1 // Copyright 2014 The Rust Project Developers. See the COPYRIGHT
2 // file at the top-level directory of this distribution and at
3 // http://rust-lang.org/COPYRIGHT.
5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
8 // option. This file may not be copied, modified, or distributed
9 // except according to those terms.
11 //! See [rustc guide] for more info on how this works.
13 //! [rustc guide]: https://rust-lang-nursery.github.io/rustc-guide/traits/resolution.html#selection
15 use self::SelectionCandidate::*;
16 use self::EvaluationResult::*;
18 use super::coherence::{self, Conflict};
19 use super::DerivedObligationCause;
20 use super::{IntercrateMode, TraitQueryMode};
22 use super::project::{normalize_with_depth, Normalized, ProjectionCacheKey};
23 use super::{PredicateObligation, TraitObligation, ObligationCause};
24 use super::{ObligationCauseCode, BuiltinDerivedObligation, ImplDerivedObligation};
25 use super::{SelectionError, Unimplemented, OutputTypeParameterMismatch, Overflow};
26 use super::{ObjectCastObligation, Obligation};
27 use super::TraitNotObjectSafe;
29 use super::SelectionResult;
30 use super::{VtableBuiltin, VtableImpl, VtableParam, VtableClosure, VtableGenerator,
31 VtableFnPointer, VtableObject, VtableAutoImpl};
32 use super::{VtableImplData, VtableObjectData, VtableBuiltinData, VtableGeneratorData,
33 VtableClosureData, VtableAutoImplData, VtableFnPointerData};
36 use dep_graph::{DepNodeIndex, DepKind};
37 use hir::def_id::DefId;
39 use infer::{InferCtxt, InferOk, TypeFreshener};
40 use ty::subst::{Subst, Substs};
41 use ty::{self, ToPredicate, ToPolyTraitRef, Ty, TyCtxt, TypeFoldable};
43 use ty::relate::TypeRelation;
44 use middle::lang_items;
45 use mir::interpret::{GlobalId};
47 use rustc_data_structures::sync::Lock;
48 use rustc_data_structures::bitvec::BitArray;
54 use rustc_target::spec::abi::Abi;
56 use util::nodemap::{FxHashMap, FxHashSet};
59 pub struct SelectionContext<'cx, 'gcx: 'cx+'tcx, 'tcx: 'cx> {
60 infcx: &'cx InferCtxt<'cx, 'gcx, 'tcx>,
62 /// Freshener used specifically for skolemizing entries on the
63 /// obligation stack. This ensures that all entries on the stack
64 /// at one time will have the same set of skolemized entries,
65 /// which is important for checking for trait bounds that
66 /// recursively require themselves.
67 freshener: TypeFreshener<'cx, 'gcx, 'tcx>,
69 /// If true, indicates that the evaluation should be conservative
70 /// and consider the possibility of types outside this crate.
71 /// This comes up primarily when resolving ambiguity. Imagine
72 /// there is some trait reference `$0 : Bar` where `$0` is an
73 /// inference variable. If `intercrate` is true, then we can never
74 /// say for sure that this reference is not implemented, even if
75 /// there are *no impls at all for `Bar`*, because `$0` could be
76 /// bound to some type that in a downstream crate that implements
77 /// `Bar`. This is the suitable mode for coherence. Elsewhere,
78 /// though, we set this to false, because we are only interested
79 /// in types that the user could actually have written --- in
80 /// other words, we consider `$0 : Bar` to be unimplemented if
81 /// there is no type that the user could *actually name* that
82 /// would satisfy it. This avoids crippling inference, basically.
83 intercrate: Option<IntercrateMode>,
85 intercrate_ambiguity_causes: Option<Vec<IntercrateAmbiguityCause>>,
87 /// Controls whether or not to filter out negative impls when selecting.
88 /// This is used in librustdoc to distinguish between the lack of an impl
89 /// and a negative impl
90 allow_negative_impls: bool,
92 /// The mode that trait queries run in, which informs our error handling
93 /// policy. In essence, canonicalized queries need their errors propagated
94 /// rather than immediately reported because we do not have accurate spans.
95 query_mode: TraitQueryMode,
98 #[derive(Clone, Debug)]
99 pub enum IntercrateAmbiguityCause {
102 self_desc: Option<String>,
104 UpstreamCrateUpdate {
106 self_desc: Option<String>,
110 impl IntercrateAmbiguityCause {
111 /// Emits notes when the overlap is caused by complex intercrate ambiguities.
112 /// See #23980 for details.
113 pub fn add_intercrate_ambiguity_hint<'a, 'tcx>(&self,
114 err: &mut ::errors::DiagnosticBuilder) {
115 err.note(&self.intercrate_ambiguity_hint());
118 pub fn intercrate_ambiguity_hint(&self) -> String {
120 &IntercrateAmbiguityCause::DownstreamCrate { ref trait_desc, ref self_desc } => {
121 let self_desc = if let &Some(ref ty) = self_desc {
122 format!(" for type `{}`", ty)
123 } else { "".to_string() };
124 format!("downstream crates may implement trait `{}`{}", trait_desc, self_desc)
126 &IntercrateAmbiguityCause::UpstreamCrateUpdate { ref trait_desc, ref self_desc } => {
127 let self_desc = if let &Some(ref ty) = self_desc {
128 format!(" for type `{}`", ty)
129 } else { "".to_string() };
130 format!("upstream crates may add new impl of trait `{}`{} \
132 trait_desc, self_desc)
138 // A stack that walks back up the stack frame.
139 struct TraitObligationStack<'prev, 'tcx: 'prev> {
140 obligation: &'prev TraitObligation<'tcx>,
142 /// Trait ref from `obligation` but skolemized with the
143 /// selection-context's freshener. Used to check for recursion.
144 fresh_trait_ref: ty::PolyTraitRef<'tcx>,
146 previous: TraitObligationStackList<'prev, 'tcx>,
150 pub struct SelectionCache<'tcx> {
151 hashmap: Lock<FxHashMap<ty::TraitRef<'tcx>,
152 WithDepNode<SelectionResult<'tcx, SelectionCandidate<'tcx>>>>>,
155 /// The selection process begins by considering all impls, where
156 /// clauses, and so forth that might resolve an obligation. Sometimes
157 /// we'll be able to say definitively that (e.g.) an impl does not
158 /// apply to the obligation: perhaps it is defined for `usize` but the
159 /// obligation is for `int`. In that case, we drop the impl out of the
160 /// list. But the other cases are considered *candidates*.
162 /// For selection to succeed, there must be exactly one matching
163 /// candidate. If the obligation is fully known, this is guaranteed
164 /// by coherence. However, if the obligation contains type parameters
165 /// or variables, there may be multiple such impls.
