1 //! Candidate selection. See the [rustc guide] for more information on how this works.
3 //! [rustc guide]: https://rust-lang.github.io/rustc-guide/traits/resolution.html#selection
5 use self::EvaluationResult::*;
6 use self::SelectionCandidate::*;
8 use super::coherence::{self, Conflict};
10 use super::project::{normalize_with_depth, Normalized, ProjectionCacheKey};
12 use super::DerivedObligationCause;
14 use super::SelectionResult;
15 use super::TraitNotObjectSafe;
16 use super::{BuiltinDerivedObligation, ImplDerivedObligation, ObligationCauseCode};
17 use super::{IntercrateMode, TraitQueryMode};
18 use super::{ObjectCastObligation, Obligation};
19 use super::{ObligationCause, PredicateObligation, TraitObligation};
20 use super::{OutputTypeParameterMismatch, Overflow, SelectionError, Unimplemented};
22 VtableAutoImpl, VtableBuiltin, VtableClosure, VtableFnPointer, VtableGenerator, VtableImpl,
23 VtableObject, VtableParam, VtableTraitAlias,
26 VtableAutoImplData, VtableBuiltinData, VtableClosureData, VtableFnPointerData,
27 VtableGeneratorData, VtableImplData, VtableObjectData, VtableTraitAliasData,
30 use crate::dep_graph::{DepKind, DepNodeIndex};
31 use crate::hir::def_id::DefId;
32 use crate::infer::{CombinedSnapshot, InferCtxt, InferOk, PlaceholderMap, TypeFreshener};
33 use crate::middle::lang_items;
34 use crate::mir::interpret::GlobalId;
35 use crate::ty::fast_reject;
36 use crate::ty::relate::TypeRelation;
37 use crate::ty::subst::{Subst, Substs};
38 use crate::ty::{self, ToPolyTraitRef, ToPredicate, Ty, TyCtxt, TypeFoldable};
41 use rustc_data_structures::bit_set::GrowableBitSet;
42 use rustc_data_structures::sync::Lock;
43 use rustc_target::spec::abi::Abi;
45 use std::fmt::{self, Display};
48 use crate::util::nodemap::{FxHashMap, FxHashSet};
50 pub struct SelectionContext<'cx, 'gcx: 'cx + 'tcx, 'tcx: 'cx> {
51 infcx: &'cx InferCtxt<'cx, 'gcx, 'tcx>,
53 /// Freshener used specifically for entries on the obligation
54 /// stack. This ensures that all entries on the stack at one time
55 /// will have the same set of placeholder entries, which is
56 /// important for checking for trait bounds that recursively
57 /// require themselves.
58 freshener: TypeFreshener<'cx, 'gcx, 'tcx>,
60 /// If `true`, indicates that the evaluation should be conservative
61 /// and consider the possibility of types outside this crate.
62 /// This comes up primarily when resolving ambiguity. Imagine
63 /// there is some trait reference `$0: Bar` where `$0` is an
64 /// inference variable. If `intercrate` is true, then we can never
65 /// say for sure that this reference is not implemented, even if
66 /// there are *no impls at all for `Bar`*, because `$0` could be
67 /// bound to some type that in a downstream crate that implements
68 /// `Bar`. This is the suitable mode for coherence. Elsewhere,
69 /// though, we set this to false, because we are only interested
70 /// in types that the user could actually have written --- in
71 /// other words, we consider `$0: Bar` to be unimplemented if
72 /// there is no type that the user could *actually name* that
73 /// would satisfy it. This avoids crippling inference, basically.
74 intercrate: Option<IntercrateMode>,
76 intercrate_ambiguity_causes: Option<Vec<IntercrateAmbiguityCause>>,
78 /// Controls whether or not to filter out negative impls when selecting.
79 /// This is used in librustdoc to distinguish between the lack of an impl
80 /// and a negative impl
81 allow_negative_impls: bool,
83 /// The mode that trait queries run in, which informs our error handling
84 /// policy. In essence, canonicalized queries need their errors propagated
85 /// rather than immediately reported because we do not have accurate spans.
86 query_mode: TraitQueryMode,
89 #[derive(Clone, Debug)]
90 pub enum IntercrateAmbiguityCause {
93 self_desc: Option<String>,
97 self_desc: Option<String>,
101 impl IntercrateAmbiguityCause {
102 /// Emits notes when the overlap is caused by complex intercrate ambiguities.
103 /// See #23980 for details.
104 pub fn add_intercrate_ambiguity_hint<'a, 'tcx>(
106 err: &mut errors::DiagnosticBuilder<'_>,
108 err.note(&self.intercrate_ambiguity_hint());
111 pub fn intercrate_ambiguity_hint(&self) -> String {
113 &IntercrateAmbiguityCause::DownstreamCrate {
117 let self_desc = if let &Some(ref ty) = self_desc {
118 format!(" for type `{}`", ty)
123 "downstream crates may implement trait `{}`{}",
124 trait_desc, self_desc
127 &IntercrateAmbiguityCause::UpstreamCrateUpdate {
131 let self_desc = if let &Some(ref ty) = self_desc {
132 format!(" for type `{}`", ty)
137 "upstream crates may add new impl of trait `{}`{} \
139 trait_desc, self_desc
146 // A stack that walks back up the stack frame.
147 struct TraitObligationStack<'prev, 'tcx: 'prev> {
148 obligation: &'prev TraitObligation<'tcx>,
150 /// Trait ref from `obligation` but "freshened" with the
151 /// selection-context's freshener. Used to check for recursion.
152 fresh_trait_ref: ty::PolyTraitRef<'tcx>,
154 previous: TraitObligationStackList<'prev, 'tcx>,
157 #[derive(Clone, Default)]
158 pub struct SelectionCache<'tcx> {
160 FxHashMap<ty::TraitRef<'tcx>, WithDepNode<SelectionResult<'tcx, SelectionCandidate<'tcx>>>>,
164 /// The selection process begins by considering all impls, where
165 /// clauses, and so forth that might resolve an obligation. Sometimes
166 /// we'll be able to say definitively that (e.g.) an impl does not
167 /// apply to the obligation: perhaps it is defined for `usize` but the
168 /// obligation is for `int`. In that case, we drop the impl out of the
169 /// list. But the other cases are considered *candidates*.
171 /// For selection to succeed, there must be exactly one matching
172 /// candidate. If the obligation is fully known, this is guaranteed
173 /// by coherence. However, if the obligation contains type parameters
174 /// or variables, there may be multiple such impls.
176 /// It is not a real problem if multiple matching impls exist because
177 /// of type variables - it just means the obligation isn't sufficiently
178 /// elaborated. In that case we report an ambiguity, and the caller can
179 /// try again after more type information has been gathered or report a
180 /// "type annotations required" error.
182 /// However, with type parameters, this can be a real problem - type
183 /// parameters don't unify with regular types, but they *can* unify
184 /// with variables from blanket impls, and (unless we know its bounds
185 /// will always be satisfied) picking the blanket impl will be wrong
186 /// for at least *some* substitutions. To make this concrete, if we have
188 /// trait AsDebug { type Out : fmt::Debug; fn debug(self) -> Self::Out; }
189 /// impl<T: fmt::Debug> AsDebug for T {
191 /// fn debug(self) -> fmt::Debug { self }
193 /// fn foo<T: AsDebug>(t: T) { println!("{:?}", <T as AsDebug>::debug(t)); }
195 /// we can't just use the impl to resolve the <T as AsDebug> obligation
196 /// - a type from another crate (that doesn't implement fmt::Debug) could
197 /// implement AsDebug.
199 /// Because where-clauses match the type exactly, multiple clauses can
200 /// only match if there are unresolved variables, and we can mostly just
201 /// report this ambiguity in that case. This is still a problem - we can't
202 /// *do anything* with ambiguities that involve only regions. This is issue
205 /// If a single where-clause matches and there are no inference
206 /// variables left, then it definitely matches and we can just select
209 /// In fact, we even select the where-clause when the obligation contains
210 /// inference variables. The can lead to inference making "leaps of logic",
211 /// for example in this situation:
213 /// pub trait Foo<T> { fn foo(&self) -> T; }
214 /// impl<T> Foo<()> for T { fn foo(&self) { } }
215 /// impl Foo<bool> for bool { fn foo(&self) -> bool { *self } }
217 /// pub fn foo<T>(t: T) where T: Foo<bool> {
218 /// println!("{:?}", <T as Foo<_>>::foo(&t));
220 /// fn main() { foo(false); }
222 /// Here the obligation <T as Foo<$0>> can be matched by both the blanket
223 /// impl and the where-clause. We select the where-clause and unify $0=bool,
224 /// so the program prints "false". However, if the where-clause is omitted,
225 /// the blanket impl is selected, we unify $0=(), and the program prints
228 /// Exactly the same issues apply to projection and object candidates, except
229 /// that we can have both a projection candidate and a where-clause candidate
230 /// for the same obligation. In that case either would do (except that
231 /// different "leaps of logic" would occur if inference variables are
232 /// present), and we just pick the where-clause. This is, for example,
233 /// required for associated types to work in default impls, as the bounds
234 /// are visible both as projection bounds and as where-clauses from the
235 /// parameter environment.
236 #[derive(PartialEq, Eq, Debug, Clone)]
237 enum SelectionCandidate<'tcx> {
238 /// If has_nested is false, there are no *further* obligations
242 ParamCandidate(ty::PolyTraitRef<'tcx>),
243 ImplCandidate(DefId),
244 AutoImplCandidate(DefId),
246 /// This is a trait matching with a projected type as `Self`, and
247 /// we found an applicable bound in the trait definition.
250 /// Implementation of a `Fn`-family trait by one of the anonymous types
251 /// generated for a `||` expression.
254 /// Implementation of a `Generator` trait by one of the anonymous types
255 /// generated for a generator.
258 /// Implementation of a `Fn`-family trait by one of the anonymous
259 /// types generated for a fn pointer type (e.g., `fn(int)->int`)
262 TraitAliasCandidate(DefId),
266 BuiltinObjectCandidate,
268 BuiltinUnsizeCandidate,
271 impl<'a, 'tcx> ty::Lift<'tcx> for SelectionCandidate<'a> {
272 type Lifted = SelectionCandidate<'tcx>;
273 fn lift_to_tcx<'b, 'gcx>(&self, tcx: TyCtxt<'b, 'gcx, 'tcx>) -> Option<Self::Lifted> {
275 BuiltinCandidate { has_nested } => BuiltinCandidate { has_nested },
276 ImplCandidate(def_id) => ImplCandidate(def_id),
277 AutoImplCandidate(def_id) => AutoImplCandidate(def_id),
278 ProjectionCandidate => ProjectionCandidate,
279 ClosureCandidate => ClosureCandidate,
280 GeneratorCandidate => GeneratorCandidate,
281 FnPointerCandidate => FnPointerCandidate,
282 TraitAliasCandidate(def_id) => TraitAliasCandidate(def_id),
283 ObjectCandidate => ObjectCandidate,
284 BuiltinObjectCandidate => BuiltinObjectCandidate,
285 BuiltinUnsizeCandidate => BuiltinUnsizeCandidate,
287 ParamCandidate(ref trait_ref) => {
288 return tcx.lift(trait_ref).map(ParamCandidate);
294 struct SelectionCandidateSet<'tcx> {
295 // a list of candidates that definitely apply to the current
296 // obligation (meaning: types unify).
297 vec: Vec<SelectionCandidate<'tcx>>,
299 // if this is true, then there were candidates that might or might
300 // not have applied, but we couldn't tell. This occurs when some
301 // of the input types are type variables, in which case there are
302 // various "builtin" rules that might or might not trigger.
306 #[derive(PartialEq, Eq, Debug, Clone)]
307 struct EvaluatedCandidate<'tcx> {
308 candidate: SelectionCandidate<'tcx>,
309 evaluation: EvaluationResult,
312 /// When does the builtin impl for `T: Trait` apply?
313 enum BuiltinImplConditions<'tcx> {
314 /// The impl is conditional on T1,T2,.. : Trait
315 Where(ty::Binder<Vec<Ty<'tcx>>>),
316 /// There is no built-in impl. There may be some other
317 /// candidate (a where-clause or user-defined impl).
319 /// It is unknown whether there is an impl.
323 #[derive(Copy, Clone, Debug, PartialOrd, Ord, PartialEq, Eq)]
324 /// The result of trait evaluation. The order is important
325 /// here as the evaluation of a list is the maximum of the
328 /// The evaluation results are ordered:
329 /// - `EvaluatedToOk` implies `EvaluatedToOkModuloRegions`
330 /// implies `EvaluatedToAmbig` implies `EvaluatedToUnknown`
331 /// - `EvaluatedToErr` implies `EvaluatedToRecur`
332 /// - the "union" of evaluation results is equal to their maximum -
333 /// all the "potential success" candidates can potentially succeed,
334 /// so they are noops when unioned with a definite error, and within
335 /// the categories it's easy to see that the unions are correct.
336 pub enum EvaluationResult {
337 /// Evaluation successful
339 /// Evaluation successful, but there were unevaluated region obligations
340 EvaluatedToOkModuloRegions,
341 /// Evaluation is known to be ambiguous - it *might* hold for some
342 /// assignment of inference variables, but it might not.
344 /// While this has the same meaning as `EvaluatedToUnknown` - we can't
345 /// know whether this obligation holds or not - it is the result we
346 /// would get with an empty stack, and therefore is cacheable.
348 /// Evaluation failed because of recursion involving inference
349 /// variables. We are somewhat imprecise there, so we don't actually
350 /// know the real result.
352 /// This can't be trivially cached for the same reason as `EvaluatedToRecur`.
354 /// Evaluation failed because we encountered an obligation we are already
355 /// trying to prove on this branch.
357 /// We know this branch can't be a part of a minimal proof-tree for
358 /// the "root" of our cycle, because then we could cut out the recursion
359 /// and maintain a valid proof tree. However, this does not mean
360 /// that all the obligations on this branch do not hold - it's possible
361 /// that we entered this branch "speculatively", and that there
362 /// might be some other way to prove this obligation that does not
363 /// go through this cycle - so we can't cache this as a failure.
365 /// For example, suppose we have this:
367 /// ```rust,ignore (pseudo-Rust)
368 /// pub trait Trait { fn xyz(); }
369 /// // This impl is "useless", but we can still have
370 /// // an `impl Trait for SomeUnsizedType` somewhere.
371 /// impl<T: Trait + Sized> Trait for T { fn xyz() {} }
373 /// pub fn foo<T: Trait + ?Sized>() {
374 /// <T as Trait>::xyz();
378 /// When checking `foo`, we have to prove `T: Trait`. This basically
379 /// translates into this:
382 /// (T: Trait + Sized →_\impl T: Trait), T: Trait ⊢ T: Trait
385 /// When we try to prove it, we first go the first option, which
386 /// recurses. This shows us that the impl is "useless" -- it won't
387 /// tell us that `T: Trait` unless it already implemented `Trait`
388 /// by some other means. However, that does not prevent `T: Trait`
389 /// does not hold, because of the bound (which can indeed be satisfied
390 /// by `SomeUnsizedType` from another crate).
392 // FIXME: when an `EvaluatedToRecur` goes past its parent root, we
393 // ought to convert it to an `EvaluatedToErr`, because we know
394 // there definitely isn't a proof tree for that obligation. Not
395 // doing so is still sound -- there isn't any proof tree, so the
396 // branch still can't be a part of a minimal one -- but does not re-enable caching.
398 /// Evaluation failed.
402 impl EvaluationResult {
403 /// Returns `true` if this evaluation result is known to apply, even
404 /// considering outlives constraints.
405 pub fn must_apply_considering_regions(self) -> bool {
406 self == EvaluatedToOk
409 /// Returns `true` if this evaluation result is known to apply, ignoring
410 /// outlives constraints.
