate_stack({:?}) --> recursive, coinductive", stack.fresh_trait_ref);
Some(EvaluatedToOk)
} else {
debug!("evaluate_stack({:?}) --> recursive, inductive", stack.fresh_trait_ref);
Some(EvaluatedToRecur)
}
} else {
None
}
}
fn evaluate_stack<'o>(
&mut self,
stack: &TraitObligationStack<'o, 'tcx>,
) -> Result<EvaluationResult, OverflowError> {
// 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_ref.skip_binder().substs.types().any(|ty| ty.is_fresh());
// This check was an imperfect workaround for a bug in the old
// intercrate mode; it should be removed when that goes away.
if unbound_input_types && self.intercrate {
debug!(
"evaluate_stack({:?}) --> unbound argument, intercrate --> ambiguous",
stack.fresh_trait_ref
);
// Heuristics: show the diagnostics when there are no candidates in crate.
if self.intercrate_ambiguity_causes.is_some() {
debug!("evaluate_stack: intercrate_ambiguity_causes is some");
if let Ok(candidate_set) = self.assemble_candidates(stack) {
if !candidate_set.ambiguous && candidate_set.vec.is_empty() {
let trait_ref = stack.obligation.predicate.skip_binder().trait_ref;
let self_ty = trait_ref.self_ty();
let cause =
with_no_trimmed_paths(|| IntercrateAmbiguityCause::DownstreamCrate {
trait_desc: trait_ref.print_only_trait_path().to_string(),
self_desc: if self_ty.has_concrete_skeleton() {
Some(self_ty.to_string())
} else {
None
},
});
debug!("evaluate_stack: pushing cause = {:?}", cause);
self.intercrate_ambiguity_causes.as_mut().unwrap().push(cause);
}
}
}
return Ok(EvaluatedToAmbig);
}
if unbound_input_types
&& stack.iter().skip(1).any(|prev| {
stack.obligation.param_env == prev.obligation.param_env
&& self.match_fresh_trait_refs(
stack.fresh_trait_ref,
prev.fresh_trait_ref,
prev.obligation.param_env,
)
})
{
debug!(
"evaluate_stack({:?}) --> unbound argument, recursive --> giving up",
stack.fresh_trait_ref
);
return Ok(EvaluatedToUnknown);
}
match self.candidate_from_obligation(stack) {
Ok(Some(c)) => self.evaluate_candidate(stack, &c),
Ok(None) => Ok(EvaluatedToAmbig),
Err(Overflow) => Err(OverflowError),
Err(..) => Ok(EvaluatedToErr),
}
}
/// For defaulted traits, we use a co-inductive strategy to solve, so
/// that recursion is ok. This routine returns `true` if the top of the
/// stack (`cycle[0]`):
///
/// - is a defaulted trait,
/// - it also appears in the backtrace at some position `X`,
/// - all the predicates at positions `X..` between `X` and the top are
/// also defaulted traits.
pub fn coinductive_match<I>(&mut self, cycle: I) -> bool
where
I: Iterator<Item = ty::Predicate<'tcx>>,
{
let mut cycle = cycle;
cycle.all(|predicate| self.coinductive_predicate(predicate))
}
fn coinductive_predicate(&self, predicate: ty::Predicate<'tcx>) -> bool {
let result = match predicate.skip_binders() {
ty::PredicateAtom::Trait(ref data, _) => self.tcx().trait_is_auto(data.def_id()),
_ => false,
};
debug!("coinductive_predicate({:?}) = {:?}", predicate, result);
result
}
/// 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.
fn evaluate_candidate<'o>(
&mut self,
stack: &TraitObligationStack<'o, 'tcx>,
candidate: &SelectionCandidate<'tcx>,
) -> Result<EvaluationResult, OverflowError> {
debug!(
"evaluate_candidate: depth={} candidate={:?}",
stack.obligation.recursion_depth, candidate
);
let result = self.evaluation_probe(|this| {
let candidate = (*candidate).clone();
match this.confirm_candidate(stack.obligation, candidate) {
Ok(selection) => this.evaluate_predicates_recursively(
stack.list(),
selection.nested_obligations().into_iter(),
),
Err(..) => Ok(EvaluatedToErr),
}
})?;
debug!(
"evaluate_candidate: depth={} result={:?}",
stack.obligation.recursion_depth, result
);
Ok(result)
}
fn check_evaluation_cache(
&self,
param_env: ty::ParamEnv<'tcx>,
trait_ref: ty::PolyTraitRef<'tcx>,
) -> Option<EvaluationResult> {
let tcx = self.tcx();
if self.can_use_global_caches(param_env) {
if let Some(res) = tcx.evaluation_cache.get(&param_env.and(trait_ref), tcx) {
return Some(res);
}
}
self.infcx.evaluation_cache.get(&param_env.and(trait_ref), tcx)
}
fn insert_evaluation_cache(
&mut self,
param_env: ty::ParamEnv<'tcx>,
trait_ref: ty::PolyTraitRef<'tcx>,
dep_node: DepNodeIndex,
result: EvaluationResult,
) {
// Avoid caching results that depend on more than just the trait-ref
// - the stack can create recursion.
if result.is_stack_dependent() {
return;
}
if self.can_use_global_caches(param_env) {
if !trait_ref.needs_infer() {
debug!(
"insert_evaluation_cache(trait_ref={:?}, candidate={:?}) global",
trait_ref, result,
);
// This may overwrite the cache with the same value
// FIXME: Due to #50507 this overwrites the different values
// This should be changed to use HashMapExt::insert_same
// when that is fixed
self.tcx().evaluation_cache.insert(param_env.and(trait_ref), dep_node, result);
return;
}
}
debug!("insert_evaluation_cache(trait_ref={:?}, candidate={:?})", trait_ref, result,);
self.infcx.evaluation_cache.insert(param_env.and(trait_ref), dep_node, result);
}
/// For various reasons, it's possible for a subobligation
/// to have a *lower* recursion_depth than the obligation used to create it.