167 /// It is not a real problem if multiple matching impls exist because
168 /// of type variables - it just means the obligation isn't sufficiently
169 /// elaborated. In that case we report an ambiguity, and the caller can
170 /// try again after more type information has been gathered or report a
171 /// "type annotations required" error.
173 /// However, with type parameters, this can be a real problem - type
174 /// parameters don't unify with regular types, but they *can* unify
175 /// with variables from blanket impls, and (unless we know its bounds
176 /// will always be satisfied) picking the blanket impl will be wrong
177 /// for at least *some* substitutions. To make this concrete, if we have
179 /// trait AsDebug { type Out : fmt::Debug; fn debug(self) -> Self::Out; }
180 /// impl<T: fmt::Debug> AsDebug for T {
182 /// fn debug(self) -> fmt::Debug { self }
184 /// fn foo<T: AsDebug>(t: T) { println!("{:?}", <T as AsDebug>::debug(t)); }
186 /// we can't just use the impl to resolve the <T as AsDebug> obligation
187 /// - a type from another crate (that doesn't implement fmt::Debug) could
188 /// implement AsDebug.
190 /// Because where-clauses match the type exactly, multiple clauses can
191 /// only match if there are unresolved variables, and we can mostly just
192 /// report this ambiguity in that case. This is still a problem - we can't
193 /// *do anything* with ambiguities that involve only regions. This is issue
196 /// If a single where-clause matches and there are no inference
197 /// variables left, then it definitely matches and we can just select
200 /// In fact, we even select the where-clause when the obligation contains
201 /// inference variables. The can lead to inference making "leaps of logic",
202 /// for example in this situation:
204 /// pub trait Foo<T> { fn foo(&self) -> T; }
205 /// impl<T> Foo<()> for T { fn foo(&self) { } }
206 /// impl Foo<bool> for bool { fn foo(&self) -> bool { *self } }
208 /// pub fn foo<T>(t: T) where T: Foo<bool> {
209 /// println!("{:?}", <T as Foo<_>>::foo(&t));
211 /// fn main() { foo(false); }
213 /// Here the obligation <T as Foo<$0>> can be matched by both the blanket
214 /// impl and the where-clause. We select the where-clause and unify $0=bool,
215 /// so the program prints "false". However, if the where-clause is omitted,
216 /// the blanket impl is selected, we unify $0=(), and the program prints
219 /// Exactly the same issues apply to projection and object candidates, except
220 /// that we can have both a projection candidate and a where-clause candidate
221 /// for the same obligation. In that case either would do (except that
222 /// different "leaps of logic" would occur if inference variables are
223 /// present), and we just pick the where-clause. This is, for example,
224 /// required for associated types to work in default impls, as the bounds
225 /// are visible both as projection bounds and as where-clauses from the
226 /// parameter environment.
227 #[derive(PartialEq,Eq,Debug,Clone)]
228 enum SelectionCandidate<'tcx> {
229 BuiltinCandidate { has_nested: bool },
230 ParamCandidate(ty::PolyTraitRef<'tcx>),
231 ImplCandidate(DefId),
232 AutoImplCandidate(DefId),
234 /// This is a trait matching with a projected type as `Self`, and
235 /// we found an applicable bound in the trait definition.
238 /// Implementation of a `Fn`-family trait by one of the anonymous types
239 /// generated for a `||` expression.
242 /// Implementation of a `Generator` trait by one of the anonymous types
243 /// generated for a generator.
246 /// Implementation of a `Fn`-family trait by one of the anonymous
247 /// types generated for a fn pointer type (e.g., `fn(int)->int`)
252 BuiltinObjectCandidate,
254 BuiltinUnsizeCandidate,
257 impl<'a, 'tcx> ty::Lift<'tcx> for SelectionCandidate<'a> {
258 type Lifted = SelectionCandidate<'tcx>;
259 fn lift_to_tcx<'b, 'gcx>(&self, tcx: TyCtxt<'b, 'gcx, 'tcx>) -> Option<Self::Lifted> {
261 BuiltinCandidate { has_nested } => {
266 ImplCandidate(def_id) => ImplCandidate(def_id),
267 AutoImplCandidate(def_id) => AutoImplCandidate(def_id),
268 ProjectionCandidate => ProjectionCandidate,
269 FnPointerCandidate => FnPointerCandidate,
270 ObjectCandidate => ObjectCandidate,
271 BuiltinObjectCandidate => BuiltinObjectCandidate,
272 BuiltinUnsizeCandidate => BuiltinUnsizeCandidate,
273 ClosureCandidate => ClosureCandidate,
274 GeneratorCandidate => GeneratorCandidate,
276 ParamCandidate(ref trait_ref) => {
277 return tcx.lift(trait_ref).map(ParamCandidate);
283 struct SelectionCandidateSet<'tcx> {
284 // a list of candidates that definitely apply to the current
285 // obligation (meaning: types unify).
286 vec: Vec<SelectionCandidate<'tcx>>,
288 // if this is true, then there were candidates that might or might
289 // not have applied, but we couldn't tell. This occurs when some
290 // of the input types are type variables, in which case there are
291 // various "builtin" rules that might or might not trigger.
295 #[derive(PartialEq,Eq,Debug,Clone)]
296 struct EvaluatedCandidate<'tcx> {
297 candidate: SelectionCandidate<'tcx>,
298 evaluation: EvaluationResult,
301 /// When does the builtin impl for `T: Trait` apply?
302 enum BuiltinImplConditions<'tcx> {
303 /// The impl is conditional on T1,T2,.. : Trait
304 Where(ty::Binder<Vec<Ty<'tcx>>>),
305 /// There is no built-in impl. There may be some other
306 /// candidate (a where-clause or user-defined impl).
308 /// It is unknown whether there is an impl.
312 #[derive(Copy, Clone, Debug, PartialOrd, Ord, PartialEq, Eq)]
313 /// The result of trait evaluation. The order is important
314 /// here as the evaluation of a list is the maximum of the
317 /// The evaluation results are ordered:
318 /// - `EvaluatedToOk` implies `EvaluatedToAmbig` implies `EvaluatedToUnknown`
319 /// - `EvaluatedToErr` implies `EvaluatedToRecur`
320 /// - the "union" of evaluation results is equal to their maximum -
321 /// all the "potential success" candidates can potentially succeed,
322 /// so they are no-ops when unioned with a definite error, and within
323 /// the categories it's easy to see that the unions are correct.
324 pub enum EvaluationResult {
325 /// Evaluation successful
327 /// Evaluation is known to be ambiguous - it *might* hold for some
328 /// assignment of inference variables, but it might not.