411 pub fn must_apply_modulo_regions(self) -> bool {
412 self <= EvaluatedToOkModuloRegions
415 pub fn may_apply(self) -> bool {
417 EvaluatedToOk | EvaluatedToOkModuloRegions | EvaluatedToAmbig | EvaluatedToUnknown => {
421 EvaluatedToErr | EvaluatedToRecur => false,
425 fn is_stack_dependent(self) -> bool {
427 EvaluatedToUnknown | EvaluatedToRecur => true,
429 EvaluatedToOk | EvaluatedToOkModuloRegions | EvaluatedToAmbig | EvaluatedToErr => false,
434 impl_stable_hash_for!(enum self::EvaluationResult {
436 EvaluatedToOkModuloRegions,
443 #[derive(Copy, Clone, Debug, PartialEq, Eq)]
444 /// Indicates that trait evaluation caused overflow.
445 pub struct OverflowError;
447 impl_stable_hash_for!(struct OverflowError {});
449 impl<'tcx> From<OverflowError> for SelectionError<'tcx> {
450 fn from(OverflowError: OverflowError) -> SelectionError<'tcx> {
451 SelectionError::Overflow
455 #[derive(Clone, Default)]
456 pub struct EvaluationCache<'tcx> {
457 hashmap: Lock<FxHashMap<ty::PolyTraitRef<'tcx>, WithDepNode<EvaluationResult>>>,
460 impl<'cx, 'gcx, 'tcx> SelectionContext<'cx, 'gcx, 'tcx> {
461 pub fn new(infcx: &'cx InferCtxt<'cx, 'gcx, 'tcx>) -> SelectionContext<'cx, 'gcx, 'tcx> {
464 freshener: infcx.freshener(),
466 intercrate_ambiguity_causes: None,
467 allow_negative_impls: false,
468 query_mode: TraitQueryMode::Standard,
473 infcx: &'cx InferCtxt<'cx, 'gcx, 'tcx>,
474 mode: IntercrateMode,
475 ) -> SelectionContext<'cx, 'gcx, 'tcx> {
476 debug!("intercrate({:?})", mode);
479 freshener: infcx.freshener(),
480 intercrate: Some(mode),
481 intercrate_ambiguity_causes: None,
482 allow_negative_impls: false,
483 query_mode: TraitQueryMode::Standard,
487 pub fn with_negative(
488 infcx: &'cx InferCtxt<'cx, 'gcx, 'tcx>,
489 allow_negative_impls: bool,
490 ) -> SelectionContext<'cx, 'gcx, 'tcx> {
491 debug!("with_negative({:?})", allow_negative_impls);
494 freshener: infcx.freshener(),
496 intercrate_ambiguity_causes: None,
497 allow_negative_impls,
498 query_mode: TraitQueryMode::Standard,
502 pub fn with_query_mode(
503 infcx: &'cx InferCtxt<'cx, 'gcx, 'tcx>,
504 query_mode: TraitQueryMode,
505 ) -> SelectionContext<'cx, 'gcx, 'tcx> {
506 debug!("with_query_mode({:?})", query_mode);
509 freshener: infcx.freshener(),
511 intercrate_ambiguity_causes: None,
512 allow_negative_impls: false,
517 /// Enables tracking of intercrate ambiguity causes. These are
518 /// used in coherence to give improved diagnostics. We don't do
519 /// this until we detect a coherence error because it can lead to
520 /// false overflow results (#47139) and because it costs
521 /// computation time.
522 pub fn enable_tracking_intercrate_ambiguity_causes(&mut self) {
523 assert!(self.intercrate.is_some());
524 assert!(self.intercrate_ambiguity_causes.is_none());
525 self.intercrate_ambiguity_causes = Some(vec![]);
526 debug!("selcx: enable_tracking_intercrate_ambiguity_causes");
529 /// Gets the intercrate ambiguity causes collected since tracking
530 /// was enabled and disables tracking at the same time. If
531 /// tracking is not enabled, just returns an empty vector.
532 pub fn take_intercrate_ambiguity_causes(&mut self) -> Vec<IntercrateAmbiguityCause> {
533 assert!(self.intercrate.is_some());
534 self.intercrate_ambiguity_causes.take().unwrap_or(vec![])
537 pub fn infcx(&self) -> &'cx InferCtxt<'cx, 'gcx, 'tcx> {
541 pub fn tcx(&self) -> TyCtxt<'cx, 'gcx, 'tcx> {
545 pub fn closure_typer(&self) -> &'cx InferCtxt<'cx, 'gcx, 'tcx> {
549 ///////////////////////////////////////////////////////////////////////////
552 // The selection phase tries to identify *how* an obligation will
553 // be resolved. For example, it will identify which impl or
554 // parameter bound is to be used. The process can be inconclusive
555 // if the self type in the obligation is not fully inferred. Selection
556 // can result in an error in one of two ways:
558 // 1. If no applicable impl or parameter bound can be found.
559 // 2. If the output type parameters in the obligation do not match
560 // those specified by the impl/bound. For example, if the obligation
561 // is `Vec<Foo>:Iterable<Bar>`, but the impl specifies
562 // `impl<T> Iterable<T> for Vec<T>`, than an error would result.
564 /// Attempts to satisfy the obligation. If successful, this will affect the surrounding
565 /// type environment by performing unification.
568 obligation: &TraitObligation<'tcx>,
569 ) -> SelectionResult<'tcx, Selection<'tcx>> {
570 debug!("select({:?})", obligation);
571 debug_assert!(!obligation.predicate.has_escaping_bound_vars());
573 let stack = self.push_stack(TraitObligationStackList::empty(), obligation);
575 let candidate = match self.candidate_from_obligation(&stack) {
576 Err(SelectionError::Overflow) => {
577 // In standard mode, overflow must have been caught and reported
579 assert!(self.query_mode == TraitQueryMode::Canonical);
580 return Err(SelectionError::Overflow);
588 Ok(Some(candidate)) => candidate,
591 match self.confirm_candidate(obligation, candidate) {
592 Err(SelectionError::Overflow) => {
593 assert!(self.query_mode == TraitQueryMode::Canonical);
594 Err(SelectionError::Overflow)
597 Ok(candidate) => Ok(Some(candidate)),
601 ///////////////////////////////////////////////////////////////////////////
604 // Tests whether an obligation can be selected or whether an impl
605 // can be applied to particular types. It skips the "confirmation"
606 // step and hence completely ignores output type parameters.
608 // The result is "true" if the obligation *may* hold and "false" if
609 // we can be sure it does not.
611 /// Evaluates whether the obligation `obligation` can be satisfied (by any means).
612 pub fn predicate_may_hold_fatal(&mut self, obligation: &PredicateObligation<'tcx>) -> bool {
613 debug!("predicate_may_hold_fatal({:?})", obligation);
615 // This fatal query is a stopgap that should only be used in standard mode,
616 // where we do not expect overflow to be propagated.
617 assert!(self.query_mode == TraitQueryMode::Standard);
619 self.evaluate_obligation_recursively(obligation)
620 .expect("Overflow should be caught earlier in standard query mode")
624 /// Evaluates whether the obligation `obligation` can be satisfied and returns
625 /// an `EvaluationResult`.
626 pub fn evaluate_obligation_recursively(
628 obligation: &PredicateObligation<'tcx>,
629 ) -> Result<EvaluationResult, OverflowError> {
630 self.evaluation_probe(|this| {
631 this.evaluate_predicate_recursively(TraitObligationStackList::empty(),
638 op: impl FnOnce(&mut Self) -> Result<EvaluationResult, OverflowError>,
639 ) -> Result<EvaluationResult, OverflowError> {
640 self.infcx.probe(|snapshot| -> Result<EvaluationResult, OverflowError> {
641 let result = op(self)?;
642 match self.infcx.region_constraints_added_in_snapshot(snapshot) {
644 Some(_) => Ok(result.max(EvaluatedToOkModuloRegions)),
649 /// Evaluates the predicates in `predicates` recursively. Note that
650 /// this applies projections in the predicates, and therefore
651 /// is run within an inference probe.
652 fn evaluate_predicates_recursively<'a, 'o, I>(
654 stack: TraitObligationStackList<'o, 'tcx>,
656 ) -> Result<EvaluationResult, OverflowError>
658 I: IntoIterator<Item = PredicateObligation<'tcx>>,
661 let mut result = EvaluatedToOk;
662 for obligation in predicates {
663 let eval = self.evaluate_predicate_recursively(stack, obligation.clone())?;
665 "evaluate_predicate_recursively({:?}) = {:?}",
668 if let EvaluatedToErr = eval {
669 // fast-path - EvaluatedToErr is the top of the lattice,
670 // so we don't need to look on the other predicates.
671 return Ok(EvaluatedToErr);
673 result = cmp::max(result, eval);
679 fn evaluate_predicate_recursively<'o>(
681 previous_stack: TraitObligationStackList<'o, 'tcx>,
682 obligation: PredicateObligation<'tcx>,
683 ) -> Result<EvaluationResult, OverflowError> {
684 debug!("evaluate_predicate_recursively(previous_stack={:?}, obligation={:?})",
685 previous_stack.head(), obligation);
687 // Previous_stack stores a TraitObligatiom, while 'obligation' is
688 // a PredicateObligation. These are distinct types, so we can't
689 // use any Option combinator method that would force them to be
691 match previous_stack.head() {
692 Some(h) => self.check_recursion_limit(&obligation, h.obligation)?,
693 None => self.check_recursion_limit(&obligation, &obligation)?
696 match obligation.predicate {
697 ty::Predicate::Trait(ref t) => {
698 debug_assert!(!t.has_escaping_bound_vars());
699 let obligation = obligation.with(t.clone());
700 self.evaluate_trait_predicate_recursively(previous_stack, obligation)
703 ty::Predicate::Subtype(ref p) => {
704 // does this code ever run?
706 .subtype_predicate(&obligation.cause, obligation.param_env, p)
708 Some(Ok(InferOk { mut obligations, .. })) => {
709 self.add_depth(obligations.iter_mut(), obligation.recursion_depth);
710 self.evaluate_predicates_recursively(previous_stack,obligations.into_iter())
712 Some(Err(_)) => Ok(EvaluatedToErr),
713 None => Ok(EvaluatedToAmbig),
717 ty::Predicate::WellFormed(ty) => match ty::wf::obligations(
719 obligation.param_env,
720 obligation.cause.body_id,
722 obligation.cause.span,
724 Some(mut obligations) => {
725 self.add_depth(obligations.iter_mut(), obligation.recursion_depth);
726 self.evaluate_predicates_recursively(previous_stack, obligations.into_iter())
728 None => Ok(EvaluatedToAmbig),
731 ty::Predicate::TypeOutlives(..) | ty::Predicate::RegionOutlives(..) => {
732 // we do not consider region relationships when
733 // evaluating trait matches
734 Ok(EvaluatedToOkModuloRegions)
737 ty::Predicate::ObjectSafe(trait_def_id) => {
738 if self.tcx().is_object_safe(trait_def_id) {
745 ty::Predicate::Projection(ref data) => {
746 let project_obligation = obligation.with(data.clone());
747 match project::poly_project_and_unify_type(self, &project_obligation) {
748 Ok(Some(mut subobligations)) => {
749 self.add_depth(subobligations.iter_mut(), obligation.recursion_depth);
750 let result = self.evaluate_predicates_recursively(
752 subobligations.into_iter(),
755 ProjectionCacheKey::from_poly_projection_predicate(self, data)
757 self.infcx.projection_cache.borrow_mut().complete(key);
761 Ok(None) => Ok(EvaluatedToAmbig),
762 Err(_) => Ok(EvaluatedToErr),
766 ty::Predicate::ClosureKind(closure_def_id, closure_substs, kind) => {
767 match self.infcx.closure_kind(closure_def_id, closure_substs) {
768 Some(closure_kind) => {
769 if closure_kind.extends(kind) {
775 None => Ok(EvaluatedToAmbig),
779 ty::Predicate::ConstEvaluatable(def_id, substs) => {
780 let tcx = self.tcx();
781 match tcx.lift_to_global(&(obligation.param_env, substs)) {
782 Some((param_env, substs)) => {
784 ty::Instance::resolve(tcx.global_tcx(), param_env, def_id, substs);
785 if let Some(instance) = instance {
790 match self.tcx().const_eval(param_env.and(cid)) {
791 Ok(_) => Ok(EvaluatedToOk),
792 Err(_) => Ok(EvaluatedToErr),
799 // Inference variables still left in param_env or substs.
807 fn evaluate_trait_predicate_recursively<'o>(
809 previous_stack: TraitObligationStackList<'o, 'tcx>,
810 mut obligation: TraitObligation<'tcx>,
811 ) -> Result<EvaluationResult, OverflowError> {
812 debug!("evaluate_trait_predicate_recursively({:?})", obligation);
814 if self.intercrate.is_none() && obligation.is_global()
819 .all(|bound| bound.needs_subst())
821 // If a param env has no global bounds, global obligations do not
822 // depend on its particular value in order to work, so we can clear
823 // out the param env and get better caching.
825 "evaluate_trait_predicate_recursively({:?}) - in global",
828 obligation.param_env = obligation.param_env.without_caller_bounds();
831 let stack = self.push_stack(previous_stack, &obligation);
832 let fresh_trait_ref = stack.fresh_trait_ref;
833 if let Some(result) = self.check_evaluation_cache(obligation.param_env, fresh_trait_ref) {
834 debug!("CACHE HIT: EVAL({:?})={:?}", fresh_trait_ref, result);
838 let (result, dep_node) = self.in_task(|this| this.evaluate_stack(&stack));
839 let result = result?;
841 debug!("CACHE MISS: EVAL({:?})={:?}", fresh_trait_ref, result);
842 self.insert_evaluation_cache(obligation.param_env, fresh_trait_ref, dep_node, result);
847 fn evaluate_stack<'o>(
849 stack: &TraitObligationStack<'o, 'tcx>,
850 ) -> Result<EvaluationResult, OverflowError> {
851 // In intercrate mode, whenever any of the types are unbound,
852 // there can always be an impl. Even if there are no impls in
853 // this crate, perhaps the type would be unified with
854 // something from another crate that does provide an impl.
856 // In intra mode, we must still be conservative. The reason is
857 // that we want to avoid cycles. Imagine an impl like:
859 // impl<T:Eq> Eq for Vec<T>
861 // and a trait reference like `$0 : Eq` where `$0` is an
862 // unbound variable. When we evaluate this trait-reference, we
863 // will unify `$0` with `Vec<$1>` (for some fresh variable
864 // `$1`), on the condition that `$1 : Eq`. We will then wind
865 // up with many candidates (since that are other `Eq` impls
866 // that apply) and try to winnow things down. This results in
867 // a recursive evaluation that `$1 : Eq` -- as you can
868 // imagine, this is just where we started. To avoid that, we
869 // check for unbound variables and return an ambiguous (hence possible)
870 // match if we've seen this trait before.
872 // This suffices to allow chains like `FnMut` implemented in
873 // terms of `Fn` etc, but we could probably make this more
875 let unbound_input_types = stack
879 .any(|ty| ty.is_fresh());
880 // this check was an imperfect workaround for a bug n the old
881 // intercrate mode, it should be removed when that goes away.
882 if unbound_input_types && self.intercrate == Some(IntercrateMode::Issue43355) {
884 "evaluate_stack({:?}) --> unbound argument, intercrate --> ambiguous",
885 stack.fresh_trait_ref
887 // Heuristics: show the diagnostics when there are no candidates in crate.