/// Projection sub-obligations may be returned from the projection cache,
/// which results in obligations with an 'old' `recursion_depth`.
/// Additionally, methods like `wf::obligations` and
/// `InferCtxt.subtype_predicate` produce subobligations without
/// taking in a 'parent' depth, causing the generated subobligations
/// to have a `recursion_depth` of `0`.
///
/// To ensure that obligation_depth never decreasees, we force all subobligations
/// to have at least the depth of the original obligation.
fn add_depth<T: 'cx, I: Iterator<Item = &'cx mut Obligation<'tcx, T>>>(
&self,
it: I,
min_depth: usize,
) {
it.for_each(|o| o.recursion_depth = cmp::max(min_depth, o.recursion_depth) + 1);
}
/// 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.
fn check_recursion_limit<T: Display + TypeFoldable<'tcx>, V: Display + TypeFoldable<'tcx>>(
&self,
obligation: &Obligation<'tcx, T>,
error_obligation: &Obligation<'tcx, V>,
) -> Result<(), OverflowError> {
if !self.infcx.tcx.sess.recursion_limit().value_within_limit(obligation.recursion_depth) {
match self.query_mode {
TraitQueryMode::Standard => {
self.infcx().report_overflow_error(error_obligation, true);
}
TraitQueryMode::Canonical => {
return Err(OverflowError);
}
}
}
Ok(())
}
fn in_task<OP, R>(&mut self, op: OP) -> (R, DepNodeIndex)
where
OP: FnOnce(&mut Self) -> R,
{
let (result, dep_node) =
self.tcx().dep_graph.with_anon_task(DepKind::TraitSelect, || op(self));
self.tcx().dep_graph.read_index(dep_node);
(result, dep_node)
}
// Treat negative impls as unimplemented, and reservation impls as ambiguity.
fn filter_negative_and_reservation_impls(
&mut self,
candidate: SelectionCandidate<'tcx>,
) -> SelectionResult<'tcx, SelectionCandidate<'tcx>> {
if let ImplCandidate(def_id) = candidate {
let tcx = self.tcx();
match tcx.impl_polarity(def_id) {
ty::ImplPolarity::Negative if !self.allow_negative_impls => {
return Err(Unimplemented);
}
ty::ImplPolarity::Reservation => {
if let Some(intercrate_ambiguity_clauses) =
&mut self.intercrate_ambiguity_causes
{
let attrs = tcx.get_attrs(def_id);
let attr = tcx.sess.find_by_name(&attrs, sym::rustc_reservation_impl);
let value = attr.and_then(|a| a.value_str());
if let Some(value) = value {
debug!(
"filter_negative_and_reservation_impls: \
reservation impl ambiguity on {:?}",
def_id
);
intercrate_ambiguity_clauses.push(
IntercrateAmbiguityCause::ReservationImpl {
message: value.to_string(),
},
);
}
}
return Ok(None);
}
_ => {}
};
}
Ok(Some(candidate))
}
fn candidate_from_obligation_no_cache<'o>(
&mut self,
stack: &TraitObligationStack<'o, 'tcx>,
) -> SelectionResult<'tcx, SelectionCandidate<'tcx>> {
if let Some(conflict) = self.is_knowable(stack) {
debug!("coherence stage: not knowable");
if self.intercrate_ambiguity_causes.is_some() {
debug!("evaluate_stack: intercrate_ambiguity_causes is some");
// Heuristics: show the diagnostics when there are no candidates in crate.
if let Ok(candidate_set) = self.assemble_candidates(stack) {
let mut no_candidates_apply = true;
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 = stack.obligation.predicate.skip_binder().trait_ref;
let self_ty = trait_ref.self_ty();
let (trait_desc, self_desc) = with_no_trimmed_paths(|| {
let trait_desc = trait_ref.print_only_trait_path().to_string();
let self_desc = if self_ty.has_concrete_skeleton() {
Some(self_ty.to_string())
} else {
None
};
(trait_desc, self_desc)
});
let cause = if let Conflict::Upstream = conflict {
IntercrateAmbiguityCause::UpstreamCrateUpdate { trait_desc, self_desc }
} else {
IntercrateAmbiguityCause::DownstreamCrate { trait_desc, self_desc }
};
debug!("evaluate_stack: pushing cause = {:?}", cause);
self.intercrate_ambiguity_causes.as_mut().unwrap().push(cause);
}
}
}
return Ok(None);
}
let candidate_set = self.assemble_candidates(stack)?;
if candidate_set.ambiguous {
debug!("candidate set contains ambig");
return Ok(None);
}
let mut candidates = candidate_set.vec;
debug!("assembled {} candidates for {:?}: {:?}", candidates.len(), stack, candidates);
// At this point, we know that each of the entries in the
// candidate set is *individually* applicable. Now we have to
// figure out if they contain mutual incompatibilities. This
// frequently arises if we have an unconstrained input type --
// for example, we are looking for `$0: Eq` where `$0` is some
// unconstrained type variable. In that case, we'll get a
// candidate which assumes $0 == int, one that assumes `$0 ==
// usize`, etc. This spells an ambiguity.
// If there is more than one candidate, first winnow them down
// by considering extra conditions (nested obligations and so
// forth). We don't winnow if there is exactly one
// candidate. This is a relatively minor distinction but it
// can lead to better inference and error-reporting. An
// example would be if there was an impl:
//
// impl<T:Clone> Vec<T> { fn push_clone(...) { ... } }
//
// and we were to see some code `foo.push_clone()` where `boo`
// is a `Vec<Bar>` and `Bar` does not implement `Clone`. If
// we were to winnow, we'd wind up with zero candidates.