330 /// While this has the same meaning as `EvaluatedToUnknown` - we can't
331 /// know whether this obligation holds or not - it is the result we
332 /// would get with an empty stack, and therefore is cacheable.
334 /// Evaluation failed because of recursion involving inference
335 /// variables. We are somewhat imprecise there, so we don't actually
336 /// know the real result.
338 /// This can't be trivially cached for the same reason as `EvaluatedToRecur`.
340 /// Evaluation failed because we encountered an obligation we are already
341 /// trying to prove on this branch.
343 /// We know this branch can't be a part of a minimal proof-tree for
344 /// the "root" of our cycle, because then we could cut out the recursion
345 /// and maintain a valid proof tree. However, this does not mean
346 /// that all the obligations on this branch do not hold - it's possible
347 /// that we entered this branch "speculatively", and that there
348 /// might be some other way to prove this obligation that does not
349 /// go through this cycle - so we can't cache this as a failure.
351 /// For example, suppose we have this:
353 /// ```rust,ignore (pseudo-Rust)
354 /// pub trait Trait { fn xyz(); }
355 /// // This impl is "useless", but we can still have
356 /// // an `impl Trait for SomeUnsizedType` somewhere.
357 /// impl<T: Trait + Sized> Trait for T { fn xyz() {} }
359 /// pub fn foo<T: Trait + ?Sized>() {
360 /// <T as Trait>::xyz();
364 /// When checking `foo`, we have to prove `T: Trait`. This basically
365 /// translates into this:
368 /// (T: Trait + Sized →_\impl T: Trait), T: Trait ⊢ T: Trait
371 /// When we try to prove it, we first go the first option, which
372 /// recurses. This shows us that the impl is "useless" - it won't
373 /// tell us that `T: Trait` unless it already implemented `Trait`
374 /// by some other means. However, that does not prevent `T: Trait`
375 /// does not hold, because of the bound (which can indeed be satisfied
376 /// by `SomeUnsizedType` from another crate).
378 /// FIXME: when an `EvaluatedToRecur` goes past its parent root, we
379 /// ought to convert it to an `EvaluatedToErr`, because we know
380 /// there definitely isn't a proof tree for that obligation. Not
381 /// doing so is still sound - there isn't any proof tree, so the
382 /// branch still can't be a part of a minimal one - but does not
383 /// re-enable caching.
385 /// Evaluation failed
389 impl EvaluationResult {
390 pub fn may_apply(self) -> bool {
394 EvaluatedToUnknown => true,
397 EvaluatedToRecur => false
401 fn is_stack_dependent(self) -> bool {
404 EvaluatedToRecur => true,
408 EvaluatedToErr => false,
413 impl_stable_hash_for!(enum self::EvaluationResult {
421 #[derive(Copy, Clone, Debug, PartialEq, Eq)]
422 /// Indicates that trait evaluation caused overflow.
423 pub struct OverflowError;
425 impl_stable_hash_for!(struct OverflowError { });
427 impl<'tcx> From<OverflowError> for SelectionError<'tcx> {
428 fn from(OverflowError: OverflowError) -> SelectionError<'tcx> {
429 SelectionError::Overflow
434 pub struct EvaluationCache<'tcx> {
435 hashmap: Lock<FxHashMap<ty::PolyTraitRef<'tcx>, WithDepNode<EvaluationResult>>>
438 impl<'cx, 'gcx, 'tcx> SelectionContext<'cx, 'gcx, 'tcx> {
439 pub fn new(infcx: &'cx InferCtxt<'cx, 'gcx, 'tcx>) -> SelectionContext<'cx, 'gcx, 'tcx> {
442 freshener: infcx.freshener(),
444 intercrate_ambiguity_causes: None,
445 allow_negative_impls: false,
446 query_mode: TraitQueryMode::Standard,
450 pub fn intercrate(infcx: &'cx InferCtxt<'cx, 'gcx, 'tcx>,
451 mode: IntercrateMode) -> SelectionContext<'cx, 'gcx, 'tcx> {
452 debug!("intercrate({:?})", mode);
455 freshener: infcx.freshener(),
456 intercrate: Some(mode),
457 intercrate_ambiguity_causes: None,
458 allow_negative_impls: false,
459 query_mode: TraitQueryMode::Standard,
463 pub fn with_negative(infcx: &'cx InferCtxt<'cx, 'gcx, 'tcx>,
464 allow_negative_impls: bool) -> SelectionContext<'cx, 'gcx, 'tcx> {
465 debug!("with_negative({:?})", allow_negative_impls);
468 freshener: infcx.freshener(),
470 intercrate_ambiguity_causes: None,
471 allow_negative_impls,
472 query_mode: TraitQueryMode::Standard,
476 pub fn with_query_mode(infcx: &'cx InferCtxt<'cx, 'gcx, 'tcx>,
477 query_mode: TraitQueryMode) -> SelectionContext<'cx, 'gcx, 'tcx> {
478 debug!("with_query_mode({:?})", query_mode);
481 freshener: infcx.freshener(),
483 intercrate_ambiguity_causes: None,
484 allow_negative_impls: false,
489 /// Enables tracking of intercrate ambiguity causes. These are
490 /// used in coherence to give improved diagnostics. We don't do
491 /// this until we detect a coherence error because it can lead to
492 /// false overflow results (#47139) and because it costs
493 /// computation time.
494 pub fn enable_tracking_intercrate_ambiguity_causes(&mut self) {
495 assert!(self.intercrate.is_some());
496 assert!(self.intercrate_ambiguity_causes.is_none());
497 self.intercrate_ambiguity_causes = Some(vec![]);
498 debug!("selcx: enable_tracking_intercrate_ambiguity_causes");
501 /// Gets the intercrate ambiguity causes collected since tracking
502 /// was enabled and disables tracking at the same time. If
503 /// tracking is not enabled, just returns an empty vector.
504 pub fn take_intercrate_ambiguity_causes(&mut self) -> Vec<IntercrateAmbiguityCause> {
505 assert!(self.intercrate.is_some());
506 self.intercrate_ambiguity_causes.take().unwrap_or(vec![])
509 pub fn infcx(&self) -> &'cx InferCtxt<'cx, 'gcx, 'tcx> {
513 pub fn tcx(&self) -> TyCtxt<'cx, 'gcx, 'tcx> {
517 pub fn closure_typer(&self) -> &'cx InferCtxt<'cx, 'gcx, 'tcx> {
521 /// Wraps the inference context's in_snapshot s.t. snapshot handling is only from the selection
523 fn in_snapshot<R, F>(&mut self, f: F) -> R
524 where F: FnOnce(&mut Self, &infer::CombinedSnapshot<'cx, 'tcx>) -> R
526 self.infcx.in_snapshot(|snapshot| f(self, snapshot))
529 /// Wraps a probe s.t. obligations collected during it are ignored and old obligations are
531 fn probe<R, F>(&mut self, f: F) -> R
532 where F: FnOnce(&mut Self, &infer::CombinedSnapshot<'cx, 'tcx>) -> R
534 self.infcx.probe(|snapshot| f(self, snapshot))
537 /// Wraps a commit_if_ok s.t. obligations collected during it are not returned in selection if
538 /// the transaction fails and s.t. old obligations are retained.