888 if self.intercrate_ambiguity_causes.is_some() {
889 debug!("evaluate_stack: intercrate_ambiguity_causes is some");
890 if let Ok(candidate_set) = self.assemble_candidates(stack) {
891 if !candidate_set.ambiguous && candidate_set.vec.is_empty() {
892 let trait_ref = stack.obligation.predicate.skip_binder().trait_ref;
893 let self_ty = trait_ref.self_ty();
894 let cause = IntercrateAmbiguityCause::DownstreamCrate {
895 trait_desc: trait_ref.to_string(),
896 self_desc: if self_ty.has_concrete_skeleton() {
897 Some(self_ty.to_string())
902 debug!("evaluate_stack: pushing cause = {:?}", cause);
903 self.intercrate_ambiguity_causes
910 return Ok(EvaluatedToAmbig);
912 if unbound_input_types && stack.iter().skip(1).any(|prev| {
913 stack.obligation.param_env == prev.obligation.param_env
914 && self.match_fresh_trait_refs(&stack.fresh_trait_ref, &prev.fresh_trait_ref)
917 "evaluate_stack({:?}) --> unbound argument, recursive --> giving up",
918 stack.fresh_trait_ref
920 return Ok(EvaluatedToUnknown);
923 // If there is any previous entry on the stack that precisely
924 // matches this obligation, then we can assume that the
925 // obligation is satisfied for now (still all other conditions
926 // must be met of course). One obvious case this comes up is
927 // marker traits like `Send`. Think of a linked list:
929 // struct List<T> { data: T, next: Option<Box<List<T>>> }
931 // `Box<List<T>>` will be `Send` if `T` is `Send` and
932 // `Option<Box<List<T>>>` is `Send`, and in turn
933 // `Option<Box<List<T>>>` is `Send` if `Box<List<T>>` is
936 // Note that we do this comparison using the `fresh_trait_ref`
937 // fields. Because these have all been freshened using
938 // `self.freshener`, we can be sure that (a) this will not
939 // affect the inferencer state and (b) that if we see two
940 // fresh regions with the same index, they refer to the same
941 // unbound type variable.
942 if let Some(rec_index) = stack.iter()
943 .skip(1) // skip top-most frame
944 .position(|prev| stack.obligation.param_env == prev.obligation.param_env &&
945 stack.fresh_trait_ref == prev.fresh_trait_ref)
947 debug!("evaluate_stack({:?}) --> recursive", stack.fresh_trait_ref);
949 // Subtle: when checking for a coinductive cycle, we do
950 // not compare using the "freshened trait refs" (which
951 // have erased regions) but rather the fully explicit
952 // trait refs. This is important because it's only a cycle
953 // if the regions match exactly.
954 let cycle = stack.iter().skip(1).take(rec_index + 1);
955 let cycle = cycle.map(|stack| ty::Predicate::Trait(stack.obligation.predicate));
956 if self.coinductive_match(cycle) {
958 "evaluate_stack({:?}) --> recursive, coinductive",
959 stack.fresh_trait_ref
961 return Ok(EvaluatedToOk);
964 "evaluate_stack({:?}) --> recursive, inductive",
965 stack.fresh_trait_ref
967 return Ok(EvaluatedToRecur);
971 match self.candidate_from_obligation(stack) {
972 Ok(Some(c)) => self.evaluate_candidate(stack, &c),
973 Ok(None) => Ok(EvaluatedToAmbig),
974 Err(Overflow) => Err(OverflowError),
975 Err(..) => Ok(EvaluatedToErr),
979 /// For defaulted traits, we use a co-inductive strategy to solve, so
980 /// that recursion is ok. This routine returns true if the top of the
981 /// stack (`cycle[0]`):
983 /// - is a defaulted trait,
984 /// - it also appears in the backtrace at some position `X`,
985 /// - all the predicates at positions `X..` between `X` an the top are
986 /// also defaulted traits.
987 pub fn coinductive_match<I>(&mut self, cycle: I) -> bool
989 I: Iterator<Item = ty::Predicate<'tcx>>,
991 let mut cycle = cycle;
992 cycle.all(|predicate| self.coinductive_predicate(predicate))
995 fn coinductive_predicate(&self, predicate: ty::Predicate<'tcx>) -> bool {
996 let result = match predicate {
997 ty::Predicate::Trait(ref data) => self.tcx().trait_is_auto(data.def_id()),
1000 debug!("coinductive_predicate({:?}) = {:?}", predicate, result);
1004 /// Further evaluate `candidate` to decide whether all type parameters match and whether nested
1005 /// obligations are met. Returns whether `candidate` remains viable after this further
1007 fn evaluate_candidate<'o>(
1009 stack: &TraitObligationStack<'o, 'tcx>,
1010 candidate: &SelectionCandidate<'tcx>,
1011 ) -> Result<EvaluationResult, OverflowError> {
1013 "evaluate_candidate: depth={} candidate={:?}",
1014 stack.obligation.recursion_depth, candidate
1016 let result = self.evaluation_probe(|this| {
1017 let candidate = (*candidate).clone();
1018 match this.confirm_candidate(stack.obligation, candidate) {
1019 Ok(selection) => this.evaluate_predicates_recursively(
1021 selection.nested_obligations().into_iter()
1023 Err(..) => Ok(EvaluatedToErr),
1027 "evaluate_candidate: depth={} result={:?}",
1028 stack.obligation.recursion_depth, result
1033 fn check_evaluation_cache(
1035 param_env: ty::ParamEnv<'tcx>,
1036 trait_ref: ty::PolyTraitRef<'tcx>,
1037 ) -> Option<EvaluationResult> {
1038 let tcx = self.tcx();
1039 if self.can_use_global_caches(param_env) {
1040 let cache = tcx.evaluation_cache.hashmap.borrow();
1041 if let Some(cached) = cache.get(&trait_ref) {
1042 return Some(cached.get(tcx));
1050 .map(|v| v.get(tcx))
1053 fn insert_evaluation_cache(
1055 param_env: ty::ParamEnv<'tcx>,
1056 trait_ref: ty::PolyTraitRef<'tcx>,
1057 dep_node: DepNodeIndex,
1058 result: EvaluationResult,
1060 // Avoid caching results that depend on more than just the trait-ref
1061 // - the stack can create recursion.
1062 if result.is_stack_dependent() {
1066 if self.can_use_global_caches(param_env) {
1067 if let Some(trait_ref) = self.tcx().lift_to_global(&trait_ref) {
1069 "insert_evaluation_cache(trait_ref={:?}, candidate={:?}) global",
1072 // This may overwrite the cache with the same value
1073 // FIXME: Due to #50507 this overwrites the different values
1074 // This should be changed to use HashMapExt::insert_same
1075 // when that is fixed
1080 .insert(trait_ref, WithDepNode::new(dep_node, result));
1086 "insert_evaluation_cache(trait_ref={:?}, candidate={:?})",
1093 .insert(trait_ref, WithDepNode::new(dep_node, result));
1096 // For various reasons, it's possible for a subobligation
1097 // to have a *lower* recursion_depth than the obligation used to create it.
1098 // Projection sub-obligations may be returned from the projection cache,
1099 // which results in obligations with an 'old' recursion_depth.
1100 // Additionally, methods like ty::wf::obligations and
1101 // InferCtxt.subtype_predicate produce subobligations without
1102 // taking in a 'parent' depth, causing the generated subobligations
1103 // to have a recursion_depth of 0
1105 // To ensure that obligation_depth never decreasees, we force all subobligations
1106 // to have at least the depth of the original obligation.
1107 fn add_depth<T: 'cx, I: Iterator<Item = &'cx mut Obligation<'tcx, T>>>(&self, it: I,
1109 it.for_each(|o| o.recursion_depth = cmp::max(min_depth, o.recursion_depth) + 1);
1112 // Check that the recursion limit has not been exceeded.
1114 // The weird return type of this function allows it to be used with the 'try' (?)
1115 // operator within certain functions
1116 fn check_recursion_limit<T: Display + TypeFoldable<'tcx>, V: Display + TypeFoldable<'tcx>>(
1118 obligation: &Obligation<'tcx, T>,
1119 error_obligation: &Obligation<'tcx, V>
1120 ) -> Result<(), OverflowError> {
1121 let recursion_limit = *self.infcx.tcx.sess.recursion_limit.get();
1122 if obligation.recursion_depth >= recursion_limit {
1123 match self.query_mode {
1124 TraitQueryMode::Standard => {
1125 self.infcx().report_overflow_error(error_obligation, true);
1127 TraitQueryMode::Canonical => {
1128 return Err(OverflowError);
1135 ///////////////////////////////////////////////////////////////////////////
1136 // CANDIDATE ASSEMBLY
1138 // The selection process begins by examining all in-scope impls,
1139 // caller obligations, and so forth and assembling a list of
1140 // candidates. See the [rustc guide] for more details.
1143 // https://rust-lang.github.io/rustc-guide/traits/resolution.html#candidate-assembly
1145 fn candidate_from_obligation<'o>(
1147 stack: &TraitObligationStack<'o, 'tcx>,
1148 ) -> SelectionResult<'tcx, SelectionCandidate<'tcx>> {
1149 // Watch out for overflow. This intentionally bypasses (and does
1150 // not update) the cache.
1151 self.check_recursion_limit(&stack.obligation, &stack.obligation)?;
1154 // Check the cache. Note that we freshen the trait-ref
1155 // separately rather than using `stack.fresh_trait_ref` --
1156 // this is because we want the unbound variables to be
1157 // replaced with fresh types starting from index 0.
1158 let cache_fresh_trait_pred = self.infcx.freshen(stack.obligation.predicate.clone());
1160 "candidate_from_obligation(cache_fresh_trait_pred={:?}, obligation={:?})",
1161 cache_fresh_trait_pred, stack
1163 debug_assert!(!stack.obligation.predicate.has_escaping_bound_vars());
1166 self.check_candidate_cache(stack.obligation.param_env, &cache_fresh_trait_pred)
1168 debug!("CACHE HIT: SELECT({:?})={:?}", cache_fresh_trait_pred, c);
1172 // If no match, compute result and insert into cache.
1173 let (candidate, dep_node) =
1174 self.in_task(|this| this.candidate_from_obligation_no_cache(stack));
1177 "CACHE MISS: SELECT({:?})={:?}",
1178 cache_fresh_trait_pred, candidate
1180 self.insert_candidate_cache(
1181 stack.obligation.param_env,
1182 cache_fresh_trait_pred,
1189 fn in_task<OP, R>(&mut self, op: OP) -> (R, DepNodeIndex)
1191 OP: FnOnce(&mut Self) -> R,
1193 let (result, dep_node) = self.tcx()
1195 .with_anon_task(DepKind::TraitSelect, || op(self));
1196 self.tcx().dep_graph.read_index(dep_node);
1200 // Treat negative impls as unimplemented
1201 fn filter_negative_impls(
1203 candidate: SelectionCandidate<'tcx>,
1204 ) -> SelectionResult<'tcx, SelectionCandidate<'tcx>> {
1205 if let ImplCandidate(def_id) = candidate {
1206 if !self.allow_negative_impls
1207 && self.tcx().impl_polarity(def_id) == hir::ImplPolarity::Negative
1209 return Err(Unimplemented);
1215 fn candidate_from_obligation_no_cache<'o>(
1217 stack: &TraitObligationStack<'o, 'tcx>,
1218 ) -> SelectionResult<'tcx, SelectionCandidate<'tcx>> {
1219 if stack.obligation.predicate.references_error() {
1220 // If we encounter a `Error`, we generally prefer the
1221 // most "optimistic" result in response -- that is, the
1222 // one least likely to report downstream errors. But
1223 // because this routine is shared by coherence and by
1224 // trait selection, there isn't an obvious "right" choice
1225 // here in that respect, so we opt to just return
1226 // ambiguity and let the upstream clients sort it out.
1230 if let Some(conflict) = self.is_knowable(stack) {
1231 debug!("coherence stage: not knowable");
1232 if self.intercrate_ambiguity_causes.is_some() {
1233 debug!("evaluate_stack: intercrate_ambiguity_causes is some");
1234 // Heuristics: show the diagnostics when there are no candidates in crate.
1235 if let Ok(candidate_set) = self.assemble_candidates(stack) {
1236 let mut no_candidates_apply = true;
1238 let evaluated_candidates = candidate_set
1241 .map(|c| self.evaluate_candidate(stack, &c));
1243 for ec in evaluated_candidates {
1247 no_candidates_apply = false;
1251 Err(e) => return Err(e.into()),
1256 if !candidate_set.ambiguous && no_candidates_apply {
1257 let trait_ref = stack.obligation.predicate.skip_binder().trait_ref;
1258 let self_ty = trait_ref.self_ty();
1259 let trait_desc = trait_ref.to_string();
1260 let self_desc = if self_ty.has_concrete_skeleton() {
1261 Some(self_ty.to_string())
1265 let cause = if let Conflict::Upstream = conflict {
1266 IntercrateAmbiguityCause::UpstreamCrateUpdate {
1271 IntercrateAmbiguityCause::DownstreamCrate {
1276 debug!("evaluate_stack: pushing cause = {:?}", cause);
1277 self.intercrate_ambiguity_causes
1287 let candidate_set = self.assemble_candidates(stack)?;
1289 if candidate_set.ambiguous {
1290 debug!("candidate set contains ambig");
1294 let mut candidates = candidate_set.vec;
1297 "assembled {} candidates for {:?}: {:?}",
1303 // At this point, we know that each of the entries in the
1304 // candidate set is *individually* applicable. Now we have to
1305 // figure out if they contain mutual incompatibilities. This
1306 // frequently arises if we have an unconstrained input type --
1307 // for example, we are looking for $0:Eq where $0 is some
1308 // unconstrained type variable. In that case, we'll get a
1309 // candidate which assumes $0 == int, one that assumes $0 ==
1310 // usize, etc. This spells an ambiguity.
1312 // If there is more than one candidate, first winnow them down
1313 // by considering extra conditions (nested obligations and so
1314 // forth). We don't winnow if there is exactly one
1315 // candidate. This is a relatively minor distinction but it
1316 // can lead to better inference and error-reporting. An
1317 // example would be if there was an impl:
1319 // impl<T:Clone> Vec<T> { fn push_clone(...) { ... } }
1321 // and we were to see some code `foo.push_clone()` where `boo`
1322 // is a `Vec<Bar>` and `Bar` does not implement `Clone`. If
1323 // we were to winnow, we'd wind up with zero candidates.
1324 // Instead, we select the right impl now but report `Bar does
1325 // not implement Clone`.
1326 if candidates.len() == 1 {
1327 return self.filter_negative_impls(candidates.pop().unwrap());
1330 // Winnow, but record the exact outcome of evaluation, which
1331 // is needed for specialization. Propagate overflow if it occurs.
1332 let mut candidates = candidates
1334 .map(|c| match self.evaluate_candidate(stack, &c) {
1335 Ok(eval) if eval.may_apply() => Ok(Some(EvaluatedCandidate {
1340 Err(OverflowError) => Err(Overflow),
1342 .flat_map(Result::transpose)
1343 .collect::<Result<Vec<_>, _>>()?;
1346 "winnowed to {} candidates for {:?}: {:?}",
1352 // If there are STILL multiple candidates, we can further
1353 // reduce the list by dropping duplicates -- including
1354 // resolving specializations.
1355 if candidates.len() > 1 {
1357 while i < candidates.len() {
1358 let is_dup = (0..candidates.len()).filter(|&j| i != j).any(|j| {
1359 self.candidate_should_be_dropped_in_favor_of(&candidates[i], &candidates[j])
1363 "Dropping candidate #{}/{}: {:?}",
1368 candidates.swap_remove(i);
1371 "Retaining candidate #{}/{}: {:?}",
1378 // If there are *STILL* multiple candidates, give up
1379 // and report ambiguity.
1381 debug!("multiple matches, ambig");
1388 // If there are *NO* candidates, then there are no impls --
1389 // that we know of, anyway. Note that in the case where there
1390 // are unbound type variables within the obligation, it might
1391 // be the case that you could still satisfy the obligation
1392 // from another crate by instantiating the type variables with
1393 // a type from another crate that does have an impl. This case
1394 // is checked for in `evaluate_stack` (and hence users
1395 // who might care about this case, like coherence, should use
1397 if candidates.is_empty() {
1398 return Err(Unimplemented);
1401 // Just one candidate left.