// Instead, we select the right impl now but report "`Bar` does
// not implement `Clone`".
if candidates.len() == 1 {
return self.filter_negative_and_reservation_impls(candidates.pop().unwrap());
}
// Winnow, but record the exact outcome of evaluation, which
// is needed for specialization. Propagate overflow if it occurs.
let mut candidates = candidates
.into_iter()
.map(|c| match self.evaluate_candidate(stack, &c) {
Ok(eval) if eval.may_apply() => {
Ok(Some(EvaluatedCandidate { candidate: c, evaluation: eval }))
}
Ok(_) => Ok(None),
Err(OverflowError) => Err(Overflow),
})
.flat_map(Result::transpose)
.collect::<Result<Vec<_>, _>>()?;
debug!("winnowed to {} candidates for {:?}: {:?}", candidates.len(), stack, candidates);
let needs_infer = stack.obligation.predicate.needs_infer();
// If there are STILL multiple candidates, we can further
// reduce the list by dropping duplicates -- including
// resolving specializations.
if candidates.len() > 1 {
let mut i = 0;
while i < candidates.len() {
let is_dup = (0..candidates.len()).filter(|&j| i != j).any(|j| {
self.candidate_should_be_dropped_in_favor_of(
&candidates[i],
&candidates[j],
needs_infer,
)
});
if is_dup {
debug!("Dropping candidate #{}/{}: {:?}", i, candidates.len(), candidates[i]);
candidates.swap_remove(i);
} else {
debug!("Retaining candidate #{}/{}: {:?}", i, candidates.len(), candidates[i]);
i += 1;
// If there are *STILL* multiple candidates, give up
// and report ambiguity.
if i > 1 {
debug!("multiple matches, ambig");
return Ok(None);
}
}
}
}
// If there are *NO* candidates, then there are no impls --
// that we know of, anyway. Note that in the case where there
// are unbound type variables within the obligation, it might
// be the case that you could still satisfy the obligation
// from another crate by instantiating the type variables with
// a type from another crate that does have an impl. This case
// is checked for in `evaluate_stack` (and hence users
// who might care about this case, like coherence, should use
// that function).
if candidates.is_empty() {
// 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.references_error() {
debug!("no results for error type, treating as ambiguous");
return Ok(None);
}
return Err(Unimplemented);
}
// Just one candidate left.
self.filter_negative_and_reservation_impls(candidates.pop().unwrap().candidate)
}
fn is_knowable<'o>(&mut self, stack: &TraitObligationStack<'o, 'tcx>) -> Option<Conflict> {
debug!("is_knowable(intercrate={:?})", self.intercrate);
if !self.intercrate {
return None;
}
let obligation = &stack.obligation;
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.tcx(), trait_ref)
}
/// Returns `true` if the global caches can be used.
/// Do note that if the type itself is not in the
/// global tcx, the local caches will be used.
fn can_use_global_caches(&self, param_env: ty::ParamEnv<'tcx>) -> bool {
// If there are any 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.needs_infer() {
return false;
}
// Avoid using the master cache during coherence and just rely
// on the local cache. This effectively disables caching
// during coherence. It is really just a simplification to
// avoid us having to fear that coherence results "pollute"
// the master cache. Since coherence executes pretty quickly,
// it's not worth going to more trouble to increase the
// hit-rate, I don't think.
if self.intercrate {
return false;
}
// Otherwise, we can use the global cache.
true
}
fn check_candidate_cache(
&mut self,
param_env: ty::ParamEnv<'tcx>,
cache_fresh_trait_pred: ty::PolyTraitPredicate<'tcx>,
) -> Option<SelectionResult<'tcx, SelectionCandidate<'tcx>>> {
let tcx = self.tcx();
let trait_ref = &cache_fresh_trait_pred.skip_binder().trait_ref;
if self.can_use_global_caches(param_env) {
if let Some(res) = tcx.selection_cache.get(&param_env.and(*trait_ref), tcx) {
return Some(res);
}
}
self.infcx.selection_cache.get(&param_env.and(*trait_ref), 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.needs_infer(),
_ => true,
}
}
fn insert_candidate_cache(
&mut self,
param_env: ty::ParamEnv<'tcx>,
cache_fresh_trait_pred: ty::PolyTraitPredicate<'tcx>,
dep_node: DepNodeIndex,
candidate: SelectionResult<'tcx, SelectionCandidate<'tcx>>,
) {
let tcx = self.tcx();
let trait_ref = cache_fresh_trait_pred.skip_binder().trait_ref;
if !self.can_cache_candidate(&candidate) {
debug!(
"insert_candidate_cache(trait_ref={:?}, candidate={:?} -\
candidate is not cacheable",
trait_ref, candidate
);
return;
}
if self.can_use_global_caches(param_env) {
if let Err(Overflow) = candidate {
// Don't cache overflow globally; we only produce this in certain modes.
} else if !trait_ref.needs_infer() {
if !candidate.needs_infer() {
debug!(
"insert_candidate_cache(trait_ref={:?}, candidate={:?}) global",
trait_ref, candidate,
);
// This may overwrite the cache with the same value.
tcx.selection_cache.insert(param_env.and(trait_ref), dep_node, candidate);
return;
}
}
}
debug!(
"insert_candidate_cache(trait_ref={:?}, candidate={:?}) local",
trait_ref, candidate,
);
self.infcx.selection_cache.insert(param_env.and(trait_ref), dep_node, candidate);
}
fn match_projection_obligation_against_definition_bounds(
&mut self,
obligation: &TraitObligation<'tcx>,
) -> bool {
let poly_trait_predicate = self.infcx().resolve_vars_if_possible(&obligation.predicate);
let (placeholder_trait_predicate, _) =
self.infcx().replace_bound_vars_with_placeholders(&poly_trait_predicate);
debug!(
"match_projection_obligation_against_definition_bounds: \
placeholder_trait_predicate={:?}",
placeholder_trait_predicate,
);
let tcx = self.infcx.tcx;
let predicates = match *placeholder_trait_predicate.trait_ref.self_ty().kind() {
ty::Projection(ref data) => {
tcx.projection_predicates(data.item_def_id).subst(tcx, data.substs)
}
ty::Opaque(def_id, substs) => tcx.projection_predicates(def_id).subst(tcx, substs),
_ => {
span_bug!(
obligation.cause.span,
"match_projection_obligation_against_definition_bounds() called \
but self-ty is not a projection: {:?}",
placeholder_trait_predicate.trait_ref.self_ty()
);
}
};
let matching_bound = predicates.iter().find_map(|bound| {
if let ty::PredicateAtom::Trait(pred, _) = bound.skip_binders() {
let bound = ty::Binder::bind(pred.trait_ref);
if self.infcx.probe(|_| {
self.match_projection(obligation, bound, placeholder_trait_predicate.trait_ref)
}) {
return Some(bound);
}
}
None
});
debug!(
"match_projection_obligation_against_definition_bounds: \
matching_bound={:?}",
matching_bound
);
match matching_bound {
None => false,
Some(bound) => {
// Repeat the successful match, if any, this time outside of a probe.