539 fn commit_if_ok<T, E, F>(&mut self, f: F) -> Result<T, E> where
540 F: FnOnce(&mut Self, &infer::CombinedSnapshot) -> Result<T, E>
542 self.infcx.commit_if_ok(|snapshot| f(self, snapshot))
546 ///////////////////////////////////////////////////////////////////////////
549 // The selection phase tries to identify *how* an obligation will
550 // be resolved. For example, it will identify which impl or
551 // parameter bound is to be used. The process can be inconclusive
552 // if the self type in the obligation is not fully inferred. Selection
553 // can result in an error in one of two ways:
555 // 1. If no applicable impl or parameter bound can be found.
556 // 2. If the output type parameters in the obligation do not match
557 // those specified by the impl/bound. For example, if the obligation
558 // is `Vec<Foo>:Iterable<Bar>`, but the impl specifies
559 // `impl<T> Iterable<T> for Vec<T>`, than an error would result.
561 /// Attempts to satisfy the obligation. If successful, this will affect the surrounding
562 /// type environment by performing unification.
563 pub fn select(&mut self, obligation: &TraitObligation<'tcx>)
564 -> SelectionResult<'tcx, Selection<'tcx>> {
565 debug!("select({:?})", obligation);
566 assert!(!obligation.predicate.has_escaping_regions());
568 let stack = self.push_stack(TraitObligationStackList::empty(), obligation);
570 let candidate = match self.candidate_from_obligation(&stack) {
571 Err(SelectionError::Overflow) => {
572 // In standard mode, overflow must have been caught and reported
574 assert!(self.query_mode == TraitQueryMode::Canonical);
575 return Err(SelectionError::Overflow);
577 Err(e) => { return Err(e); },
578 Ok(None) => { return Ok(None); },
579 Ok(Some(candidate)) => candidate
582 match self.confirm_candidate(obligation, candidate) {
583 Err(SelectionError::Overflow) => {
584 assert!(self.query_mode == TraitQueryMode::Canonical);
585 return Err(SelectionError::Overflow);
588 Ok(candidate) => Ok(Some(candidate))
592 ///////////////////////////////////////////////////////////////////////////
595 // Tests whether an obligation can be selected or whether an impl
596 // can be applied to particular types. It skips the "confirmation"
597 // step and hence completely ignores output type parameters.
599 // The result is "true" if the obligation *may* hold and "false" if
600 // we can be sure it does not.
602 /// Evaluates whether the obligation `obligation` can be satisfied (by any means).
603 pub fn predicate_may_hold_fatal(&mut self,
604 obligation: &PredicateObligation<'tcx>)
607 debug!("predicate_may_hold_fatal({:?})",
610 // This fatal query is a stopgap that should only be used in standard mode,
611 // where we do not expect overflow to be propagated.
612 assert!(self.query_mode == TraitQueryMode::Standard);
614 self.evaluate_obligation_recursively(obligation)
615 .expect("Overflow should be caught earlier in standard query mode")
619 /// Evaluates whether the obligation `obligation` can be satisfied and returns
620 /// an `EvaluationResult`.
621 pub fn evaluate_obligation_recursively(&mut self,
622 obligation: &PredicateObligation<'tcx>)
623 -> Result<EvaluationResult, OverflowError>
625 self.probe(|this, _| {
626 this.evaluate_predicate_recursively(TraitObligationStackList::empty(), obligation)
630 /// Evaluates the predicates in `predicates` recursively. Note that
631 /// this applies projections in the predicates, and therefore
632 /// is run within an inference probe.
633 fn evaluate_predicates_recursively<'a,'o,I>(&mut self,
634 stack: TraitObligationStackList<'o, 'tcx>,
636 -> Result<EvaluationResult, OverflowError>
637 where I : IntoIterator<Item=&'a PredicateObligation<'tcx>>, 'tcx:'a
639 let mut result = EvaluatedToOk;
640 for obligation in predicates {
641 let eval = self.evaluate_predicate_recursively(stack, obligation)?;
642 debug!("evaluate_predicate_recursively({:?}) = {:?}",
644 if let EvaluatedToErr = eval {
645 // fast-path - EvaluatedToErr is the top of the lattice,
646 // so we don't need to look on the other predicates.
647 return Ok(EvaluatedToErr);
649 result = cmp::max(result, eval);
655 fn evaluate_predicate_recursively<'o>(&mut self,
656 previous_stack: TraitObligationStackList<'o, 'tcx>,
657 obligation: &PredicateObligation<'tcx>)
658 -> Result<EvaluationResult, OverflowError>
660 debug!("evaluate_predicate_recursively({:?})",
663 match obligation.predicate {
664 ty::Predicate::Trait(ref t) => {
665 assert!(!t.has_escaping_regions());
666 let obligation = obligation.with(t.clone());
667 self.evaluate_trait_predicate_recursively(previous_stack, obligation)
670 ty::Predicate::Subtype(ref p) => {
671 // does this code ever run?
672 match self.infcx.subtype_predicate(&obligation.cause, obligation.param_env, p) {
673 Some(Ok(InferOk { obligations, .. })) => {
674 self.evaluate_predicates_recursively(previous_stack, &obligations)
676 Some(Err(_)) => Ok(EvaluatedToErr),
677 None => Ok(EvaluatedToAmbig),
681 ty::Predicate::WellFormed(ty) => {
682 match ty::wf::obligations(self.infcx,
683 obligation.param_env,
684 obligation.cause.body_id,
685 ty, obligation.cause.span) {
687 self.evaluate_predicates_recursively(previous_stack, obligations.iter()),
689 Ok(EvaluatedToAmbig),
693 ty::Predicate::TypeOutlives(..) | ty::Predicate::RegionOutlives(..) => {
694 // we do not consider region relationships when
695 // evaluating trait matches
699 ty::Predicate::ObjectSafe(trait_def_id) => {
700 if self.tcx().is_object_safe(trait_def_id) {
707 ty::Predicate::Projection(ref data) => {
708 let project_obligation = obligation.with(data.clone());
709 match project::poly_project_and_unify_type(self, &project_obligation) {
710 Ok(Some(subobligations)) => {
711 let result = self.evaluate_predicates_recursively(previous_stack,
712 subobligations.iter());
714 ProjectionCacheKey::from_poly_projection_predicate(self, data)
716 self.infcx.projection_cache.borrow_mut().complete(key);
729 ty::Predicate::ClosureKind(closure_def_id, closure_substs, kind) => {
730 match self.infcx.closure_kind(closure_def_id, closure_substs) {
731 Some(closure_kind) => {
732 if closure_kind.extends(kind) {
744 ty::Predicate::ConstEvaluatable(def_id, substs) => {
745 let tcx = self.tcx();
746 match tcx.lift_to_global(&(obligation.param_env, substs)) {
747 Some((param_env, substs)) => {
748 let instance = ty::Instance::resolve(
754 if let Some(instance) = instance {
759 match self.tcx().const_eval(param_env.and(cid)) {
760 Ok(_) => Ok(EvaluatedToOk),
761 Err(_) => Ok(EvaluatedToErr)
768 // Inference variables still left in param_env or substs.