1402 self.filter_negative_impls(candidates.pop().unwrap().candidate)
1405 fn is_knowable<'o>(&mut self, stack: &TraitObligationStack<'o, 'tcx>) -> Option<Conflict> {
1406 debug!("is_knowable(intercrate={:?})", self.intercrate);
1408 if !self.intercrate.is_some() {
1412 let obligation = &stack.obligation;
1413 let predicate = self.infcx()
1414 .resolve_type_vars_if_possible(&obligation.predicate);
1416 // OK to skip binder because of the nature of the
1417 // trait-ref-is-knowable check, which does not care about
1419 let trait_ref = predicate.skip_binder().trait_ref;
1421 let result = coherence::trait_ref_is_knowable(self.tcx(), trait_ref);
1423 Some(Conflict::Downstream {
1424 used_to_be_broken: true,
1426 Some(IntercrateMode::Issue43355),
1427 ) = (result, self.intercrate)
1429 debug!("is_knowable: IGNORING conflict to be bug-compatible with #43355");
1436 /// Returns `true` if the global caches can be used.
1437 /// Do note that if the type itself is not in the
1438 /// global tcx, the local caches will be used.
1439 fn can_use_global_caches(&self, param_env: ty::ParamEnv<'tcx>) -> bool {
1440 // If there are any where-clauses in scope, then we always use
1441 // a cache local to this particular scope. Otherwise, we
1442 // switch to a global cache. We used to try and draw
1443 // finer-grained distinctions, but that led to a serious of
1444 // annoying and weird bugs like #22019 and #18290. This simple
1445 // rule seems to be pretty clearly safe and also still retains
1446 // a very high hit rate (~95% when compiling rustc).
1447 if !param_env.caller_bounds.is_empty() {
1451 // Avoid using the master cache during coherence and just rely
1452 // on the local cache. This effectively disables caching
1453 // during coherence. It is really just a simplification to
1454 // avoid us having to fear that coherence results "pollute"
1455 // the master cache. Since coherence executes pretty quickly,
1456 // it's not worth going to more trouble to increase the
1457 // hit-rate I don't think.
1458 if self.intercrate.is_some() {
1462 // Otherwise, we can use the global cache.
1466 fn check_candidate_cache(
1468 param_env: ty::ParamEnv<'tcx>,
1469 cache_fresh_trait_pred: &ty::PolyTraitPredicate<'tcx>,
1470 ) -> Option<SelectionResult<'tcx, SelectionCandidate<'tcx>>> {
1471 let tcx = self.tcx();
1472 let trait_ref = &cache_fresh_trait_pred.skip_binder().trait_ref;
1473 if self.can_use_global_caches(param_env) {
1474 let cache = tcx.selection_cache.hashmap.borrow();
1475 if let Some(cached) = cache.get(&trait_ref) {
1476 return Some(cached.get(tcx));
1484 .map(|v| v.get(tcx))
1487 /// Determines whether can we safely cache the result
1488 /// of selecting an obligation. This is almost always 'true',
1489 /// except when dealing with certain ParamCandidates.
1491 /// Ordinarily, a ParamCandidate will contain no inference variables,
1492 /// since it was usually produced directly from a DefId. However,
1493 /// certain cases (currently only librustdoc's blanket impl finder),
1494 /// a ParamEnv may be explicitly constructed with inference types.
1495 /// When this is the case, we do *not* want to cache the resulting selection
1496 /// candidate. This is due to the fact that it might not always be possible
1497 /// to equate the obligation's trait ref and the candidate's trait ref,
1498 /// if more constraints end up getting added to an inference variable.
1500 /// Because of this, we always want to re-run the full selection
1501 /// process for our obligation the next time we see it, since
1502 /// we might end up picking a different SelectionCandidate (or none at all)
1503 fn can_cache_candidate(&self,
1504 result: &SelectionResult<'tcx, SelectionCandidate<'tcx>>
1507 Ok(Some(SelectionCandidate::ParamCandidate(trait_ref))) => {
1508 !trait_ref.skip_binder().input_types().any(|t| t.walk().any(|t_| t_.is_ty_infer()))
1514 fn insert_candidate_cache(
1516 param_env: ty::ParamEnv<'tcx>,
1517 cache_fresh_trait_pred: ty::PolyTraitPredicate<'tcx>,
1518 dep_node: DepNodeIndex,
1519 candidate: SelectionResult<'tcx, SelectionCandidate<'tcx>>,
1521 let tcx = self.tcx();
1522 let trait_ref = cache_fresh_trait_pred.skip_binder().trait_ref;
1524 if !self.can_cache_candidate(&candidate) {
1525 debug!("insert_candidate_cache(trait_ref={:?}, candidate={:?} -\
1526 candidate is not cacheable", trait_ref, candidate);
1531 if self.can_use_global_caches(param_env) {
1532 if let Err(Overflow) = candidate {
1533 // Don't cache overflow globally; we only produce this
1534 // in certain modes.
1535 } else if let Some(trait_ref) = tcx.lift_to_global(&trait_ref) {
1536 if let Some(candidate) = tcx.lift_to_global(&candidate) {
1538 "insert_candidate_cache(trait_ref={:?}, candidate={:?}) global",
1539 trait_ref, candidate,
1541 // This may overwrite the cache with the same value
1545 .insert(trait_ref, WithDepNode::new(dep_node, candidate));
1552 "insert_candidate_cache(trait_ref={:?}, candidate={:?}) local",
1553 trait_ref, candidate,
1559 .insert(trait_ref, WithDepNode::new(dep_node, candidate));
1562 fn assemble_candidates<'o>(
1564 stack: &TraitObligationStack<'o, 'tcx>,
1565 ) -> Result<SelectionCandidateSet<'tcx>, SelectionError<'tcx>> {
1566 let TraitObligationStack { obligation, .. } = *stack;
1567 let ref obligation = Obligation {
1568 param_env: obligation.param_env,
1569 cause: obligation.cause.clone(),
1570 recursion_depth: obligation.recursion_depth,
1571 predicate: self.infcx()
1572 .resolve_type_vars_if_possible(&obligation.predicate),
1575 if obligation.predicate.skip_binder().self_ty().is_ty_var() {
1576 // Self is a type variable (e.g., `_: AsRef<str>`).
1578 // This is somewhat problematic, as the current scheme can't really
1579 // handle it turning to be a projection. This does end up as truly
1580 // ambiguous in most cases anyway.
1582 // Take the fast path out - this also improves
1583 // performance by preventing assemble_candidates_from_impls from
1584 // matching every impl for this trait.
1585 return Ok(SelectionCandidateSet {
1591 let mut candidates = SelectionCandidateSet {
1596 self.assemble_candidates_for_trait_alias(obligation, &mut candidates)?;
1598 // Other bounds. Consider both in-scope bounds from fn decl
1599 // and applicable impls. There is a certain set of precedence rules here.
1600 let def_id = obligation.predicate.def_id();
1601 let lang_items = self.tcx().lang_items();
1603 if lang_items.copy_trait() == Some(def_id) {
1605 "obligation self ty is {:?}",
1606 obligation.predicate.skip_binder().self_ty()
1609 // User-defined copy impls are permitted, but only for
1610 // structs and enums.
1611 self.assemble_candidates_from_impls(obligation, &mut candidates)?;
1613 // For other types, we'll use the builtin rules.
1614 let copy_conditions = self.copy_clone_conditions(obligation);
1615 self.assemble_builtin_bound_candidates(copy_conditions, &mut candidates)?;
1616 } else if lang_items.sized_trait() == Some(def_id) {
1617 // Sized is never implementable by end-users, it is
1618 // always automatically computed.
1619 let sized_conditions = self.sized_conditions(obligation);
1620 self.assemble_builtin_bound_candidates(sized_conditions, &mut candidates)?;
1621 } else if lang_items.unsize_trait() == Some(def_id) {
1622 self.assemble_candidates_for_unsizing(obligation, &mut candidates);
1624 if lang_items.clone_trait() == Some(def_id) {
1625 // Same builtin conditions as `Copy`, i.e., every type which has builtin support
1626 // for `Copy` also has builtin support for `Clone`, + tuples and arrays of `Clone`
1627 // types have builtin support for `Clone`.
1628 let clone_conditions = self.copy_clone_conditions(obligation);
1629 self.assemble_builtin_bound_candidates(clone_conditions, &mut candidates)?;
1632 self.assemble_generator_candidates(obligation, &mut candidates)?;
1633 self.assemble_closure_candidates(obligation, &mut candidates)?;
1634 self.assemble_fn_pointer_candidates(obligation, &mut candidates)?;
1635 self.assemble_candidates_from_impls(obligation, &mut candidates)?;
1636 self.assemble_candidates_from_object_ty(obligation, &mut candidates);
1639 self.assemble_candidates_from_projected_tys(obligation, &mut candidates);
1640 self.assemble_candidates_from_caller_bounds(stack, &mut candidates)?;
1641 // Auto implementations have lower priority, so we only
1642 // consider triggering a default if there is no other impl that can apply.
1643 if candidates.vec.is_empty() {
1644 self.assemble_candidates_from_auto_impls(obligation, &mut candidates)?;
1646 debug!("candidate list size: {}", candidates.vec.len());
1650 fn assemble_candidates_from_projected_tys(
1652 obligation: &TraitObligation<'tcx>,
1653 candidates: &mut SelectionCandidateSet<'tcx>,
1655 debug!("assemble_candidates_for_projected_tys({:?})", obligation);
1657 // before we go into the whole placeholder thing, just
1658 // quickly check if the self-type is a projection at all.
1659 match obligation.predicate.skip_binder().trait_ref.self_ty().sty {
1660 ty::Projection(_) | ty::Opaque(..) => {}
1661 ty::Infer(ty::TyVar(_)) => {
1663 obligation.cause.span,
1664 "Self=_ should have been handled by assemble_candidates"
1670 let result = self.infcx.probe(|snapshot| {
1671 self.match_projection_obligation_against_definition_bounds(
1678 candidates.vec.push(ProjectionCandidate);
1682 fn match_projection_obligation_against_definition_bounds(
1684 obligation: &TraitObligation<'tcx>,
1685 snapshot: &CombinedSnapshot<'_, 'tcx>,
1687 let poly_trait_predicate = self.infcx()
1688 .resolve_type_vars_if_possible(&obligation.predicate);
1689 let (placeholder_trait_predicate, placeholder_map) = self.infcx()
1690 .replace_bound_vars_with_placeholders(&poly_trait_predicate);
1692 "match_projection_obligation_against_definition_bounds: \
1693 placeholder_trait_predicate={:?}",
1694 placeholder_trait_predicate,
1697 let (def_id, substs) = match placeholder_trait_predicate.trait_ref.self_ty().sty {
1698 ty::Projection(ref data) => (data.trait_ref(self.tcx()).def_id, data.substs),
1699 ty::Opaque(def_id, substs) => (def_id, substs),
1702 obligation.cause.span,
1703 "match_projection_obligation_against_definition_bounds() called \
1704 but self-ty is not a projection: {:?}",
1705 placeholder_trait_predicate.trait_ref.self_ty()
1710 "match_projection_obligation_against_definition_bounds: \
1711 def_id={:?}, substs={:?}",
1715 let predicates_of = self.tcx().predicates_of(def_id);
1716 let bounds = predicates_of.instantiate(self.tcx(), substs);
1718 "match_projection_obligation_against_definition_bounds: \
1723 let matching_bound = util::elaborate_predicates(self.tcx(), bounds.predicates)
1726 self.infcx.probe(|_| {
1727 self.match_projection(
1730 placeholder_trait_predicate.trait_ref.clone(),
1738 "match_projection_obligation_against_definition_bounds: \
1739 matching_bound={:?}",
1742 match matching_bound {
1745 // Repeat the successful match, if any, this time outside of a probe.
1746 let result = self.match_projection(
1749 placeholder_trait_predicate.trait_ref.clone(),
1760 fn match_projection(
1762 obligation: &TraitObligation<'tcx>,
1763 trait_bound: ty::PolyTraitRef<'tcx>,
1764 placeholder_trait_ref: ty::TraitRef<'tcx>,
1765 placeholder_map: &PlaceholderMap<'tcx>,
1766 snapshot: &CombinedSnapshot<'_, 'tcx>,
1768 debug_assert!(!placeholder_trait_ref.has_escaping_bound_vars());
1770 .at(&obligation.cause, obligation.param_env)
1771 .sup(ty::Binder::dummy(placeholder_trait_ref), trait_bound)
1774 self.infcx.leak_check(false, placeholder_map, snapshot).is_ok()
1777 /// Given an obligation like `<SomeTrait for T>`, search the obligations that the caller
1778 /// supplied to find out whether it is listed among them.
1780 /// Never affects inference environment.
1781 fn assemble_candidates_from_caller_bounds<'o>(
1783 stack: &TraitObligationStack<'o, 'tcx>,
1784 candidates: &mut SelectionCandidateSet<'tcx>,
1785 ) -> Result<(), SelectionError<'tcx>> {
1787 "assemble_candidates_from_caller_bounds({:?})",
1791 let all_bounds = stack
1796 .filter_map(|o| o.to_opt_poly_trait_ref());
1798 // micro-optimization: filter out predicates relating to different
1800 let matching_bounds =
1801 all_bounds.filter(|p| p.def_id() == stack.obligation.predicate.def_id());
1803 // keep only those bounds which may apply, and propagate overflow if it occurs
1804 let mut param_candidates = vec![];
1805 for bound in matching_bounds {
1806 let wc = self.evaluate_where_clause(stack, bound.clone())?;
1808 param_candidates.push(ParamCandidate(bound));
1812 candidates.vec.extend(param_candidates);
1817 fn evaluate_where_clause<'o>(
1819 stack: &TraitObligationStack<'o, 'tcx>,
1820 where_clause_trait_ref: ty::PolyTraitRef<'tcx>,
1821 ) -> Result<EvaluationResult, OverflowError> {
1822 self.evaluation_probe(|this| {
1823 match this.match_where_clause_trait_ref(stack.obligation, where_clause_trait_ref) {
1824 Ok(obligations) => {
1825 this.evaluate_predicates_recursively(stack.list(), obligations.into_iter())
1827 Err(()) => Ok(EvaluatedToErr),
1832 fn assemble_generator_candidates(
1834 obligation: &TraitObligation<'tcx>,
1835 candidates: &mut SelectionCandidateSet<'tcx>,
1836 ) -> Result<(), SelectionError<'tcx>> {
1837 if self.tcx().lang_items().gen_trait() != Some(obligation.predicate.def_id()) {
1841 // OK to skip binder because the substs on generator types never
1842 // touch bound regions, they just capture the in-scope
1843 // type/region parameters
1844 let self_ty = *obligation.self_ty().skip_binder();
1846 ty::Generator(..) => {
1848 "assemble_generator_candidates: self_ty={:?} obligation={:?}",
1852 candidates.vec.push(GeneratorCandidate);
1854 ty::Infer(ty::TyVar(_)) => {
1855 debug!("assemble_generator_candidates: ambiguous self-type");
1856 candidates.ambiguous = true;
1864 /// Checks for the artificial impl that the compiler will create for an obligation like `X :
1865 /// FnMut<..>` where `X` is a closure type.
1867 /// Note: the type parameters on a closure candidate are modeled as *output* type
1868 /// parameters and hence do not affect whether this trait is a match or not. They will be
1869 /// unified during the confirmation step.
1870 fn assemble_closure_candidates(
1872 obligation: &TraitObligation<'tcx>,
1873 candidates: &mut SelectionCandidateSet<'tcx>,
1874 ) -> Result<(), SelectionError<'tcx>> {
1875 let kind = match self.tcx()
1877 .fn_trait_kind(obligation.predicate.def_id())
1885 // OK to skip binder because the substs on closure types never
1886 // touch bound regions, they just capture the in-scope
1887 // type/region parameters
1888 match obligation.self_ty().skip_binder().sty {
1889 ty::Closure(closure_def_id, closure_substs) => {
1891 "assemble_unboxed_candidates: kind={:?} obligation={:?}",
1894 match self.infcx.closure_kind(closure_def_id, closure_substs) {
1895 Some(closure_kind) => {
1897 "assemble_unboxed_candidates: closure_kind = {:?}",
1900 if closure_kind.extends(kind) {
1901 candidates.vec.push(ClosureCandidate);
1905 debug!("assemble_unboxed_candidates: closure_kind not yet known");
1906 candidates.vec.push(ClosureCandidate);
1910 ty::Infer(ty::TyVar(_)) => {
1911 debug!("assemble_unboxed_closure_candidates: ambiguous self-type");
1912 candidates.ambiguous = true;
1920 /// Implement one of the `Fn()` family for a fn pointer.