let result =
self.match_projection(obligation, bound, placeholder_trait_predicate.trait_ref);
assert!(result);
true
}
}
}
fn match_projection(
&mut self,
obligation: &TraitObligation<'tcx>,
trait_bound: ty::PolyTraitRef<'tcx>,
placeholder_trait_ref: ty::TraitRef<'tcx>,
) -> bool {
debug_assert!(!placeholder_trait_ref.has_escaping_bound_vars());
self.infcx
.at(&obligation.cause, obligation.param_env)
.sup(ty::Binder::dummy(placeholder_trait_ref), trait_bound)
.is_ok()
}
fn evaluate_where_clause<'o>(
&mut self,
stack: &TraitObligationStack<'o, 'tcx>,
where_clause_trait_ref: ty::PolyTraitRef<'tcx>,
) -> Result<EvaluationResult, OverflowError> {
self.evaluation_probe(|this| {
match this.match_where_clause_trait_ref(stack.obligation, where_clause_trait_ref) {
Ok(obligations) => {
this.evaluate_predicates_recursively(stack.list(), obligations.into_iter())
}
Err(()) => Ok(EvaluatedToErr),
}
})
}
///////////////////////////////////////////////////////////////////////////
// WINNOW
//
// Winnowing is the process of attempting to resolve ambiguity by
// probing further. During the winnowing process, we unify all
// type variables and then we also attempt to evaluate recursive
// bounds to see if they are satisfied.
/// Returns `true` if `victim` should be dropped in favor of
/// `other`. Generally speaking we will drop duplicate
/// candidates and prefer where-clause candidates.
///
/// See the comment for "SelectionCandidate" for more details.
fn candidate_should_be_dropped_in_favor_of(
&mut self,
victim: &EvaluatedCandidate<'tcx>,
other: &EvaluatedCandidate<'tcx>,
needs_infer: bool,
) -> bool {
if victim.candidate == other.candidate {
return true;
}
// Check if a bound would previously have been removed when normalizing
// the param_env so that it can be given the lowest priority. See
// #50825 for the motivation for this.
let is_global =
|cand: &ty::PolyTraitRef<'_>| cand.is_global() && !cand.has_late_bound_regions();
// (*) Prefer `BuiltinCandidate { has_nested: false }` and `DiscriminantKindCandidate`
// to anything else.
//
// This is a fix for #53123 and prevents winnowing from accidentally extending the
// lifetime of a variable.
match other.candidate {
// (*)
BuiltinCandidate { has_nested: false } | DiscriminantKindCandidate => true,
ParamCandidate(ref cand) => match victim.candidate {
AutoImplCandidate(..) => {
bug!(
"default implementations shouldn't be recorded \
when there are other valid candidates"
);
}
// (*)
BuiltinCandidate { has_nested: false } | DiscriminantKindCandidate => false,
ImplCandidate(..)
| ClosureCandidate
| GeneratorCandidate
| FnPointerCandidate
| BuiltinObjectCandidate
| BuiltinUnsizeCandidate
| BuiltinCandidate { .. }
| TraitAliasCandidate(..) => {
// Global bounds from the where clause should be ignored
// here (see issue #50825). Otherwise, we have a where
// clause so don't go around looking for impls.
!is_global(cand)
}
ObjectCandidate | ProjectionCandidate => {
// Arbitrarily give param candidates priority
// over projection and object candidates.
!is_global(cand)
}
ParamCandidate(..) => false,
},
ObjectCandidate | ProjectionCandidate => match victim.candidate {
AutoImplCandidate(..) => {
bug!(
"default implementations shouldn't be recorded \
when there are other valid candidates"
);
}
// (*)
BuiltinCandidate { has_nested: false } | DiscriminantKindCandidate => false,
ImplCandidate(..)
| ClosureCandidate
| GeneratorCandidate
| FnPointerCandidate
| BuiltinObjectCandidate
| BuiltinUnsizeCandidate
| BuiltinCandidate { .. }
| TraitAliasCandidate(..) => true,
ObjectCandidate | ProjectionCandidate => {
// Arbitrarily give param candidates priority
// over projection and object candidates.
true
}
ParamCandidate(ref cand) => is_global(cand),
},
ImplCandidate(other_def) => {
// See if we can toss out `victim` based on specialization.
// This requires us to know *for sure* that the `other` impl applies
// i.e., `EvaluatedToOk`.
if other.evaluation.must_apply_modulo_regions() {
match victim.candidate {
ImplCandidate(victim_def) => {
let tcx = self.tcx();
if tcx.specializes((other_def, victim_def)) {
return true;
}
return match tcx.impls_are_allowed_to_overlap(other_def, victim_def) {
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
// constrainting 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 constrin)
// 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.