776 fn evaluate_trait_predicate_recursively<'o>(&mut self,
777 previous_stack: TraitObligationStackList<'o, 'tcx>,
778 mut obligation: TraitObligation<'tcx>)
779 -> Result<EvaluationResult, OverflowError>
781 debug!("evaluate_trait_predicate_recursively({:?})", obligation);
783 if self.intercrate.is_none() && obligation.is_global()
784 && obligation.param_env.caller_bounds.iter().all(|bound| bound.needs_subst()) {
785 // If a param env has no global bounds, global obligations do not
786 // depend on its particular value in order to work, so we can clear
787 // out the param env and get better caching.
788 debug!("evaluate_trait_predicate_recursively({:?}) - in global", obligation);
789 obligation.param_env = obligation.param_env.without_caller_bounds();
792 let stack = self.push_stack(previous_stack, &obligation);
793 let fresh_trait_ref = stack.fresh_trait_ref;
794 if let Some(result) = self.check_evaluation_cache(obligation.param_env, fresh_trait_ref) {
795 debug!("CACHE HIT: EVAL({:?})={:?}",
801 let (result, dep_node) = self.in_task(|this| this.evaluate_stack(&stack));
802 let result = result?;
804 debug!("CACHE MISS: EVAL({:?})={:?}",
807 self.insert_evaluation_cache(obligation.param_env, fresh_trait_ref, dep_node, result);
812 fn evaluate_stack<'o>(&mut self,
813 stack: &TraitObligationStack<'o, 'tcx>)
814 -> Result<EvaluationResult, OverflowError>
816 // In intercrate mode, whenever any of the types are unbound,
817 // there can always be an impl. Even if there are no impls in
818 // this crate, perhaps the type would be unified with
819 // something from another crate that does provide an impl.
821 // In intra mode, we must still be conservative. The reason is
822 // that we want to avoid cycles. Imagine an impl like:
824 // impl<T:Eq> Eq for Vec<T>
826 // and a trait reference like `$0 : Eq` where `$0` is an
827 // unbound variable. When we evaluate this trait-reference, we
828 // will unify `$0` with `Vec<$1>` (for some fresh variable
829 // `$1`), on the condition that `$1 : Eq`. We will then wind
830 // up with many candidates (since that are other `Eq` impls
831 // that apply) and try to winnow things down. This results in
832 // a recursive evaluation that `$1 : Eq` -- as you can
833 // imagine, this is just where we started. To avoid that, we
834 // check for unbound variables and return an ambiguous (hence possible)
835 // match if we've seen this trait before.
837 // This suffices to allow chains like `FnMut` implemented in
838 // terms of `Fn` etc, but we could probably make this more
840 let unbound_input_types =
841 stack.fresh_trait_ref.skip_binder().input_types().any(|ty| ty.is_fresh());
842 // this check was an imperfect workaround for a bug n the old
843 // intercrate mode, it should be removed when that goes away.
844 if unbound_input_types &&
845 self.intercrate == Some(IntercrateMode::Issue43355)
847 debug!("evaluate_stack({:?}) --> unbound argument, intercrate --> ambiguous",
848 stack.fresh_trait_ref);
849 // Heuristics: show the diagnostics when there are no candidates in crate.
850 if self.intercrate_ambiguity_causes.is_some() {
851 debug!("evaluate_stack: intercrate_ambiguity_causes is some");
852 if let Ok(candidate_set) = self.assemble_candidates(stack) {
853 if !candidate_set.ambiguous && candidate_set.vec.is_empty() {
854 let trait_ref = stack.obligation.predicate.skip_binder().trait_ref;
855 let self_ty = trait_ref.self_ty();
856 let cause = IntercrateAmbiguityCause::DownstreamCrate {
857 trait_desc: trait_ref.to_string(),
858 self_desc: if self_ty.has_concrete_skeleton() {
859 Some(self_ty.to_string())
864 debug!("evaluate_stack: pushing cause = {:?}", cause);
865 self.intercrate_ambiguity_causes.as_mut().unwrap().push(cause);
869 return Ok(EvaluatedToAmbig);
871 if unbound_input_types &&
872 stack.iter().skip(1).any(
873 |prev| stack.obligation.param_env == prev.obligation.param_env &&
874 self.match_fresh_trait_refs(&stack.fresh_trait_ref,
875 &prev.fresh_trait_ref))
877 debug!("evaluate_stack({:?}) --> unbound argument, recursive --> giving up",
878 stack.fresh_trait_ref);
879 return Ok(EvaluatedToUnknown);
882 // If there is any previous entry on the stack that precisely
883 // matches this obligation, then we can assume that the
884 // obligation is satisfied for now (still all other conditions
885 // must be met of course). One obvious case this comes up is
886 // marker traits like `Send`. Think of a linked list:
888 // struct List<T> { data: T, next: Option<Box<List<T>>> {
890 // `Box<List<T>>` will be `Send` if `T` is `Send` and
891 // `Option<Box<List<T>>>` is `Send`, and in turn
892 // `Option<Box<List<T>>>` is `Send` if `Box<List<T>>` is
895 // Note that we do this comparison using the `fresh_trait_ref`
896 // fields. Because these have all been skolemized using
897 // `self.freshener`, we can be sure that (a) this will not
898 // affect the inferencer state and (b) that if we see two
899 // skolemized types with the same index, they refer to the
900 // same unbound type variable.