1921 fn assemble_fn_pointer_candidates(
1923 obligation: &TraitObligation<'tcx>,
1924 candidates: &mut SelectionCandidateSet<'tcx>,
1925 ) -> Result<(), SelectionError<'tcx>> {
1926 // We provide impl of all fn traits for fn pointers.
1929 .fn_trait_kind(obligation.predicate.def_id())
1935 // OK to skip binder because what we are inspecting doesn't involve bound regions
1936 let self_ty = *obligation.self_ty().skip_binder();
1938 ty::Infer(ty::TyVar(_)) => {
1939 debug!("assemble_fn_pointer_candidates: ambiguous self-type");
1940 candidates.ambiguous = true; // could wind up being a fn() type
1942 // provide an impl, but only for suitable `fn` pointers
1943 ty::FnDef(..) | ty::FnPtr(_) => {
1945 unsafety: hir::Unsafety::Normal,
1949 } = self_ty.fn_sig(self.tcx()).skip_binder()
1951 candidates.vec.push(FnPointerCandidate);
1960 /// Search for impls that might apply to `obligation`.
1961 fn assemble_candidates_from_impls(
1963 obligation: &TraitObligation<'tcx>,
1964 candidates: &mut SelectionCandidateSet<'tcx>,
1965 ) -> Result<(), SelectionError<'tcx>> {
1967 "assemble_candidates_from_impls(obligation={:?})",
1971 self.tcx().for_each_relevant_impl(
1972 obligation.predicate.def_id(),
1973 obligation.predicate.skip_binder().trait_ref.self_ty(),
1975 self.infcx.probe(|snapshot| {
1976 if let Ok(_substs) = self.match_impl(impl_def_id, obligation, snapshot)
1978 candidates.vec.push(ImplCandidate(impl_def_id));
1987 fn assemble_candidates_from_auto_impls(
1989 obligation: &TraitObligation<'tcx>,
1990 candidates: &mut SelectionCandidateSet<'tcx>,
1991 ) -> Result<(), SelectionError<'tcx>> {
1992 // OK to skip binder here because the tests we do below do not involve bound regions
1993 let self_ty = *obligation.self_ty().skip_binder();
1994 debug!("assemble_candidates_from_auto_impls(self_ty={:?})", self_ty);
1996 let def_id = obligation.predicate.def_id();
1998 if self.tcx().trait_is_auto(def_id) {
2000 ty::Dynamic(..) => {
2001 // For object types, we don't know what the closed
2002 // over types are. This means we conservatively
2003 // say nothing; a candidate may be added by
2004 // `assemble_candidates_from_object_ty`.
2006 ty::Foreign(..) => {
2007 // Since the contents of foreign types is unknown,
2008 // we don't add any `..` impl. Default traits could
2009 // still be provided by a manual implementation for
2010 // this trait and type.
2012 ty::Param(..) | ty::Projection(..) => {
2013 // In these cases, we don't know what the actual
2014 // type is. Therefore, we cannot break it down
2015 // into its constituent types. So we don't
2016 // consider the `..` impl but instead just add no
2017 // candidates: this means that typeck will only
2018 // succeed if there is another reason to believe
2019 // that this obligation holds. That could be a
2020 // where-clause or, in the case of an object type,
2021 // it could be that the object type lists the
2022 // trait (e.g., `Foo+Send : Send`). See
2023 // `compile-fail/typeck-default-trait-impl-send-param.rs`
2024 // for an example of a test case that exercises
2027 ty::Infer(ty::TyVar(_)) => {
2028 // the auto impl might apply, we don't know
2029 candidates.ambiguous = true;
2031 ty::Generator(_, _, movability)
2032 if self.tcx().lang_items().unpin_trait() == Some(def_id) =>
2035 hir::GeneratorMovability::Static => {
2036 // Immovable generators are never `Unpin`, so
2037 // suppress the normal auto-impl candidate for it.
2039 hir::GeneratorMovability::Movable => {
2040 // Movable generators are always `Unpin`, so add an
2041 // unconditional builtin candidate.
2042 candidates.vec.push(BuiltinCandidate {
2049 _ => candidates.vec.push(AutoImplCandidate(def_id.clone())),
2056 /// Search for impls that might apply to `obligation`.
2057 fn assemble_candidates_from_object_ty(
2059 obligation: &TraitObligation<'tcx>,
2060 candidates: &mut SelectionCandidateSet<'tcx>,
2063 "assemble_candidates_from_object_ty(self_ty={:?})",
2064 obligation.self_ty().skip_binder()
2067 self.infcx.probe(|_snapshot| {
2068 // The code below doesn't care about regions, and the
2069 // self-ty here doesn't escape this probe, so just erase
2071 let self_ty = self.tcx().erase_late_bound_regions(&obligation.self_ty());
2072 let poly_trait_ref = match self_ty.sty {
2073 ty::Dynamic(ref data, ..) => {
2074 if data.auto_traits()
2075 .any(|did| did == obligation.predicate.def_id())
2078 "assemble_candidates_from_object_ty: matched builtin bound, \
2081 candidates.vec.push(BuiltinObjectCandidate);
2085 if let Some(principal) = data.principal() {
2086 principal.with_self_ty(self.tcx(), self_ty)
2088 // Only auto-trait bounds exist.
2092 ty::Infer(ty::TyVar(_)) => {
2093 debug!("assemble_candidates_from_object_ty: ambiguous");
2094 candidates.ambiguous = true; // could wind up being an object type
2101 "assemble_candidates_from_object_ty: poly_trait_ref={:?}",
2105 // Count only those upcast versions that match the trait-ref
2106 // we are looking for. Specifically, do not only check for the
2107 // correct trait, but also the correct type parameters.
2108 // For example, we may be trying to upcast `Foo` to `Bar<i32>`,
2109 // but `Foo` is declared as `trait Foo : Bar<u32>`.
2110 let upcast_trait_refs = util::supertraits(self.tcx(), poly_trait_ref)
2111 .filter(|upcast_trait_ref| {
2112 self.infcx.probe(|_| {
2113 let upcast_trait_ref = upcast_trait_ref.clone();
2114 self.match_poly_trait_ref(obligation, upcast_trait_ref)
2120 if upcast_trait_refs > 1 {
2121 // Can be upcast in many ways; need more type information.
2122 candidates.ambiguous = true;
2123 } else if upcast_trait_refs == 1 {
2124 candidates.vec.push(ObjectCandidate);
2129 /// Search for unsizing that might apply to `obligation`.
2130 fn assemble_candidates_for_unsizing(
2132 obligation: &TraitObligation<'tcx>,
2133 candidates: &mut SelectionCandidateSet<'tcx>,
2135 // We currently never consider higher-ranked obligations e.g.
2136 // `for<'a> &'a T: Unsize<Trait+'a>` to be implemented. This is not
2137 // because they are a priori invalid, and we could potentially add support
2138 // for them later, it's just that there isn't really a strong need for it.
2139 // A `T: Unsize<U>` obligation is always used as part of a `T: CoerceUnsize<U>`
2140 // impl, and those are generally applied to concrete types.
2142 // That said, one might try to write a fn with a where clause like
2143 // for<'a> Foo<'a, T>: Unsize<Foo<'a, Trait>>
2144 // where the `'a` is kind of orthogonal to the relevant part of the `Unsize`.
2145 // Still, you'd be more likely to write that where clause as
2147 // so it seems ok if we (conservatively) fail to accept that `Unsize`
2148 // obligation above. Should be possible to extend this in the future.
2149 let source = match obligation.self_ty().no_bound_vars() {
2152 // Don't add any candidates if there are bound regions.
2156 let target = obligation
2164 "assemble_candidates_for_unsizing(source={:?}, target={:?})",
2168 let may_apply = match (&source.sty, &target.sty) {
2169 // Trait+Kx+'a -> Trait+Ky+'b (upcasts).
2170 (&ty::Dynamic(ref data_a, ..), &ty::Dynamic(ref data_b, ..)) => {
2171 // Upcasts permit two things:
2173 // 1. Dropping builtin bounds, e.g., `Foo+Send` to `Foo`
2174 // 2. Tightening the region bound, e.g., `Foo+'a` to `Foo+'b` if `'a : 'b`
2176 // Note that neither of these changes requires any
2177 // change at runtime. Eventually this will be
2180 // We always upcast when we can because of reason
2181 // #2 (region bounds).
2182 data_a.principal_def_id() == data_b.principal_def_id()
2183 && data_b.auto_traits()
2184 // All of a's auto traits need to be in b's auto traits.
2185 .all(|b| data_a.auto_traits().any(|a| a == b))
2189 (_, &ty::Dynamic(..)) => true,
2191 // Ambiguous handling is below T -> Trait, because inference
2192 // variables can still implement Unsize<Trait> and nested
2193 // obligations will have the final say (likely deferred).
2194 (&ty::Infer(ty::TyVar(_)), _) | (_, &ty::Infer(ty::TyVar(_))) => {
2195 debug!("assemble_candidates_for_unsizing: ambiguous");
2196 candidates.ambiguous = true;
2201 (&ty::Array(..), &ty::Slice(_)) => true,
2203 // Struct<T> -> Struct<U>.
2204 (&ty::Adt(def_id_a, _), &ty::Adt(def_id_b, _)) if def_id_a.is_struct() => {
2205 def_id_a == def_id_b
2208 // (.., T) -> (.., U).
2209 (&ty::Tuple(tys_a), &ty::Tuple(tys_b)) => tys_a.len() == tys_b.len(),
2215 candidates.vec.push(BuiltinUnsizeCandidate);
2219 fn assemble_candidates_for_trait_alias(
2221 obligation: &TraitObligation<'tcx>,
2222 candidates: &mut SelectionCandidateSet<'tcx>,
2223 ) -> Result<(), SelectionError<'tcx>> {
2224 // OK to skip binder here because the tests we do below do not involve bound regions
2225 let self_ty = *obligation.self_ty().skip_binder();
2226 debug!("assemble_candidates_for_trait_alias(self_ty={:?})", self_ty);
2228 let def_id = obligation.predicate.def_id();
2230 if self.tcx().is_trait_alias(def_id) {
2231 candidates.vec.push(TraitAliasCandidate(def_id.clone()));
2237 ///////////////////////////////////////////////////////////////////////////
2240 // Winnowing is the process of attempting to resolve ambiguity by
2241 // probing further. During the winnowing process, we unify all
2242 // type variables and then we also attempt to evaluate recursive
2243 // bounds to see if they are satisfied.
2245 /// Returns `true` if `victim` should be dropped in favor of
2246 /// `other`. Generally speaking we will drop duplicate
2247 /// candidates and prefer where-clause candidates.
2249 /// See the comment for "SelectionCandidate" for more details.
2250 fn candidate_should_be_dropped_in_favor_of<'o>(
2252 victim: &EvaluatedCandidate<'tcx>,
2253 other: &EvaluatedCandidate<'tcx>,
2255 if victim.candidate == other.candidate {
2259 // Check if a bound would previously have been removed when normalizing
2260 // the param_env so that it can be given the lowest priority. See
2261 // #50825 for the motivation for this.
2263 |cand: &ty::PolyTraitRef<'_>| cand.is_global() && !cand.has_late_bound_regions();
2265 match other.candidate {
2266 // Prefer BuiltinCandidate { has_nested: false } to anything else.
2267 // This is a fix for #53123 and prevents winnowing from accidentally extending the
2268 // lifetime of a variable.
2269 BuiltinCandidate { has_nested: false } => true,
2270 ParamCandidate(ref cand) => match victim.candidate {
2271 AutoImplCandidate(..) => {
2273 "default implementations shouldn't be recorded \
2274 when there are other valid candidates"
2277 // Prefer BuiltinCandidate { has_nested: false } to anything else.
2278 // This is a fix for #53123 and prevents winnowing from accidentally extending the
2279 // lifetime of a variable.
2280 BuiltinCandidate { has_nested: false } => false,
2283 | GeneratorCandidate
2284 | FnPointerCandidate
2285 | BuiltinObjectCandidate
2286 | BuiltinUnsizeCandidate
2287 | BuiltinCandidate { .. }
2288 | TraitAliasCandidate(..) => {
2289 // Global bounds from the where clause should be ignored
2290 // here (see issue #50825). Otherwise, we have a where
2291 // clause so don't go around looking for impls.
2294 ObjectCandidate | ProjectionCandidate => {
2295 // Arbitrarily give param candidates priority
2296 // over projection and object candidates.
2299 ParamCandidate(..) => false,
2301 ObjectCandidate | ProjectionCandidate => match victim.candidate {
2302 AutoImplCandidate(..) => {
2304 "default implementations shouldn't be recorded \
2305 when there are other valid candidates"
2308 // Prefer BuiltinCandidate { has_nested: false } to anything else.
2309 // This is a fix for #53123 and prevents winnowing from accidentally extending the
2310 // lifetime of a variable.
2311 BuiltinCandidate { has_nested: false } => false,
2314 | GeneratorCandidate
2315 | FnPointerCandidate
2316 | BuiltinObjectCandidate
2317 | BuiltinUnsizeCandidate
2318 | BuiltinCandidate { .. }
2319 | TraitAliasCandidate(..) => true,
2320 ObjectCandidate | ProjectionCandidate => {
2321 // Arbitrarily give param candidates priority
2322 // over projection and object candidates.
2325 ParamCandidate(ref cand) => is_global(cand),
2327 ImplCandidate(other_def) => {
2328 // See if we can toss out `victim` based on specialization.
2329 // This requires us to know *for sure* that the `other` impl applies
2330 // i.e., EvaluatedToOk:
2331 if other.evaluation.must_apply_modulo_regions() {
2332 match victim.candidate {
2333 ImplCandidate(victim_def) => {
2334 let tcx = self.tcx().global_tcx();
2335 return tcx.specializes((other_def, victim_def))
2336 || tcx.impls_are_allowed_to_overlap(
2337 other_def, victim_def).is_some();
2339 ParamCandidate(ref cand) => {
2340 // Prefer the impl to a global where clause candidate.
2341 return is_global(cand);
2350 | GeneratorCandidate
2351 | FnPointerCandidate
2352 | BuiltinObjectCandidate
2353 | BuiltinUnsizeCandidate
2354 | BuiltinCandidate { has_nested: true } => {
2355 match victim.candidate {
2356 ParamCandidate(ref cand) => {
2357 // Prefer these to a global where-clause bound
2358 // (see issue #50825)
2359 is_global(cand) && other.evaluation.must_apply_modulo_regions()
2368 ///////////////////////////////////////////////////////////////////////////
2371 // These cover the traits that are built-in to the language
2372 // itself: `Copy`, `Clone` and `Sized`.
2374 fn assemble_builtin_bound_candidates<'o>(
2376 conditions: BuiltinImplConditions<'tcx>,
2377 candidates: &mut SelectionCandidateSet<'tcx>,
2378 ) -> Result<(), SelectionError<'tcx>> {
2380 BuiltinImplConditions::Where(nested) => {
2381 debug!("builtin_bound: nested={:?}", nested);
2382 candidates.vec.push(BuiltinCandidate {
2383 has_nested: nested.skip_binder().len() > 0,
2386 BuiltinImplConditions::None => {}
2387 BuiltinImplConditions::Ambiguous => {
2388 debug!("assemble_builtin_bound_candidates: ambiguous builtin");
2389 candidates.ambiguous = true;
2396 fn sized_conditions(
2398 obligation: &TraitObligation<'tcx>,
2399 ) -> BuiltinImplConditions<'tcx> {
2400 use self::BuiltinImplConditions::{Ambiguous, None, Where};
2402 // NOTE: binder moved to (*)
2403 let self_ty = self.infcx
2404 .shallow_resolve(obligation.predicate.skip_binder().self_ty());
2407 ty::Infer(ty::IntVar(_))
2408 | ty::Infer(ty::FloatVar(_))
2419 | ty::GeneratorWitness(..)