!needs_infer
}
Some(_) => true,
None => false,
};
}
ParamCandidate(ref cand) => {
// Prefer the impl to a global where clause candidate.
return is_global(cand);
}
_ => (),
}
}
false
}
ClosureCandidate
| GeneratorCandidate
| FnPointerCandidate
| BuiltinObjectCandidate
| BuiltinUnsizeCandidate
| BuiltinCandidate { has_nested: true } => {
match victim.candidate {
ParamCandidate(ref cand) => {
// Prefer these to a global where-clause bound
// (see issue #50825).
is_global(cand) && other.evaluation.must_apply_modulo_regions()
}
_ => false,
}
}
_ => false,
}
}
fn sized_conditions(
&mut self,
obligation: &TraitObligation<'tcx>,
) -> BuiltinImplConditions<'tcx> {
use self::BuiltinImplConditions::{Ambiguous, None, Where};
// NOTE: binder moved to (*)
let self_ty = self.infcx.shallow_resolve(obligation.predicate.skip_binder().self_ty());
match self_ty.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::Generator(..)
| ty::GeneratorWitness(..)
| ty::Array(..)
| ty::Closure(..)
| ty::Never
| ty::Error(_) => {
// safe for everything
Where(ty::Binder::dummy(Vec::new()))
}
ty::Str | ty::Slice(_) | ty::Dynamic(..) | ty::Foreign(..) => None,
ty::Tuple(tys) => {
Where(ty::Binder::bind(tys.last().into_iter().map(|k| k.expect_ty()).collect()))
}
ty::Adt(def, substs) => {
let sized_crit = def.sized_constraint(self.tcx());
// (*) binder moved here
Where(ty::Binder::bind(
sized_crit.iter().map(|ty| ty.subst(self.tcx(), substs)).collect(),
))
}
ty::Projection(_) | ty::Param(_) | ty::Opaque(..) => None,
ty::Infer(ty::TyVar(_)) => Ambiguous,
ty::Placeholder(..)
| ty::Bound(..)
| ty::Infer(ty::FreshTy(_) | ty::FreshIntTy(_) | ty::FreshFloatTy(_)) => {
bug!("asked to assemble builtin bounds of unexpected type: {:?}", self_ty);
}
}
}
fn copy_clone_conditions(
&mut self,
obligation: &TraitObligation<'tcx>,
) -> BuiltinImplConditions<'tcx> {
// NOTE: binder moved to (*)
let self_ty = self.infcx.shallow_resolve(obligation.predicate.skip_binder().self_ty());
use self::BuiltinImplConditions::{Ambiguous, None, Where};
match self_ty.kind() {
ty::Infer(ty::IntVar(_))
| ty::Infer(ty::FloatVar(_))
| ty::FnDef(..)
| ty::FnPtr(_)
| ty::Error(_) => Where(ty::Binder::dummy(Vec::new())),
ty::Uint(_)
| ty::Int(_)
| ty::Bool
| ty::Float(_)
| ty::Char
| ty::RawPtr(..)
| ty::Never
| ty::Ref(_, _, hir::Mutability::Not) => {
// Implementations provided in libcore
None
}
ty::Dynamic(..)
| ty::Str
| ty::Slice(..)
| ty::Generator(..)
| ty::GeneratorWitness(..)
| ty::Foreign(..)
| ty::Ref(_, _, hir::Mutability::Mut) => None,
ty::Array(element_ty, _) => {
// (*) binder moved here
Where(ty::Binder::bind(vec![element_ty]))
}
ty::Tuple(tys) => {
// (*) binder moved here
Where(ty::Binder::bind(tys.iter().map(|k| k.expect_ty()).collect()))
}
ty::Closure(_, substs) => {
// (*) binder moved here
Where(ty::Binder::bind(substs.as_closure().upvar_tys().collect()))
}
ty::Adt(..) | ty::Projection(..) | ty::Param(..) | ty::Opaque(..) => {
// Fallback to whatever user-defined impls exist in this case.
None
}
ty::Infer(ty::TyVar(_)) => {
// Unbound type variable. Might or might not have
// applicable impls and so forth, depending on what
// those type variables wind up being bound to.
Ambiguous
}
ty::Placeholder(..)
| ty::Bound(..)
| ty::Infer(ty::FreshTy(_) | ty::FreshIntTy(_) | ty::FreshFloatTy(_)) => {
bug!("asked to assemble builtin bounds of unexpected type: {:?}", self_ty);
}
}
}
/// For default impls, we need to break apart a type into its
/// "constituent types" -- meaning, the types that it contains.
///
/// Here are some (simple) examples:
///
/// ```
/// (i32, u32) -> [i32, u32]
/// Foo where struct Foo { x: i32, y: u32 } -> [i32, u32]
/// Bar<i32> where struct Bar<T> { x: T, y: u32 } -> [i32, u32]
/// Zed<i32> where enum Zed { A(T), B(u32) } -> [i32, u32]
/// ```
fn constituent_types_for_ty(&self, t: Ty<'tcx>) -> Vec<Ty<'tcx>> {
match *t.kind() {
ty::Uint(_)
| ty::Int(_)
| ty::Bool
| ty::Float(_)
| ty::FnDef(..)
| ty::FnPtr(_)
| ty::Str
| ty::Error(_)
| ty::Infer(ty::IntVar(_) | ty::FloatVar(_))
| ty::Never
| ty::Char => Vec::new(),
ty::Placeholder(..)
| ty::Dynamic(..)
| ty::Param(..)
| ty::Foreign(..)
| ty::Projection(..)
| ty::Bound(..)
| ty::Infer(ty::TyVar(_) | ty::FreshTy(_) | ty::FreshIntTy(_) | ty::FreshFloatTy(_)) => {
bug!("asked to assemble constituent types of unexpected type: {:?}", t);
}
ty::RawPtr(ty::TypeAndMut { ty: element_ty, .. }) | ty::Ref(_, element_ty, _) => {
vec![element_ty]
}
ty::Array(element_ty, _) | ty::Slice(element_ty) => vec![element_ty],
ty::Tuple(ref tys) => {
// (T1, ..., Tn) -- meets any bound that all of T1...Tn meet
tys.iter().map(|k| k.expect_ty()).collect()
}
ty::Closure(_, ref substs) => substs.as_closure().upvar_tys().collect(),
ty::Generator(_, ref substs, _) => {
let witness = substs.as_generator().witness();
substs.as_generator().upvar_tys().chain(iter::once(witness)).collect()
}
ty::GeneratorWitness(types) => {
// This is sound because no regions in the witness can refer to
// the binder outside the witness. So we'll effectivly reuse
// the implicit binder around the witness.