901 if let Some(rec_index) =
903 .skip(1) // skip top-most frame
904 .position(|prev| stack.obligation.param_env == prev.obligation.param_env &&
905 stack.fresh_trait_ref == prev.fresh_trait_ref)
907 debug!("evaluate_stack({:?}) --> recursive",
908 stack.fresh_trait_ref);
909 let cycle = stack.iter().skip(1).take(rec_index+1);
910 let cycle = cycle.map(|stack| ty::Predicate::Trait(stack.obligation.predicate));
911 if self.coinductive_match(cycle) {
912 debug!("evaluate_stack({:?}) --> recursive, coinductive",
913 stack.fresh_trait_ref);
914 return Ok(EvaluatedToOk);
916 debug!("evaluate_stack({:?}) --> recursive, inductive",
917 stack.fresh_trait_ref);
918 return Ok(EvaluatedToRecur);
922 match self.candidate_from_obligation(stack) {
923 Ok(Some(c)) => self.evaluate_candidate(stack, &c),
924 Ok(None) => Ok(EvaluatedToAmbig),
925 Err(Overflow) => Err(OverflowError),
926 Err(..) => Ok(EvaluatedToErr)
930 /// For defaulted traits, we use a co-inductive strategy to solve, so
931 /// that recursion is ok. This routine returns true if the top of the
932 /// stack (`cycle[0]`):
934 /// - is a defaulted trait, and
935 /// - it also appears in the backtrace at some position `X`; and,
936 /// - all the predicates at positions `X..` between `X` an the top are
937 /// also defaulted traits.
938 pub fn coinductive_match<I>(&mut self, cycle: I) -> bool
939 where I: Iterator<Item=ty::Predicate<'tcx>>
941 let mut cycle = cycle;
942 cycle.all(|predicate| self.coinductive_predicate(predicate))
945 fn coinductive_predicate(&self, predicate: ty::Predicate<'tcx>) -> bool {
946 let result = match predicate {
947 ty::Predicate::Trait(ref data) => {
948 self.tcx().trait_is_auto(data.def_id())
954 debug!("coinductive_predicate({:?}) = {:?}", predicate, result);
958 /// Further evaluate `candidate` to decide whether all type parameters match and whether nested
959 /// obligations are met. Returns true if `candidate` remains viable after this further
961 fn evaluate_candidate<'o>(&mut self,
962 stack: &TraitObligationStack<'o, 'tcx>,
963 candidate: &SelectionCandidate<'tcx>)
964 -> Result<EvaluationResult, OverflowError>
966 debug!("evaluate_candidate: depth={} candidate={:?}",
967 stack.obligation.recursion_depth, candidate);
968 let result = self.probe(|this, _| {
969 let candidate = (*candidate).clone();
970 match this.confirm_candidate(stack.obligation, candidate) {
972 this.evaluate_predicates_recursively(
974 selection.nested_obligations().iter())
976 Err(..) => Ok(EvaluatedToErr)
979 debug!("evaluate_candidate: depth={} result={:?}",
980 stack.obligation.recursion_depth, result);
984 fn check_evaluation_cache(&self,
985 param_env: ty::ParamEnv<'tcx>,
986 trait_ref: ty::PolyTraitRef<'tcx>)
987 -> Option<EvaluationResult>
989 let tcx = self.tcx();
990 if self.can_use_global_caches(param_env) {
991 let cache = tcx.evaluation_cache.hashmap.borrow();
992 if let Some(cached) = cache.get(&trait_ref) {
993 return Some(cached.get(tcx));
996 self.infcx.evaluation_cache.hashmap
1002 fn insert_evaluation_cache(&mut self,
1003 param_env: ty::ParamEnv<'tcx>,
1004 trait_ref: ty::PolyTraitRef<'tcx>,
1005 dep_node: DepNodeIndex,
1006 result: EvaluationResult)
1008 // Avoid caching results that depend on more than just the trait-ref
1009 // - the stack can create recursion.
1010 if result.is_stack_dependent() {
1014 if self.can_use_global_caches(param_env) {
1015 if let Some(trait_ref) = self.tcx().lift_to_global(&trait_ref) {
1017 "insert_evaluation_cache(trait_ref={:?}, candidate={:?}) global",
1021 // This may overwrite the cache with the same value
1022 // FIXME: Due to #50507 this overwrites the different values
1023 // This should be changed to use HashMapExt::insert_same
1024 // when that is fixed
1025 self.tcx().evaluation_cache
1026 .hashmap.borrow_mut()
1027 .insert(trait_ref, WithDepNode::new(dep_node, result));
1033 "insert_evaluation_cache(trait_ref={:?}, candidate={:?})",
1037 self.infcx.evaluation_cache.hashmap
1039 .insert(trait_ref, WithDepNode::new(dep_node, result));
1042 ///////////////////////////////////////////////////////////////////////////
1043 // CANDIDATE ASSEMBLY
1045 // The selection process begins by examining all in-scope impls,
1046 // caller obligations, and so forth and assembling a list of
1047 // candidates. See [rustc guide] for more details.
1050 // https://rust-lang-nursery.github.io/rustc-guide/traits/resolution.html#candidate-assembly
1052 fn candidate_from_obligation<'o>(&mut self,
1053 stack: &TraitObligationStack<'o, 'tcx>)
1054 -> SelectionResult<'tcx, SelectionCandidate<'tcx>>
1056 // Watch out for overflow. This intentionally bypasses (and does
1057 // not update) the cache.
1058 let recursion_limit = *self.infcx.tcx.sess.recursion_limit.get();
1059 if stack.obligation.recursion_depth >= recursion_limit {
1060 match self.query_mode {
1061 TraitQueryMode::Standard => {
1062 self.infcx().report_overflow_error(&stack.obligation, true);
1064 TraitQueryMode::Canonical => {
1065 return Err(Overflow);
1070 // Check the cache. Note that we skolemize the trait-ref
1071 // separately rather than using `stack.fresh_trait_ref` -- this
1072 // is because we want the unbound variables to be replaced
1073 // with fresh skolemized types starting from index 0.
1074 let cache_fresh_trait_pred =
1075 self.infcx.freshen(stack.obligation.predicate.clone());
1076 debug!("candidate_from_obligation(cache_fresh_trait_pred={:?}, obligation={:?})",
1077 cache_fresh_trait_pred,
1079 assert!(!stack.obligation.predicate.has_escaping_regions());
1081 if let Some(c) = self.check_candidate_cache(stack.obligation.param_env,
1082 &cache_fresh_trait_pred) {
1083 debug!("CACHE HIT: SELECT({:?})={:?}",
1084 cache_fresh_trait_pred,
1089 // If no match, compute result and insert into cache.