2424 // safe for everything
2425 Where(ty::Binder::dummy(Vec::new()))
2428 ty::Str | ty::Slice(_) | ty::Dynamic(..) | ty::Foreign(..) => None,
2430 ty::Tuple(tys) => Where(ty::Binder::bind(tys.last().into_iter().cloned().collect())),
2432 ty::Adt(def, substs) => {
2433 let sized_crit = def.sized_constraint(self.tcx());
2434 // (*) binder moved here
2435 Where(ty::Binder::bind(
2438 .map(|ty| ty.subst(self.tcx(), substs))
2443 ty::Projection(_) | ty::Param(_) | ty::Opaque(..) => None,
2444 ty::Infer(ty::TyVar(_)) => Ambiguous,
2446 ty::UnnormalizedProjection(..)
2447 | ty::Placeholder(..)
2449 | ty::Infer(ty::FreshTy(_))
2450 | ty::Infer(ty::FreshIntTy(_))
2451 | ty::Infer(ty::FreshFloatTy(_)) => {
2453 "asked to assemble builtin bounds of unexpected type: {:?}",
2460 fn copy_clone_conditions(
2462 obligation: &TraitObligation<'tcx>,
2463 ) -> BuiltinImplConditions<'tcx> {
2464 // NOTE: binder moved to (*)
2465 let self_ty = self.infcx
2466 .shallow_resolve(obligation.predicate.skip_binder().self_ty());
2468 use self::BuiltinImplConditions::{Ambiguous, None, Where};
2471 ty::Infer(ty::IntVar(_))
2472 | ty::Infer(ty::FloatVar(_))
2475 | ty::Error => Where(ty::Binder::dummy(Vec::new())),
2484 | ty::Ref(_, _, hir::MutImmutable) => {
2485 // Implementations provided in libcore
2493 | ty::GeneratorWitness(..)
2495 | ty::Ref(_, _, hir::MutMutable) => None,
2497 ty::Array(element_ty, _) => {
2498 // (*) binder moved here
2499 Where(ty::Binder::bind(vec![element_ty]))
2503 // (*) binder moved here
2504 Where(ty::Binder::bind(tys.to_vec()))
2507 ty::Closure(def_id, substs) => {
2508 let trait_id = obligation.predicate.def_id();
2509 let is_copy_trait = Some(trait_id) == self.tcx().lang_items().copy_trait();
2510 let is_clone_trait = Some(trait_id) == self.tcx().lang_items().clone_trait();
2511 if is_copy_trait || is_clone_trait {
2512 Where(ty::Binder::bind(
2513 substs.upvar_tys(def_id, self.tcx()).collect(),
2520 ty::Adt(..) | ty::Projection(..) | ty::Param(..) | ty::Opaque(..) => {
2521 // Fallback to whatever user-defined impls exist in this case.
2525 ty::Infer(ty::TyVar(_)) => {
2526 // Unbound type variable. Might or might not have
2527 // applicable impls and so forth, depending on what
2528 // those type variables wind up being bound to.
2532 ty::UnnormalizedProjection(..)
2533 | ty::Placeholder(..)
2535 | ty::Infer(ty::FreshTy(_))
2536 | ty::Infer(ty::FreshIntTy(_))
2537 | ty::Infer(ty::FreshFloatTy(_)) => {
2539 "asked to assemble builtin bounds of unexpected type: {:?}",
2546 /// For default impls, we need to break apart a type into its
2547 /// "constituent types" -- meaning, the types that it contains.
2549 /// Here are some (simple) examples:
2552 /// (i32, u32) -> [i32, u32]
2553 /// Foo where struct Foo { x: i32, y: u32 } -> [i32, u32]
2554 /// Bar<i32> where struct Bar<T> { x: T, y: u32 } -> [i32, u32]
2555 /// Zed<i32> where enum Zed { A(T), B(u32) } -> [i32, u32]
2557 fn constituent_types_for_ty(&self, t: Ty<'tcx>) -> Vec<Ty<'tcx>> {
2567 | ty::Infer(ty::IntVar(_))
2568 | ty::Infer(ty::FloatVar(_))
2570 | ty::Char => Vec::new(),
2572 ty::UnnormalizedProjection(..)
2573 | ty::Placeholder(..)
2577 | ty::Projection(..)
2579 | ty::Infer(ty::TyVar(_))
2580 | ty::Infer(ty::FreshTy(_))
2581 | ty::Infer(ty::FreshIntTy(_))
2582 | ty::Infer(ty::FreshFloatTy(_)) => {
2584 "asked to assemble constituent types of unexpected type: {:?}",
2589 ty::RawPtr(ty::TypeAndMut { ty: element_ty, .. }) | ty::Ref(_, element_ty, _) => {
2593 ty::Array(element_ty, _) | ty::Slice(element_ty) => vec![element_ty],
2595 ty::Tuple(ref tys) => {
2596 // (T1, ..., Tn) -- meets any bound that all of T1...Tn meet
2600 ty::Closure(def_id, ref substs) => substs.upvar_tys(def_id, self.tcx()).collect(),
2602 ty::Generator(def_id, ref substs, _) => {
2603 let witness = substs.witness(def_id, self.tcx());
2605 .upvar_tys(def_id, self.tcx())
2606 .chain(iter::once(witness))
2610 ty::GeneratorWitness(types) => {
2611 // This is sound because no regions in the witness can refer to
2612 // the binder outside the witness. So we'll effectivly reuse
2613 // the implicit binder around the witness.
2614 types.skip_binder().to_vec()
2617 // for `PhantomData<T>`, we pass `T`
2618 ty::Adt(def, substs) if def.is_phantom_data() => substs.types().collect(),
2620 ty::Adt(def, substs) => def.all_fields().map(|f| f.ty(self.tcx(), substs)).collect(),
2622 ty::Opaque(def_id, substs) => {
2623 // We can resolve the `impl Trait` to its concrete type,
2624 // which enforces a DAG between the functions requiring
2625 // the auto trait bounds in question.
2626 vec![self.tcx().type_of(def_id).subst(self.tcx(), substs)]
2631 fn collect_predicates_for_types(
2633 param_env: ty::ParamEnv<'tcx>,
2634 cause: ObligationCause<'tcx>,
2635 recursion_depth: usize,
2636 trait_def_id: DefId,
2637 types: ty::Binder<Vec<Ty<'tcx>>>,
2638 ) -> Vec<PredicateObligation<'tcx>> {
2639 // Because the types were potentially derived from
2640 // higher-ranked obligations they may reference late-bound
2641 // regions. For example, `for<'a> Foo<&'a int> : Copy` would
2642 // yield a type like `for<'a> &'a int`. In general, we
2643 // maintain the invariant that we never manipulate bound
2644 // regions, so we have to process these bound regions somehow.
2646 // The strategy is to:
2648 // 1. Instantiate those regions to placeholder regions (e.g.,
2649 // `for<'a> &'a int` becomes `&0 int`.
2650 // 2. Produce something like `&'0 int : Copy`
2651 // 3. Re-bind the regions back to `for<'a> &'a int : Copy`
2658 let ty: ty::Binder<Ty<'tcx>> = ty::Binder::bind(ty); // <----/
2660 self.infcx.in_snapshot(|_| {
2661 let (skol_ty, _) = self.infcx
2662 .replace_bound_vars_with_placeholders(&ty);
2664 value: normalized_ty,
2666 } = project::normalize_with_depth(
2673 let skol_obligation = self.tcx().predicate_for_trait_def(
2681 obligations.push(skol_obligation);
2688 ///////////////////////////////////////////////////////////////////////////
2691 // Confirmation unifies the output type parameters of the trait
2692 // with the values found in the obligation, possibly yielding a
2693 // type error. See the [rustc guide] for more details.
2696 // https://rust-lang.github.io/rustc-guide/traits/resolution.html#confirmation
2698 fn confirm_candidate(
2700 obligation: &TraitObligation<'tcx>,
2701 candidate: SelectionCandidate<'tcx>,
2702 ) -> Result<Selection<'tcx>, SelectionError<'tcx>> {
2703 debug!("confirm_candidate({:?}, {:?})", obligation, candidate);
2706 BuiltinCandidate { has_nested } => {
2707 let data = self.confirm_builtin_candidate(obligation, has_nested);
2708 Ok(VtableBuiltin(data))
2711 ParamCandidate(param) => {
2712 let obligations = self.confirm_param_candidate(obligation, param);
2713 Ok(VtableParam(obligations))
2716 ImplCandidate(impl_def_id) => Ok(VtableImpl(self.confirm_impl_candidate(
2721 AutoImplCandidate(trait_def_id) => {
2722 let data = self.confirm_auto_impl_candidate(obligation, trait_def_id);
2723 Ok(VtableAutoImpl(data))
2726 ProjectionCandidate => {
2727 self.confirm_projection_candidate(obligation);
2728 Ok(VtableParam(Vec::new()))
2731 ClosureCandidate => {
2732 let vtable_closure = self.confirm_closure_candidate(obligation)?;
2733 Ok(VtableClosure(vtable_closure))
2736 GeneratorCandidate => {
2737 let vtable_generator = self.confirm_generator_candidate(obligation)?;
2738 Ok(VtableGenerator(vtable_generator))
2741 FnPointerCandidate => {
2742 let data = self.confirm_fn_pointer_candidate(obligation)?;
2743 Ok(VtableFnPointer(data))
2746 TraitAliasCandidate(alias_def_id) => {
2747 let data = self.confirm_trait_alias_candidate(obligation, alias_def_id);
2748 Ok(VtableTraitAlias(data))
2751 ObjectCandidate => {
2752 let data = self.confirm_object_candidate(obligation);
2753 Ok(VtableObject(data))
2756 BuiltinObjectCandidate => {
2757 // This indicates something like `(Trait+Send) :
2758 // Send`. In this case, we know that this holds
2759 // because that's what the object type is telling us,
2760 // and there's really no additional obligations to
2761 // prove and no types in particular to unify etc.
2762 Ok(VtableParam(Vec::new()))
2765 BuiltinUnsizeCandidate => {
2766 let data = self.confirm_builtin_unsize_candidate(obligation)?;
2767 Ok(VtableBuiltin(data))
2772 fn confirm_projection_candidate(&mut self, obligation: &TraitObligation<'tcx>) {
2773 self.infcx.in_snapshot(|snapshot| {
2775 self.match_projection_obligation_against_definition_bounds(
2783 fn confirm_param_candidate(
2785 obligation: &TraitObligation<'tcx>,
2786 param: ty::PolyTraitRef<'tcx>,
2787 ) -> Vec<PredicateObligation<'tcx>> {
2788 debug!("confirm_param_candidate({:?},{:?})", obligation, param);
2790 // During evaluation, we already checked that this
2791 // where-clause trait-ref could be unified with the obligation
2792 // trait-ref. Repeat that unification now without any
2793 // transactional boundary; it should not fail.
2794 match self.match_where_clause_trait_ref(obligation, param.clone()) {
2795 Ok(obligations) => obligations,
2798 "Where clause `{:?}` was applicable to `{:?}` but now is not",
2806 fn confirm_builtin_candidate(
2808 obligation: &TraitObligation<'tcx>,
2810 ) -> VtableBuiltinData<PredicateObligation<'tcx>> {
2812 "confirm_builtin_candidate({:?}, {:?})",
2813 obligation, has_nested
2816 let lang_items = self.tcx().lang_items();
2817 let obligations = if has_nested {
2818 let trait_def = obligation.predicate.def_id();
2819 let conditions = if Some(trait_def) == lang_items.sized_trait() {
2820 self.sized_conditions(obligation)
2821 } else if Some(trait_def) == lang_items.copy_trait() {
2822 self.copy_clone_conditions(obligation)
2823 } else if Some(trait_def) == lang_items.clone_trait() {
2824 self.copy_clone_conditions(obligation)
2826 bug!("unexpected builtin trait {:?}", trait_def)
2828 let nested = match conditions {
2829 BuiltinImplConditions::Where(nested) => nested,
2831 "obligation {:?} had matched a builtin impl but now doesn't",
2836 let cause = obligation.derived_cause(BuiltinDerivedObligation);
2837 self.collect_predicates_for_types(
2838 obligation.param_env,
2840 obligation.recursion_depth + 1,
2848 debug!("confirm_builtin_candidate: obligations={:?}", obligations);
2851 nested: obligations,
2855 /// This handles the case where a `auto trait Foo` impl is being used.
2856 /// The idea is that the impl applies to `X : Foo` if the following conditions are met:
2858 /// 1. For each constituent type `Y` in `X`, `Y : Foo` holds
2859 /// 2. For each where-clause `C` declared on `Foo`, `[Self => X] C` holds.
2860 fn confirm_auto_impl_candidate(
2862 obligation: &TraitObligation<'tcx>,
2863 trait_def_id: DefId,
2864 ) -> VtableAutoImplData<PredicateObligation<'tcx>> {
2866 "confirm_auto_impl_candidate({:?}, {:?})",
2867 obligation, trait_def_id
2870 let types = obligation.predicate.map_bound(|inner| {
2871 let self_ty = self.infcx.shallow_resolve(inner.self_ty());
2872 self.constituent_types_for_ty(self_ty)
2874 self.vtable_auto_impl(obligation, trait_def_id, types)
2877 /// See `confirm_auto_impl_candidate`.
2878 fn vtable_auto_impl(
2880 obligation: &TraitObligation<'tcx>,
2881 trait_def_id: DefId,
2882 nested: ty::Binder<Vec<Ty<'tcx>>>,
2883 ) -> VtableAutoImplData<PredicateObligation<'tcx>> {
2884 debug!("vtable_auto_impl: nested={:?}", nested);
2886 let cause = obligation.derived_cause(BuiltinDerivedObligation);
2887 let mut obligations = self.collect_predicates_for_types(
2888 obligation.param_env,
2890 obligation.recursion_depth + 1,
2895 let trait_obligations: Vec<PredicateObligation<'_>> = self.infcx.in_snapshot(|_| {
2896 let poly_trait_ref = obligation.predicate.to_poly_trait_ref();
2897 let (trait_ref, _) = self.infcx
2898 .replace_bound_vars_with_placeholders(&poly_trait_ref);
2899 let cause = obligation.derived_cause(ImplDerivedObligation);
2900 self.impl_or_trait_obligations(
2902 obligation.recursion_depth + 1,
2903 obligation.param_env,
2909 // Adds the predicates from the trait. Note that this contains a `Self: Trait`
2910 // predicate as usual. It won't have any effect since auto traits are coinductive.
2911 obligations.extend(trait_obligations);
2913 debug!("vtable_auto_impl: obligations={:?}", obligations);
2915 VtableAutoImplData {
2917 nested: obligations,
2921 fn confirm_impl_candidate(
2923 obligation: &TraitObligation<'tcx>,
2925 ) -> VtableImplData<'tcx, PredicateObligation<'tcx>> {
2926 debug!("confirm_impl_candidate({:?},{:?})", obligation, impl_def_id);
2928 // First, create the substitutions by matching the impl again,
2929 // this time not in a probe.