types.skip_binder().to_vec()
}
// For `PhantomData<T>`, we pass `T`.
ty::Adt(def, substs) if def.is_phantom_data() => substs.types().collect(),
ty::Adt(def, substs) => def.all_fields().map(|f| f.ty(self.tcx(), substs)).collect(),
ty::Opaque(def_id, substs) => {
// We can resolve the `impl Trait` to its concrete type,
// which enforces a DAG between the functions requiring
// the auto trait bounds in question.
vec![self.tcx().type_of(def_id).subst(self.tcx(), substs)]
}
}
}
fn collect_predicates_for_types(
&mut self,
param_env: ty::ParamEnv<'tcx>,
cause: ObligationCause<'tcx>,
recursion_depth: usize,
trait_def_id: DefId,
types: ty::Binder<Vec<Ty<'tcx>>>,
) -> Vec<PredicateObligation<'tcx>> {
// Because the types were potentially derived from
// higher-ranked obligations they may reference late-bound
// regions. For example, `for<'a> Foo<&'a 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
.skip_binder() // binder moved -\
.iter()
.flat_map(|ty| {
let ty: ty::Binder<Ty<'tcx>> = ty::Binder::bind(ty); // <----/
self.infcx.commit_unconditionally(|_| {
let (placeholder_ty, _) = self.infcx.replace_bound_vars_with_placeholders(&ty);
let Normalized { value: normalized_ty, mut obligations } =
ensure_sufficient_stack(|| {
project::normalize_with_depth(
self,
param_env,
cause.clone(),
recursion_depth,
&placeholder_ty,
)
});
let placeholder_obligation = predicate_for_trait_def(
self.tcx(),
param_env,
cause.clone(),
trait_def_id,
recursion_depth,
normalized_ty,
&[],
);
obligations.push(placeholder_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: &TraitObligation<'tcx>,
) -> Normalized<'tcx, SubstsRef<'tcx>> {
match self.match_impl(impl_def_id, obligation) {
Ok(substs) => substs,
Err(()) => {
bug!(
"Impl {:?} was matchable against {:?} but now is not",
impl_def_id,
obligation
);
}
}
}
fn match_impl(
&mut self,
impl_def_id: DefId,
obligation: &TraitObligation<'tcx>,
) -> Result<Normalized<'tcx, SubstsRef<'tcx>>, ()> {
let impl_trait_ref = self.tcx().impl_trait_ref(impl_def_id).unwrap();
// Before we create the substitutions and everything, first
// consider a "quick reject". This avoids creating more types
// and so forth that we need to.
if self.fast_reject_trait_refs(obligation, &impl_trait_ref) {
return Err(());
}
let (placeholder_obligation, _) =
self.infcx().replace_bound_vars_with_placeholders(&obligation.predicate);
let placeholder_obligation_trait_ref = placeholder_obligation.trait_ref;
let impl_substs = self.infcx.fresh_substs_for_item(obligation.cause.span, impl_def_id);
let impl_trait_ref = impl_trait_ref.subst(self.tcx(), impl_substs);
let Normalized { value: impl_trait_ref, obligations: mut nested_obligations } =
ensure_sufficient_stack(|| {
project::normalize_with_depth(
self,
obligation.param_env,
obligation.cause.clone(),
obligation.recursion_depth + 1,
&impl_trait_ref,
)
});
debug!(
"match_impl(impl_def_id={:?}, obligation={:?}, \
impl_trait_ref={:?}, placeholder_obligation_trait_ref={:?})",
impl_def_id, obligation, impl_trait_ref, placeholder_obligation_trait_ref
);
let InferOk { obligations, .. } = self
.infcx
.at(&obligation.cause, obligation.param_env)
.eq(placeholder_obligation_trait_ref, impl_trait_ref)
.map_err(|e| debug!("match_impl: failed eq_trait_refs due to `{}`", e))?;
nested_obligations.extend(obligations);
if !self.intercrate
&& self.tcx().impl_polarity(impl_def_id) == ty::ImplPolarity::Reservation
{
debug!("match_impl: reservation impls only apply in intercrate mode");
return Err(());
}
debug!("match_impl: success impl_substs={:?}", impl_substs);
Ok(Normalized { value: impl_substs, obligations: nested_obligations })
}
fn fast_reject_trait_refs(
&mut self,
obligation: &TraitObligation<'_>,
impl_trait_ref: &ty::TraitRef<'_>,
) -> bool {
// We can avoid creating type variables and doing the full
// substitution if we find that any of the input types, when
// simplified, do not match.
obligation.predicate.skip_binder().trait_ref.substs.iter().zip(impl_trait_ref.substs).any(
|(obligation_arg, impl_arg)| {
match (obligation_arg.unpack(), impl_arg.unpack()) {
(GenericArgKind::Type(obligation_ty), GenericArgKind::Type(impl_ty)) => {
let simplified_obligation_ty =
fast_reject::simplify_type(self.tcx(), obligation_ty, true);
let simplified_impl_ty =
fast_reject::simplify_type(self.tcx(), impl_ty, false);
simplified_obligation_ty.is_some()
&& simplified_impl_ty.is_some()
&& simplified_obligation_ty != simplified_impl_ty
}
(GenericArgKind::Lifetime(_), GenericArgKind::Lifetime(_)) => {
// Lifetimes can never cause a rejection.