1090 let (candidate, dep_node) = self.in_task(|this| {
1091 this.candidate_from_obligation_no_cache(stack)
1094 debug!("CACHE MISS: SELECT({:?})={:?}",
1095 cache_fresh_trait_pred, candidate);
1096 self.insert_candidate_cache(stack.obligation.param_env,
1097 cache_fresh_trait_pred,
1103 fn in_task<OP, R>(&mut self, op: OP) -> (R, DepNodeIndex)
1104 where OP: FnOnce(&mut Self) -> R
1106 let (result, dep_node) = self.tcx().dep_graph.with_anon_task(DepKind::TraitSelect, || {
1109 self.tcx().dep_graph.read_index(dep_node);
1113 // Treat negative impls as unimplemented
1114 fn filter_negative_impls(&self, candidate: SelectionCandidate<'tcx>)
1115 -> SelectionResult<'tcx, SelectionCandidate<'tcx>> {
1116 if let ImplCandidate(def_id) = candidate {
1117 if !self.allow_negative_impls &&
1118 self.tcx().impl_polarity(def_id) == hir::ImplPolarity::Negative {
1119 return Err(Unimplemented)
1125 fn candidate_from_obligation_no_cache<'o>(&mut self,
1126 stack: &TraitObligationStack<'o, 'tcx>)
1127 -> SelectionResult<'tcx, SelectionCandidate<'tcx>>
1129 if stack.obligation.predicate.references_error() {
1130 // If we encounter a `TyError`, we generally prefer the
1131 // most "optimistic" result in response -- that is, the
1132 // one least likely to report downstream errors. But
1133 // because this routine is shared by coherence and by
1134 // trait selection, there isn't an obvious "right" choice
1135 // here in that respect, so we opt to just return
1136 // ambiguity and let the upstream clients sort it out.
1140 match self.is_knowable(stack) {
1143 debug!("coherence stage: not knowable");
1144 if self.intercrate_ambiguity_causes.is_some() {
1145 debug!("evaluate_stack: intercrate_ambiguity_causes is some");
1146 // Heuristics: show the diagnostics when there are no candidates in crate.
1147 if let Ok(candidate_set) = self.assemble_candidates(stack) {
1148 let no_candidates_apply =
1152 .map(|c| self.evaluate_candidate(stack, &c))
1153 .collect::<Result<Vec<_>, OverflowError>>()?
1155 .all(|r| !r.may_apply());
1156 if !candidate_set.ambiguous && no_candidates_apply {
1157 let trait_ref = stack.obligation.predicate.skip_binder().trait_ref;
1158 let self_ty = trait_ref.self_ty();
1159 let trait_desc = trait_ref.to_string();
1160 let self_desc = if self_ty.has_concrete_skeleton() {
1161 Some(self_ty.to_string())
1165 let cause = if let Conflict::Upstream = conflict {
1166 IntercrateAmbiguityCause::UpstreamCrateUpdate {
1171 IntercrateAmbiguityCause::DownstreamCrate { trait_desc, self_desc }
1173 debug!("evaluate_stack: pushing cause = {:?}", cause);
1174 self.intercrate_ambiguity_causes.as_mut().unwrap().push(cause);
1182 let candidate_set = self.assemble_candidates(stack)?;
1184 if candidate_set.ambiguous {
1185 debug!("candidate set contains ambig");
1189 let mut candidates = candidate_set.vec;
1191 debug!("assembled {} candidates for {:?}: {:?}",
1196 // At this point, we know that each of the entries in the
1197 // candidate set is *individually* applicable. Now we have to
1198 // figure out if they contain mutual incompatibilities. This
1199 // frequently arises if we have an unconstrained input type --
1200 // for example, we are looking for $0:Eq where $0 is some
1201 // unconstrained type variable. In that case, we'll get a
1202 // candidate which assumes $0 == int, one that assumes $0 ==
1203 // usize, etc. This spells an ambiguity.
1205 // If there is more than one candidate, first winnow them down
1206 // by considering extra conditions (nested obligations and so
1207 // forth). We don't winnow if there is exactly one
1208 // candidate. This is a relatively minor distinction but it
1209 // can lead to better inference and error-reporting. An
1210 // example would be if there was an impl:
1212 // impl<T:Clone> Vec<T> { fn push_clone(...) { ... } }
1214 // and we were to see some code `foo.push_clone()` where `boo`
1215 // is a `Vec<Bar>` and `Bar` does not implement `Clone`. If
1216 // we were to winnow, we'd wind up with zero candidates.
1217 // Instead, we select the right impl now but report `Bar does
1218 // not implement Clone`.
1219 if candidates.len() == 1 {
1220 return self.filter_negative_impls(candidates.pop().unwrap());
1223 // Winnow, but record the exact outcome of evaluation, which
1224 // is needed for specialization. Propagate overflow if it occurs.
1225 let candidates: Result<Vec<Option<EvaluatedCandidate>>, _> = candidates
1227 .map(|c| match self.evaluate_candidate(stack, &c) {
1228 Ok(eval) if eval.may_apply() => Ok(Some(EvaluatedCandidate {
1233 Err(OverflowError) => Err(Overflow),
1237 let mut candidates: Vec<EvaluatedCandidate> =
1238 candidates?.into_iter().filter_map(|c| c).collect();
1240 debug!("winnowed to {} candidates for {:?}: {:?}",
1245 // If there are STILL multiple candidate, we can further
1246 // reduce the list by dropping duplicates -- including
1247 // resolving specializations.
1248 if candidates.len() > 1 {
1250 while i < candidates.len() {
1252 (0..candidates.len())
1253 .filter(|&j| i != j)
1254 .any(|j| self.candidate_should_be_dropped_in_favor_of(&candidates[i],
1257 debug!("Dropping candidate #{}/{}: {:?}",
1258 i, candidates.len(), candidates[i]);
1259 candidates.swap_remove(i);
1261 debug!("Retaining candidate #{}/{}: {:?}",
1262 i, candidates.len(), candidates[i]);
1265 // If there are *STILL* multiple candidates, give up
1266 // and report ambiguity.
1268 debug!("multiple matches, ambig");
1275 // If there are *NO* candidates, then there are no impls --
1276 // that we know of, anyway. Note that in the case where there
1277 // are unbound type variables within the obligation, it might
1278 // be the case that you could still satisfy the obligation
1279 // from another crate by instantiating the type variables with
1280 // a type from another crate that does have an impl. This case
1281 // is checked for in `evaluate_stack` (and hence users
1282 // who might care about this case, like coherence, should use
1284 if candidates.is_empty() {
1285 return Err(Unimplemented);
1288 // Just one candidate left.
1289 self.filter_negative_impls(candidates.pop().unwrap().candidate)
1292 fn is_knowable<'o>(&mut self,
1293 stack: &TraitObligationStack<'o, 'tcx>)
1296 debug!("is_knowable(intercrate={:?})", self.intercrate);
1298 if !self.intercrate.is_some() {
1302 let obligation = &stack.obligation;
1303 let predicate = self.infcx().resolve_type_vars_if_possible(&obligation.predicate);
1305 // ok to skip binder because of the nature of the
1306 // trait-ref-is-knowable check, which does not care about
1308 let trait_ref = predicate.skip_binder().trait_ref;
1310 let result = coherence::trait_ref_is_knowable(self.tcx(), trait_ref);
1311 if let (Some(Conflict::Downstream { used_to_be_broken: true }),
1312 Some(IntercrateMode::Issue43355)) = (result, self.intercrate) {
1313 debug!("is_knowable: IGNORING conflict to be bug-compatible with #43355");
1320 /// Returns true if the global caches can be used.