2930 self.infcx.in_snapshot(|snapshot| {
2931 let substs = self.rematch_impl(impl_def_id, obligation, snapshot);
2932 debug!("confirm_impl_candidate: substs={:?}", substs);
2933 let cause = obligation.derived_cause(ImplDerivedObligation);
2938 obligation.recursion_depth + 1,
2939 obligation.param_env,
2947 mut substs: Normalized<'tcx, &'tcx Substs<'tcx>>,
2948 cause: ObligationCause<'tcx>,
2949 recursion_depth: usize,
2950 param_env: ty::ParamEnv<'tcx>,
2951 ) -> VtableImplData<'tcx, PredicateObligation<'tcx>> {
2953 "vtable_impl(impl_def_id={:?}, substs={:?}, recursion_depth={})",
2954 impl_def_id, substs, recursion_depth,
2957 let mut impl_obligations = self.impl_or_trait_obligations(
2966 "vtable_impl: impl_def_id={:?} impl_obligations={:?}",
2967 impl_def_id, impl_obligations
2970 // Because of RFC447, the impl-trait-ref and obligations
2971 // are sufficient to determine the impl substs, without
2972 // relying on projections in the impl-trait-ref.
2974 // e.g., `impl<U: Tr, V: Iterator<Item=U>> Foo<<U as Tr>::T> for V`
2975 impl_obligations.append(&mut substs.obligations);
2979 substs: substs.value,
2980 nested: impl_obligations,
2984 fn confirm_object_candidate(
2986 obligation: &TraitObligation<'tcx>,
2987 ) -> VtableObjectData<'tcx, PredicateObligation<'tcx>> {
2988 debug!("confirm_object_candidate({:?})", obligation);
2990 // FIXME(nmatsakis) skipping binder here seems wrong -- we should
2991 // probably flatten the binder from the obligation and the binder
2992 // from the object. Have to try to make a broken test case that
2994 let self_ty = self.infcx
2995 .shallow_resolve(*obligation.self_ty().skip_binder());
2996 let poly_trait_ref = match self_ty.sty {
2997 ty::Dynamic(ref data, ..) =>
2998 data.principal().unwrap_or_else(|| {
2999 span_bug!(obligation.cause.span, "object candidate with no principal")
3000 }).with_self_ty(self.tcx(), self_ty),
3001 _ => span_bug!(obligation.cause.span, "object candidate with non-object"),
3004 let mut upcast_trait_ref = None;
3005 let mut nested = vec![];
3009 let tcx = self.tcx();
3011 // We want to find the first supertrait in the list of
3012 // supertraits that we can unify with, and do that
3013 // unification. We know that there is exactly one in the list
3014 // where we can unify because otherwise select would have
3015 // reported an ambiguity. (When we do find a match, also
3016 // record it for later.)
3017 let nonmatching = util::supertraits(tcx, poly_trait_ref).take_while(
3018 |&t| match self.infcx.commit_if_ok(|_| self.match_poly_trait_ref(obligation, t)) {
3019 Ok(obligations) => {
3020 upcast_trait_ref = Some(t);
3021 nested.extend(obligations);
3028 // Additionally, for each of the nonmatching predicates that
3029 // we pass over, we sum up the set of number of vtable
3030 // entries, so that we can compute the offset for the selected
3032 vtable_base = nonmatching.map(|t| tcx.count_own_vtable_entries(t)).sum();
3036 upcast_trait_ref: upcast_trait_ref.unwrap(),
3042 fn confirm_fn_pointer_candidate(
3044 obligation: &TraitObligation<'tcx>,
3045 ) -> Result<VtableFnPointerData<'tcx, PredicateObligation<'tcx>>, SelectionError<'tcx>> {
3046 debug!("confirm_fn_pointer_candidate({:?})", obligation);
3048 // OK to skip binder; it is reintroduced below
3049 let self_ty = self.infcx
3050 .shallow_resolve(*obligation.self_ty().skip_binder());
3051 let sig = self_ty.fn_sig(self.tcx());
3052 let trait_ref = self.tcx()
3053 .closure_trait_ref_and_return_type(
3054 obligation.predicate.def_id(),
3057 util::TupleArgumentsFlag::Yes,
3059 .map_bound(|(trait_ref, _)| trait_ref);
3064 } = project::normalize_with_depth(
3066 obligation.param_env,
3067 obligation.cause.clone(),
3068 obligation.recursion_depth + 1,
3072 self.confirm_poly_trait_refs(
3073 obligation.cause.clone(),
3074 obligation.param_env,
3075 obligation.predicate.to_poly_trait_ref(),
3078 Ok(VtableFnPointerData {
3080 nested: obligations,
3084 fn confirm_trait_alias_candidate(
3086 obligation: &TraitObligation<'tcx>,
3087 alias_def_id: DefId,
3088 ) -> VtableTraitAliasData<'tcx, PredicateObligation<'tcx>> {
3090 "confirm_trait_alias_candidate({:?}, {:?})",
3091 obligation, alias_def_id
3094 self.infcx.in_snapshot(|_| {
3095 let (predicate, _) = self.infcx()
3096 .replace_bound_vars_with_placeholders(&obligation.predicate);
3097 let trait_ref = predicate.trait_ref;
3098 let trait_def_id = trait_ref.def_id;
3099 let substs = trait_ref.substs;
3101 let trait_obligations = self.impl_or_trait_obligations(
3102 obligation.cause.clone(),
3103 obligation.recursion_depth,
3104 obligation.param_env,
3110 "confirm_trait_alias_candidate: trait_def_id={:?} trait_obligations={:?}",
3111 trait_def_id, trait_obligations
3114 VtableTraitAliasData {
3117 nested: trait_obligations,
3122 fn confirm_generator_candidate(
3124 obligation: &TraitObligation<'tcx>,
3125 ) -> Result<VtableGeneratorData<'tcx, PredicateObligation<'tcx>>, SelectionError<'tcx>> {
3126 // OK to skip binder because the substs on generator types never
3127 // touch bound regions, they just capture the in-scope
3128 // type/region parameters
3129 let self_ty = self.infcx
3130 .shallow_resolve(obligation.self_ty().skip_binder());
3131 let (generator_def_id, substs) = match self_ty.sty {
3132 ty::Generator(id, substs, _) => (id, substs),
3133 _ => bug!("closure candidate for non-closure {:?}", obligation),
3137 "confirm_generator_candidate({:?},{:?},{:?})",
3138 obligation, generator_def_id, substs
3141 let trait_ref = self.generator_trait_ref_unnormalized(obligation, generator_def_id, substs);
3145 } = normalize_with_depth(
3147 obligation.param_env,
3148 obligation.cause.clone(),
3149 obligation.recursion_depth + 1,
3154 "confirm_generator_candidate(generator_def_id={:?}, \
3155 trait_ref={:?}, obligations={:?})",
3156 generator_def_id, trait_ref, obligations
3159 obligations.extend(self.confirm_poly_trait_refs(
3160 obligation.cause.clone(),
3161 obligation.param_env,
3162 obligation.predicate.to_poly_trait_ref(),
3166 Ok(VtableGeneratorData {
3167 generator_def_id: generator_def_id,
3168 substs: substs.clone(),
3169 nested: obligations,
3173 fn confirm_closure_candidate(
3175 obligation: &TraitObligation<'tcx>,
3176 ) -> Result<VtableClosureData<'tcx, PredicateObligation<'tcx>>, SelectionError<'tcx>> {
3177 debug!("confirm_closure_candidate({:?})", obligation);
3179 let kind = self.tcx()
3181 .fn_trait_kind(obligation.predicate.def_id())
3182 .unwrap_or_else(|| bug!("closure candidate for non-fn trait {:?}", obligation));
3184 // OK to skip binder because the substs on closure types never
3185 // touch bound regions, they just capture the in-scope
3186 // type/region parameters
3187 let self_ty = self.infcx
3188 .shallow_resolve(obligation.self_ty().skip_binder());
3189 let (closure_def_id, substs) = match self_ty.sty {
3190 ty::Closure(id, substs) => (id, substs),
3191 _ => bug!("closure candidate for non-closure {:?}", obligation),
3194 let trait_ref = self.closure_trait_ref_unnormalized(obligation, closure_def_id, substs);
3198 } = normalize_with_depth(
3200 obligation.param_env,
3201 obligation.cause.clone(),
3202 obligation.recursion_depth + 1,
3207 "confirm_closure_candidate(closure_def_id={:?}, trait_ref={:?}, obligations={:?})",
3208 closure_def_id, trait_ref, obligations
3211 obligations.extend(self.confirm_poly_trait_refs(
3212 obligation.cause.clone(),
3213 obligation.param_env,
3214 obligation.predicate.to_poly_trait_ref(),
3219 if !self.tcx().sess.opts.debugging_opts.chalk {
3220 obligations.push(Obligation::new(
3221 obligation.cause.clone(),
3222 obligation.param_env,
3223 ty::Predicate::ClosureKind(closure_def_id, substs, kind),
3227 Ok(VtableClosureData {
3229 substs: substs.clone(),
3230 nested: obligations,
3234 /// In the case of closure types and fn pointers,
3235 /// we currently treat the input type parameters on the trait as
3236 /// outputs. This means that when we have a match we have only
3237 /// considered the self type, so we have to go back and make sure
3238 /// to relate the argument types too. This is kind of wrong, but
3239 /// since we control the full set of impls, also not that wrong,
3240 /// and it DOES yield better error messages (since we don't report
3241 /// errors as if there is no applicable impl, but rather report
3242 /// errors are about mismatched argument types.
3244 /// Here is an example. Imagine we have a closure expression
3245 /// and we desugared it so that the type of the expression is
3246 /// `Closure`, and `Closure` expects an int as argument. Then it
3247 /// is "as if" the compiler generated this impl:
3249 /// impl Fn(int) for Closure { ... }
3251 /// Now imagine our obligation is `Fn(usize) for Closure`. So far
3252 /// we have matched the self type `Closure`. At this point we'll
3253 /// compare the `int` to `usize` and generate an error.
3255 /// Note that this checking occurs *after* the impl has selected,
3256 /// because these output type parameters should not affect the
3257 /// selection of the impl. Therefore, if there is a mismatch, we
3258 /// report an error to the user.
3259 fn confirm_poly_trait_refs(
3261 obligation_cause: ObligationCause<'tcx>,
3262 obligation_param_env: ty::ParamEnv<'tcx>,
3263 obligation_trait_ref: ty::PolyTraitRef<'tcx>,
3264 expected_trait_ref: ty::PolyTraitRef<'tcx>,
3265 ) -> Result<Vec<PredicateObligation<'tcx>>, SelectionError<'tcx>> {
3266 let obligation_trait_ref = obligation_trait_ref.clone();
3268 .at(&obligation_cause, obligation_param_env)
3269 .sup(obligation_trait_ref, expected_trait_ref)
3270 .map(|InferOk { obligations, .. }| obligations)
3271 .map_err(|e| OutputTypeParameterMismatch(expected_trait_ref, obligation_trait_ref, e))
3274 fn confirm_builtin_unsize_candidate(
3276 obligation: &TraitObligation<'tcx>,
3277 ) -> Result<VtableBuiltinData<PredicateObligation<'tcx>>, SelectionError<'tcx>> {
3278 let tcx = self.tcx();
3280 // assemble_candidates_for_unsizing should ensure there are no late bound
3281 // regions here. See the comment there for more details.
3282 let source = self.infcx
3283 .shallow_resolve(obligation.self_ty().no_bound_vars().unwrap());
3284 let target = obligation
3290 let target = self.infcx.shallow_resolve(target);
3293 "confirm_builtin_unsize_candidate(source={:?}, target={:?})",
3297 let mut nested = vec![];
3298 match (&source.sty, &target.sty) {
3299 // Trait+Kx+'a -> Trait+Ky+'b (upcasts).
3300 (&ty::Dynamic(ref data_a, r_a), &ty::Dynamic(ref data_b, r_b)) => {
3301 // See assemble_candidates_for_unsizing for more info.
3302 let existential_predicates = data_a.map_bound(|data_a| {
3304 data_a.principal().map(|x| ty::ExistentialPredicate::Trait(x))
3307 .projection_bounds()
3308 .map(|x| ty::ExistentialPredicate::Projection(x)),
3313 .map(ty::ExistentialPredicate::AutoTrait),
3315 tcx.mk_existential_predicates(iter)
3317 let source_trait = tcx.mk_dynamic(existential_predicates, r_b);
3318 let InferOk { obligations, .. } = self.infcx
3319 .at(&obligation.cause, obligation.param_env)
3320 .sup(target, source_trait)
3321 .map_err(|_| Unimplemented)?;
3322 nested.extend(obligations);
3324 // Register one obligation for 'a: 'b.
3325 let cause = ObligationCause::new(
3326 obligation.cause.span,
3327 obligation.cause.body_id,
3328 ObjectCastObligation(target),
3330 let outlives = ty::OutlivesPredicate(r_a, r_b);
3331 nested.push(Obligation::with_depth(
3333 obligation.recursion_depth + 1,
3334 obligation.param_env,
3335 ty::Binder::bind(outlives).to_predicate(),
3340 (_, &ty::Dynamic(ref data, r)) => {
3341 let mut object_dids = data.auto_traits()
3342 .chain(data.principal_def_id());
3343 if let Some(did) = object_dids.find(|did| !tcx.is_object_safe(*did)) {
3344 return Err(TraitNotObjectSafe(did));
3347 let cause = ObligationCause::new(
3348 obligation.cause.span,
3349 obligation.cause.body_id,
3350 ObjectCastObligation(target),
3353 let predicate_to_obligation = |predicate| {
3354 Obligation::with_depth(
3356 obligation.recursion_depth + 1,
3357 obligation.param_env,
3362 // Create obligations:
3363 // - Casting T to Trait
3364 // - For all the various builtin bounds attached to the object cast. (In other
3365 // words, if the object type is Foo+Send, this would create an obligation for the
3367 // - Projection predicates
3370 .map(|d| predicate_to_obligation(d.with_self_ty(tcx, source))),
3373 // We can only make objects from sized types.
3374 let tr = ty::TraitRef {
3375 def_id: tcx.require_lang_item(lang_items::SizedTraitLangItem),
3376 substs: tcx.mk_substs_trait(source, &[]),
3378 nested.push(predicate_to_obligation(tr.to_predicate()));
3380 // If the type is `Foo+'a`, ensures that the type
3381 // being cast to `Foo+'a` outlives `'a`:
3382 let outlives = ty::OutlivesPredicate(source, r);
3383 nested.push(predicate_to_obligation(
3384 ty::Binder::dummy(outlives).to_predicate(),
3389 (&ty::Array(a, _), &ty::Slice(b)) => {
3390 let InferOk { obligations, .. } = self.infcx
3391 .at(&obligation.cause, obligation.param_env)
3393 .map_err(|_| Unimplemented)?;
3394 nested.extend(obligations);
3397 // Struct<T> -> Struct<U>.
3398 (&ty::Adt(def, substs_a), &ty::Adt(_, substs_b)) => {
3399 let fields = def.all_fields()
3400 .map(|f| tcx.type_of(f.did))
3401 .collect::<Vec<_>>();
3403 // The last field of the structure has to exist and contain type parameters.
3404 let field = if let Some(&field) = fields.last() {
3407 return Err(Unimplemented);
3409 let mut ty_params = GrowableBitSet::new_empty();
3410 let mut found = false;
3411 for ty in field.walk() {
3412 if let ty::Param(p) = ty.sty {
3413 ty_params.insert(p.idx as usize);
3418 return Err(Unimplemented);
3421 // Replace type parameters used in unsizing with
3422 // Error and ensure they do not affect any other fields.
3423 // This could be checked after type collection for any struct
3424 // with a potentially unsized trailing field.
3425 let params = substs_a.iter().enumerate().map(|(i, &k)| {
3426 if ty_params.contains(i) {
3427 tcx.types.err.into()
3432 let substs = tcx.mk_substs(params);
3433 for &ty in fields.split_last().unwrap().1 {
3434 if ty.subst(tcx, substs).references_error() {
3435 return Err(Unimplemented);
3439 // Extract Field<T> and Field<U> from Struct<T> and Struct<U>.
3440 let inner_source = field.subst(tcx, substs_a);
3441 let inner_target = field.subst(tcx, substs_b);
3443 // Check that the source struct with the target's
3444 // unsized parameters is equal to the target.