false
}
(GenericArgKind::Const(_), GenericArgKind::Const(_)) => {
// Conservatively ignore consts (i.e. assume they might
// unify later) until we have `fast_reject` support for
// them (if we'll ever need it, even).
false
}
_ => unreachable!(),
}
},
)
}
/// Normalize `where_clause_trait_ref` and try to match it against
/// `obligation`. If successful, return any predicates that
/// result from the normalization. Normalization is necessary
/// because where-clauses are stored in the parameter environment
/// unnormalized.
fn match_where_clause_trait_ref(
&mut self,
obligation: &TraitObligation<'tcx>,
where_clause_trait_ref: ty::PolyTraitRef<'tcx>,
) -> Result<Vec<PredicateObligation<'tcx>>, ()> {
self.match_poly_trait_ref(obligation, where_clause_trait_ref)
}
/// Returns `Ok` if `poly_trait_ref` being true implies that the
/// obligation is satisfied.
fn match_poly_trait_ref(
&mut self,
obligation: &TraitObligation<'tcx>,
poly_trait_ref: ty::PolyTraitRef<'tcx>,
) -> Result<Vec<PredicateObligation<'tcx>>, ()> {
debug!(
"match_poly_trait_ref: obligation={:?} poly_trait_ref={:?}",
obligation, poly_trait_ref
);
self.infcx
.at(&obligation.cause, obligation.param_env)
.sup(obligation.predicate.to_poly_trait_ref(), poly_trait_ref)
.map(|InferOk { obligations, .. }| obligations)
.map_err(|_| ())
}
///////////////////////////////////////////////////////////////////////////
// Miscellany
fn match_fresh_trait_refs(
&self,
previous: ty::PolyTraitRef<'tcx>,
current: ty::PolyTraitRef<'tcx>,
param_env: ty::ParamEnv<'tcx>,
) -> bool {
let mut matcher = ty::_match::Match::new(self.tcx(), param_env);
matcher.relate(previous, current).is_ok()
}
fn push_stack<'o>(
&mut self,
previous_stack: TraitObligationStackList<'o, 'tcx>,
obligation: &'o TraitObligation<'tcx>,
) -> TraitObligationStack<'o, 'tcx> {
let fresh_trait_ref =
obligation.predicate.to_poly_trait_ref().fold_with(&mut self.freshener);
let dfn = previous_stack.cache.next_dfn();
let depth = previous_stack.depth() + 1;
TraitObligationStack {
obligation,
fresh_trait_ref,
reached_depth: Cell::new(depth),
previous: previous_stack,
dfn,
depth,
}
}
fn closure_trait_ref_unnormalized(
&mut self,
obligation: &TraitObligation<'tcx>,
substs: SubstsRef<'tcx>,
) -> ty::PolyTraitRef<'tcx> {
debug!("closure_trait_ref_unnormalized(obligation={:?}, substs={:?})", obligation, substs);
let closure_sig = substs.as_closure().sig();
debug!("closure_trait_ref_unnormalized: closure_sig = {:?}", closure_sig);
// (1) Feels icky to skip the binder here, but OTOH we know
// that the self-type is an unboxed closure type and hence is
// in fact unparameterized (or at least does not reference any
// regions bound in the obligation). Still probably some
// refactoring could make this nicer.
closure_trait_ref_and_return_type(
self.tcx(),
obligation.predicate.def_id(),
obligation.predicate.skip_binder().self_ty(), // (1)
closure_sig,
util::TupleArgumentsFlag::No,
)
.map_bound(|(trait_ref, _)| trait_ref)
}
fn generator_trait_ref_unnormalized(
&mut self,
obligation: &TraitObligation<'tcx>,
substs: SubstsRef<'tcx>,
) -> ty::PolyTraitRef<'tcx> {
let gen_sig = substs.as_generator().poly_sig();
// (1) Feels icky to skip the binder here, but OTOH we know
// that the self-type is an generator type and hence is
// in fact unparameterized (or at least does not reference any
// regions bound in the obligation). Still probably some
// refactoring could make this nicer.
super::util::generator_trait_ref_and_outputs(
self.tcx(),
obligation.predicate.def_id(),
obligation.predicate.skip_binder().self_ty(), // (1)
gen_sig,
)
.map_bound(|(trait_ref, ..)| trait_ref)
}
/// Returns the obligations that are implied by instantiating an
/// impl or trait. The obligations are substituted and fully
/// normalized. This is used when confirming an impl or default
/// impl.
fn impl_or_trait_obligations(
&mut self,
cause: ObligationCause<'tcx>,
recursion_depth: usize,
param_env: ty::ParamEnv<'tcx>,
def_id: DefId, // of impl or trait
substs: SubstsRef<'tcx>, // for impl or trait
) -> Vec<PredicateObligation<'tcx>> {
debug!("impl_or_trait_obligations(def_id={:?})", def_id);
let tcx = self.tcx();
// To allow for one-pass evaluation of the nested obligation,
// each predicate must be preceded by the obligations required
// to normalize it.
// for example, if we have:
// impl<U: Iterator<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 substitute, say, `V => IntoIter<u32>, U => $0`, the last
// obligation will normalize to `<$0 as Iterator>::Item = $1` and
// `$1: Copy`, so we must ensure the obligations are emitted in
// that order.
let predicates = tcx.predicates_of(def_id);
assert_eq!(predicates.parent, None);
let mut obligations = Vec::with_capacity(predicates.predicates.len());
for (predicate, _) in predicates.predicates {
let predicate = normalize_with_depth_to(
self,
param_env,
cause.clone(),
recursion_depth,
&predicate.subst(tcx, substs),
&mut obligations,
);
obligations.push(Obligation {
cause: cause.clone(),
recursion_depth,
param_env,
predicate,
});
}
// We are performing deduplication here to avoid exponential blowups
// (#38528) from happening, but the real cause of the duplication is
// unknown. What we know is that the deduplication avoids exponential
// amount of predicates being propagated when processing deeply nested
// types.
//
// This code is hot enough that it's worth avoiding the allocation
// required for the FxHashSet when possible. Special-casing lengths 0,
// 1 and 2 covers roughly 75-80% of the cases.
if obligations.len() <= 1 {
// No possibility of duplicates.
} else if obligations.len() == 2 {
// Only two elements. Drop the second if they are equal.
if obligations[0] == obligations[1] {
obligations.truncate(1);
}
} else {
// Three or more elements. Use a general deduplication process.
let mut seen = FxHashSet::default();
obligations.retain(|i| seen.insert(i.clone()));
}
obligations
}
}
trait TraitObligationExt<'tcx> {
fn derived_cause(
&self,
variant: fn(DerivedObligationCause<'tcx>) -> ObligationCauseCode<'tcx>,
) -> ObligationCause<'tcx>;
}
impl<'tcx> TraitObligationExt<'tcx> for TraitObligation<'tcx> {
fn derived_cause(
&self,
variant: fn(DerivedObligationCause<'tcx>) -> ObligationCauseCode<'tcx>,
) -> ObligationCause<'tcx> {
/*!
* Creates a cause for obligations that are derived from
* `obligation` by a recursive search (e.g., for a builtin
* bound, or eventually a `auto trait Foo`). If `obligation`
* is itself a derived obligation, this is just a clone, but
* otherwise we create a "derived obligation" cause so as to
* keep track of the original root obligation for error
* reporting.
*/
let obligation = self;
// NOTE(flaper87): As of now, it keeps track of the whole error
// chain. Ideally, we should have a way to configure this either
// by using -Z verbose or just a CLI argument.
let derived_cause = DerivedObligationCause {
parent_trait_ref: obligation.predicate.to_poly_trait_ref(),
parent_code: Rc::new(obligation.cause.code.clone()),
};
let derived_code = variant(derived_cause);
ObligationCause::new(obligation.cause.span, obligation.cause.body_id, derived_code)
}
}
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!("update_reached_depth(reached_depth={})", reached_depth);
let mut p = self;
while reached_depth < p.depth {
debug!("update_reached_depth: marking {:?} as cycle participant", p.fresh_trait_ref);
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>,
/// Stores the "coldest" depth (bottom of stack) reached by any of
/// the evaluation entries. The idea here is that all things in the provisional
/// cache are always dependent on *something* that is colder in the stack:
/// therefore, if we add a new entry that is dependent on something *colder still*,
/// we have to modify the depth for all entries at once.
///
/// Example:
///
/// Imagine we have a stack `A B C D E` (with `E` being the top of
/// the stack). We cache something with depth 2, which means that
/// it was dependent on C. Then we pop E but go on and process a
/// new node F: A B C D F. Now F adds something to the cache with
/// depth 1, meaning it is dependent on B. Our original cache
/// entry is also dependent on B, because there is a path from E
/// to C and then from C to F and from F to B.
reached_depth: 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 dependices 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<FxHashMap<ty::PolyTraitRef<'tcx>, ProvisionalEvaluation>>,
}
/// A cache value for the provisional cache: contains the depth-first
/// number (DFN) and result.
#[derive(Copy, Clone, Debug)]
struct ProvisionalEvaluation {
from_dfn: usize,
result: EvaluationResult,
}
impl<'tcx> Default for ProvisionalEvaluationCache<'tcx> {
fn default() -> Self {
Self { dfn: Cell::new(0), reached_depth: Cell::new(usize::MAX), map: 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
/// `self.current_reached_depth()` and above.
fn get_provisional(&self, fresh_trait_ref: ty::PolyTraitRef<'tcx>) -> Option<EvaluationResult> {
debug!(
"get_provisional(fresh_trait_ref={:?}) = {:#?} with reached-depth {}",
fresh_trait_ref,
self.map.borrow().get(&fresh_trait_ref),
self.reached_depth.get(),
);
Some(self.map.borrow().get(&fresh_trait_ref)?.result)
}
/// Current value of the `reached_depth` counter -- all the
/// provisional cache entries are dependent on the item at this
/// depth.
fn current_reached_depth(&self) -> usize {
self.reached_depth.get()
}
/// 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_ref`
/// and resulted in `result`.
fn insert_provisional(
&self,
from_dfn: usize,
reached_depth: usize,
fresh_trait_ref: ty::PolyTraitRef<'tcx>,
result: EvaluationResult,
) {
debug!(
"insert_provisional(from_dfn={}, reached_depth={}, fresh_trait_ref={:?}, result={:?})",
from_dfn, reached_depth, fresh_trait_ref, result,
);
let r_d = self.reached_depth.get();
self.reached_depth.set(r_d.min(reached_depth));
debug!("insert_provisional: reached_depth={:?}", self.reached_depth.get());
self.map.borrow_mut().insert(fresh_trait_ref, ProvisionalEvaluation { from_dfn, 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!("on_failure(dfn={:?})", dfn,);
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). The callback `op` will be invoked for each
/// provisional entry that we can now confirm.
fn on_completion(
&self,
depth: usize,
mut op: impl FnMut(ty::PolyTraitRef<'tcx>, EvaluationResult),
) {
debug!("on_completion(depth={}, reached_depth={})", depth, self.reached_depth.get(),);
if self.reached_depth.get() < depth {
debug!("on_completion: did not yet reach depth to complete");
return;
}
for (fresh_trait_ref, eval) in self.map.borrow_mut().drain() {
debug!("on_completion: fresh_trait_ref={:?} eval={:?}", fresh_trait_ref, eval,);
op(fresh_trait_ref, eval.result);
}
self.reached_depth.set(usize::MAX);
}
}
#[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>> {
match self.head {
Some(o) => {
*self = o.previous;
Some(o)
}
None => None,
}
}
}
impl<'o, 'tcx> fmt::Debug for TraitObligationStack<'o, 'tcx> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
write!(f, "TraitObligationStack({:?})", self.obligation)
}
}