3445 let params = substs_a.iter().enumerate().map(|(i, &k)| {
3446 if ty_params.contains(i) {
3447 substs_b.type_at(i).into()
3452 let new_struct = tcx.mk_adt(def, tcx.mk_substs(params));
3453 let InferOk { obligations, .. } = self.infcx
3454 .at(&obligation.cause, obligation.param_env)
3455 .eq(target, new_struct)
3456 .map_err(|_| Unimplemented)?;
3457 nested.extend(obligations);
3459 // Construct the nested Field<T>: Unsize<Field<U>> predicate.
3460 nested.push(tcx.predicate_for_trait_def(
3461 obligation.param_env,
3462 obligation.cause.clone(),
3463 obligation.predicate.def_id(),
3464 obligation.recursion_depth + 1,
3466 &[inner_target.into()],
3470 // (.., T) -> (.., U).
3471 (&ty::Tuple(tys_a), &ty::Tuple(tys_b)) => {
3472 assert_eq!(tys_a.len(), tys_b.len());
3474 // The last field of the tuple has to exist.
3475 let (&a_last, a_mid) = if let Some(x) = tys_a.split_last() {
3478 return Err(Unimplemented);
3480 let &b_last = tys_b.last().unwrap();
3482 // Check that the source tuple with the target's
3483 // last element is equal to the target.
3484 let new_tuple = tcx.mk_tup(a_mid.iter().cloned().chain(iter::once(b_last)));
3485 let InferOk { obligations, .. } = self.infcx
3486 .at(&obligation.cause, obligation.param_env)
3487 .eq(target, new_tuple)
3488 .map_err(|_| Unimplemented)?;
3489 nested.extend(obligations);
3491 // Construct the nested T: Unsize<U> predicate.
3492 nested.push(tcx.predicate_for_trait_def(
3493 obligation.param_env,
3494 obligation.cause.clone(),
3495 obligation.predicate.def_id(),
3496 obligation.recursion_depth + 1,
3505 Ok(VtableBuiltinData { nested })
3508 ///////////////////////////////////////////////////////////////////////////
3511 // Matching is a common path used for both evaluation and
3512 // confirmation. It basically unifies types that appear in impls
3513 // and traits. This does affect the surrounding environment;
3514 // therefore, when used during evaluation, match routines must be
3515 // run inside of a `probe()` so that their side-effects are
3521 obligation: &TraitObligation<'tcx>,
3522 snapshot: &CombinedSnapshot<'_, 'tcx>,
3523 ) -> Normalized<'tcx, &'tcx Substs<'tcx>> {
3524 match self.match_impl(impl_def_id, obligation, snapshot) {
3525 Ok(substs) => substs,
3528 "Impl {:?} was matchable against {:?} but now is not",
3539 obligation: &TraitObligation<'tcx>,
3540 snapshot: &CombinedSnapshot<'_, 'tcx>,
3541 ) -> Result<Normalized<'tcx, &'tcx Substs<'tcx>>, ()> {
3542 let impl_trait_ref = self.tcx().impl_trait_ref(impl_def_id).unwrap();
3544 // Before we create the substitutions and everything, first
3545 // consider a "quick reject". This avoids creating more types
3546 // and so forth that we need to.
3547 if self.fast_reject_trait_refs(obligation, &impl_trait_ref) {
3551 let (skol_obligation, placeholder_map) = self.infcx()
3552 .replace_bound_vars_with_placeholders(&obligation.predicate);
3553 let skol_obligation_trait_ref = skol_obligation.trait_ref;
3555 let impl_substs = self.infcx
3556 .fresh_substs_for_item(obligation.cause.span, impl_def_id);
3558 let impl_trait_ref = impl_trait_ref.subst(self.tcx(), impl_substs);
3561 value: impl_trait_ref,
3562 obligations: mut nested_obligations,
3563 } = project::normalize_with_depth(
3565 obligation.param_env,
3566 obligation.cause.clone(),
3567 obligation.recursion_depth + 1,
3572 "match_impl(impl_def_id={:?}, obligation={:?}, \
3573 impl_trait_ref={:?}, skol_obligation_trait_ref={:?})",
3574 impl_def_id, obligation, impl_trait_ref, skol_obligation_trait_ref
3577 let InferOk { obligations, .. } = self.infcx
3578 .at(&obligation.cause, obligation.param_env)
3579 .eq(skol_obligation_trait_ref, impl_trait_ref)
3580 .map_err(|e| debug!("match_impl: failed eq_trait_refs due to `{}`", e))?;
3581 nested_obligations.extend(obligations);
3583 if let Err(e) = self.infcx.leak_check(false, &placeholder_map, snapshot) {
3584 debug!("match_impl: failed leak check due to `{}`", e);
3588 debug!("match_impl: success impl_substs={:?}", impl_substs);
3591 obligations: nested_obligations,
3595 fn fast_reject_trait_refs(
3597 obligation: &TraitObligation<'_>,
3598 impl_trait_ref: &ty::TraitRef<'_>,
3600 // We can avoid creating type variables and doing the full
3601 // substitution if we find that any of the input types, when
3602 // simplified, do not match.
3608 .zip(impl_trait_ref.input_types())
3609 .any(|(obligation_ty, impl_ty)| {
3610 let simplified_obligation_ty =
3611 fast_reject::simplify_type(self.tcx(), obligation_ty, true);
3612 let simplified_impl_ty = fast_reject::simplify_type(self.tcx(), impl_ty, false);
3614 simplified_obligation_ty.is_some()
3615 && simplified_impl_ty.is_some()
3616 && simplified_obligation_ty != simplified_impl_ty
3620 /// Normalize `where_clause_trait_ref` and try to match it against
3621 /// `obligation`. If successful, return any predicates that
3622 /// result from the normalization. Normalization is necessary
3623 /// because where-clauses are stored in the parameter environment
3625 fn match_where_clause_trait_ref(
3627 obligation: &TraitObligation<'tcx>,
3628 where_clause_trait_ref: ty::PolyTraitRef<'tcx>,
3629 ) -> Result<Vec<PredicateObligation<'tcx>>, ()> {
3630 self.match_poly_trait_ref(obligation, where_clause_trait_ref)
3633 /// Returns `Ok` if `poly_trait_ref` being true implies that the
3634 /// obligation is satisfied.
3635 fn match_poly_trait_ref(
3637 obligation: &TraitObligation<'tcx>,
3638 poly_trait_ref: ty::PolyTraitRef<'tcx>,
3639 ) -> Result<Vec<PredicateObligation<'tcx>>, ()> {
3641 "match_poly_trait_ref: obligation={:?} poly_trait_ref={:?}",
3642 obligation, poly_trait_ref
3646 .at(&obligation.cause, obligation.param_env)
3647 .sup(obligation.predicate.to_poly_trait_ref(), poly_trait_ref)
3648 .map(|InferOk { obligations, .. }| obligations)
3652 ///////////////////////////////////////////////////////////////////////////
3655 fn match_fresh_trait_refs(
3657 previous: &ty::PolyTraitRef<'tcx>,
3658 current: &ty::PolyTraitRef<'tcx>,
3660 let mut matcher = ty::_match::Match::new(self.tcx());
3661 matcher.relate(previous, current).is_ok()
3664 fn push_stack<'o, 's: 'o>(
3666 previous_stack: TraitObligationStackList<'s, 'tcx>,
3667 obligation: &'o TraitObligation<'tcx>,
3668 ) -> TraitObligationStack<'o, 'tcx> {
3669 let fresh_trait_ref = obligation
3671 .to_poly_trait_ref()
3672 .fold_with(&mut self.freshener);
3674 TraitObligationStack {
3677 previous: previous_stack,
3681 fn closure_trait_ref_unnormalized(
3683 obligation: &TraitObligation<'tcx>,
3684 closure_def_id: DefId,
3685 substs: ty::ClosureSubsts<'tcx>,
3686 ) -> ty::PolyTraitRef<'tcx> {
3688 "closure_trait_ref_unnormalized(obligation={:?}, closure_def_id={:?}, substs={:?})",
3689 obligation, closure_def_id, substs,
3691 let closure_type = self.infcx.closure_sig(closure_def_id, substs);
3694 "closure_trait_ref_unnormalized: closure_type = {:?}",
3698 // (1) Feels icky to skip the binder here, but OTOH we know
3699 // that the self-type is an unboxed closure type and hence is
3700 // in fact unparameterized (or at least does not reference any
3701 // regions bound in the obligation). Still probably some
3702 // refactoring could make this nicer.
3704 .closure_trait_ref_and_return_type(
3705 obligation.predicate.def_id(),
3706 obligation.predicate.skip_binder().self_ty(), // (1)
3708 util::TupleArgumentsFlag::No,
3710 .map_bound(|(trait_ref, _)| trait_ref)
3713 fn generator_trait_ref_unnormalized(
3715 obligation: &TraitObligation<'tcx>,
3716 closure_def_id: DefId,
3717 substs: ty::GeneratorSubsts<'tcx>,
3718 ) -> ty::PolyTraitRef<'tcx> {
3719 let gen_sig = substs.poly_sig(closure_def_id, self.tcx());
3721 // (1) Feels icky to skip the binder here, but OTOH we know
3722 // that the self-type is an generator type and hence is
3723 // in fact unparameterized (or at least does not reference any
3724 // regions bound in the obligation). Still probably some
3725 // refactoring could make this nicer.
3728 .generator_trait_ref_and_outputs(
3729 obligation.predicate.def_id(),
3730 obligation.predicate.skip_binder().self_ty(), // (1)
3733 .map_bound(|(trait_ref, ..)| trait_ref)
3736 /// Returns the obligations that are implied by instantiating an
3737 /// impl or trait. The obligations are substituted and fully
3738 /// normalized. This is used when confirming an impl or default
3740 fn impl_or_trait_obligations(
3742 cause: ObligationCause<'tcx>,
3743 recursion_depth: usize,
3744 param_env: ty::ParamEnv<'tcx>,
3745 def_id: DefId, // of impl or trait
3746 substs: &Substs<'tcx>, // for impl or trait
3747 ) -> Vec<PredicateObligation<'tcx>> {
3748 debug!("impl_or_trait_obligations(def_id={:?})", def_id);
3749 let tcx = self.tcx();
3751 // To allow for one-pass evaluation of the nested obligation,
3752 // each predicate must be preceded by the obligations required
3754 // for example, if we have:
3755 // impl<U: Iterator, V: Iterator<Item=U>> Foo for V where U::Item: Copy
3756 // the impl will have the following predicates:
3757 // <V as Iterator>::Item = U,
3758 // U: Iterator, U: Sized,
3759 // V: Iterator, V: Sized,
3760 // <U as Iterator>::Item: Copy
3761 // When we substitute, say, `V => IntoIter<u32>, U => $0`, the last
3762 // obligation will normalize to `<$0 as Iterator>::Item = $1` and
3763 // `$1: Copy`, so we must ensure the obligations are emitted in
3765 let predicates = tcx.predicates_of(def_id);
3766 assert_eq!(predicates.parent, None);
3767 let mut predicates: Vec<_> = predicates
3770 .flat_map(|(predicate, _)| {
3771 let predicate = normalize_with_depth(
3776 &predicate.subst(tcx, substs),
3778 predicate.obligations.into_iter().chain(Some(Obligation {
3779 cause: cause.clone(),
3782 predicate: predicate.value,
3787 // We are performing deduplication here to avoid exponential blowups
3788 // (#38528) from happening, but the real cause of the duplication is
3789 // unknown. What we know is that the deduplication avoids exponential
3790 // amount of predicates being propagated when processing deeply nested
3793 // This code is hot enough that it's worth avoiding the allocation
3794 // required for the FxHashSet when possible. Special-casing lengths 0,
3795 // 1 and 2 covers roughly 75--80% of the cases.
3796 if predicates.len() <= 1 {
3797 // No possibility of duplicates.
3798 } else if predicates.len() == 2 {
3799 // Only two elements. Drop the second if they are equal.
3800 if predicates[0] == predicates[1] {
3801 predicates.truncate(1);
3804 // Three or more elements. Use a general deduplication process.
3805 let mut seen = FxHashSet::default();
3806 predicates.retain(|i| seen.insert(i.clone()));
3813 impl<'tcx> TraitObligation<'tcx> {
3814 #[allow(unused_comparisons)]
3815 pub fn derived_cause(
3817 variant: fn(DerivedObligationCause<'tcx>) -> ObligationCauseCode<'tcx>,
3818 ) -> ObligationCause<'tcx> {
3820 * Creates a cause for obligations that are derived from
3821 * `obligation` by a recursive search (e.g., for a builtin
3822 * bound, or eventually a `auto trait Foo`). If `obligation`
3823 * is itself a derived obligation, this is just a clone, but
3824 * otherwise we create a "derived obligation" cause so as to
3825 * keep track of the original root obligation for error
3829 let obligation = self;
3831 // NOTE(flaper87): As of now, it keeps track of the whole error
3832 // chain. Ideally, we should have a way to configure this either
3833 // by using -Z verbose or just a CLI argument.
3834 if obligation.recursion_depth >= 0 {
3835 let derived_cause = DerivedObligationCause {
3836 parent_trait_ref: obligation.predicate.to_poly_trait_ref(),
3837 parent_code: Rc::new(obligation.cause.code.clone()),
3839 let derived_code = variant(derived_cause);
3840 ObligationCause::new(
3841 obligation.cause.span,
3842 obligation.cause.body_id,
3846 obligation.cause.clone()
3851 impl<'tcx> SelectionCache<'tcx> {
3852 /// Actually frees the underlying memory in contrast to what stdlib containers do on `clear`
3853 pub fn clear(&self) {
3854 *self.hashmap.borrow_mut() = Default::default();
3858 impl<'tcx> EvaluationCache<'tcx> {
3859 /// Actually frees the underlying memory in contrast to what stdlib containers do on `clear`
3860 pub fn clear(&self) {
3861 *self.hashmap.borrow_mut() = Default::default();
3865 impl<'o, 'tcx> TraitObligationStack<'o, 'tcx> {
3866 fn list(&'o self) -> TraitObligationStackList<'o, 'tcx> {
3867 TraitObligationStackList::with(self)
3870 fn iter(&'o self) -> TraitObligationStackList<'o, 'tcx> {
3875 #[derive(Copy, Clone)]
3876 struct TraitObligationStackList<'o, 'tcx: 'o> {
3877 head: Option<&'o TraitObligationStack<'o, 'tcx>>,
3880 impl<'o, 'tcx> TraitObligationStackList<'o, 'tcx> {
3881 fn empty() -> TraitObligationStackList<'o, 'tcx> {
3882 TraitObligationStackList { head: None }
3885 fn with(r: &'o TraitObligationStack<'o, 'tcx>) -> TraitObligationStackList<'o, 'tcx> {
3886 TraitObligationStackList { head: Some(r) }
3889 fn head(&self) -> Option<&'o TraitObligationStack<'o, 'tcx>> {
3894 impl<'o, 'tcx> Iterator for TraitObligationStackList<'o, 'tcx> {
3895 type Item = &'o TraitObligationStack<'o, 'tcx>;
3897 fn next(&mut self) -> Option<&'o TraitObligationStack<'o, 'tcx>> {
3908 impl<'o, 'tcx> fmt::Debug for TraitObligationStack<'o, 'tcx> {
3909 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
3910 write!(f, "TraitObligationStack({:?})", self.obligation)
3914 #[derive(Clone, Eq, PartialEq)]
3915 pub struct WithDepNode<T> {
3916 dep_node: DepNodeIndex,
3920 impl<T: Clone> WithDepNode<T> {
3921 pub fn new(dep_node: DepNodeIndex, cached_value: T) -> Self {
3928 pub fn get(&self, tcx: TyCtxt<'_, '_, '_>) -> T {
3929 tcx.dep_graph.read_index(self.dep_node);
3930 self.cached_value.clone()