1321 /// Do note that if the type itself is not in the
1322 /// global tcx, the local caches will be used.
1323 fn can_use_global_caches(&self, param_env: ty::ParamEnv<'tcx>) -> bool {
1324 // If there are any where-clauses in scope, then we always use
1325 // a cache local to this particular scope. Otherwise, we
1326 // switch to a global cache. We used to try and draw
1327 // finer-grained distinctions, but that led to a serious of
1328 // annoying and weird bugs like #22019 and #18290. This simple
1329 // rule seems to be pretty clearly safe and also still retains
1330 // a very high hit rate (~95% when compiling rustc).
1331 if !param_env.caller_bounds.is_empty() {
1335 // Avoid using the master cache during coherence and just rely
1336 // on the local cache. This effectively disables caching
1337 // during coherence. It is really just a simplification to
1338 // avoid us having to fear that coherence results "pollute"
1339 // the master cache. Since coherence executes pretty quickly,
1340 // it's not worth going to more trouble to increase the
1341 // hit-rate I don't think.
1342 if self.intercrate.is_some() {
1346 // Otherwise, we can use the global cache.
1350 fn check_candidate_cache(&mut self,
1351 param_env: ty::ParamEnv<'tcx>,
1352 cache_fresh_trait_pred: &ty::PolyTraitPredicate<'tcx>)
1353 -> Option<SelectionResult<'tcx, SelectionCandidate<'tcx>>>
1355 let tcx = self.tcx();
1356 let trait_ref = &cache_fresh_trait_pred.skip_binder().trait_ref;
1357 if self.can_use_global_caches(param_env) {
1358 let cache = tcx.selection_cache.hashmap.borrow();
1359 if let Some(cached) = cache.get(&trait_ref) {
1360 return Some(cached.get(tcx));
1363 self.infcx.selection_cache.hashmap
1366 .map(|v| v.get(tcx))
1369 fn insert_candidate_cache(&mut self,
1370 param_env: ty::ParamEnv<'tcx>,
1371 cache_fresh_trait_pred: ty::PolyTraitPredicate<'tcx>,
1372 dep_node: DepNodeIndex,
1373 candidate: SelectionResult<'tcx, SelectionCandidate<'tcx>>)
1375 let tcx = self.tcx();
1376 let trait_ref = cache_fresh_trait_pred.skip_binder().trait_ref;
1377 if self.can_use_global_caches(param_env) {
1378 if let Some(trait_ref) = tcx.lift_to_global(&trait_ref) {
1379 if let Some(candidate) = tcx.lift_to_global(&candidate) {
1381 "insert_candidate_cache(trait_ref={:?}, candidate={:?}) global",
1385 // This may overwrite the cache with the same value
1387 .hashmap.borrow_mut()
1388 .insert(trait_ref, WithDepNode::new(dep_node, candidate));
1395 "insert_candidate_cache(trait_ref={:?}, candidate={:?}) local",
1399 self.infcx.selection_cache.hashmap
1401 .insert(trait_ref, WithDepNode::new(dep_node, candidate));
1404 fn assemble_candidates<'o>(&mut self,
1405 stack: &TraitObligationStack<'o, 'tcx>)
1406 -> Result<SelectionCandidateSet<'tcx>, SelectionError<'tcx>>
1408 let TraitObligationStack { obligation, .. } = *stack;
1409 let ref obligation = Obligation {
1410 param_env: obligation.param_env,
1411 cause: obligation.cause.clone(),
1412 recursion_depth: obligation.recursion_depth,
1413 predicate: self.infcx().resolve_type_vars_if_possible(&obligation.predicate)
1416 if obligation.predicate.skip_binder().self_ty().is_ty_var() {
1417 // Self is a type variable (e.g. `_: AsRef<str>`).
1419 // This is somewhat problematic, as the current scheme can't really
1420 // handle it turning to be a projection. This does end up as truly
1421 // ambiguous in most cases anyway.
1423 // Take the fast path out - this also improves
1424 // performance by preventing assemble_candidates_from_impls from
1425 // matching every impl for this trait.
1426 return Ok(SelectionCandidateSet { vec: vec![], ambiguous: true });
1429 let mut candidates = SelectionCandidateSet {
1434 // Other bounds. Consider both in-scope bounds from fn decl
1435 // and applicable impls. There is a certain set of precedence rules here.
1437 let def_id = obligation.predicate.def_id();
1438 let lang_items = self.tcx().lang_items();
1439 if lang_items.copy_trait() == Some(def_id) {
1440 debug!("obligation self ty is {:?}",
1441 obligation.predicate.skip_binder().self_ty());
1443 // User-defined copy impls are permitted, but only for
1444 // structs and enums.
1445 self.assemble_candidates_from_impls(obligation, &mut candidates)?;
1447 // For other types, we'll use the builtin rules.
1448 let copy_conditions = self.copy_clone_conditions(obligation);
1449 self.assemble_builtin_bound_candidates(copy_conditions, &mut candidates)?;
1450 } else if lang_items.sized_trait() == Some(def_id) {
1451 // Sized is never implementable by end-users, it is
1452 // always automatically computed.
1453 let sized_conditions = self.sized_conditions(obligation);
1454 self.assemble_builtin_bound_candidates(sized_conditions,
1456 } else if lang_items.unsize_trait() == Some(def_id) {
1457 self.assemble_candidates_for_unsizing(obligation, &mut candidates);
1459 if lang_items.clone_trait() == Some(def_id) {
1460 // Same builtin conditions as `Copy`, i.e. every type which has builtin support
1461 // for `Copy` also has builtin support for `Clone`, + tuples and arrays of `Clone`
1462 // types have builtin support for `Clone`.
1463 let clone_conditions = self.copy_clone_conditions(obligation);
1464 self.assemble_builtin_bound_candidates(clone_conditions, &mut candidates)?;
1467 self.assemble_generator_candidates(obligation, &mut candidates)?;
1468 self.assemble_closure_candidates(obligation, &mut candidates)?;
1469 self.assemble_fn_pointer_candidates(obligation, &mut candidates)?;
1470 self.assemble_candidates_from_impls(obligation, &mut candidates)?;
1471 self.assemble_candidates_from_object_ty(obligation, &mut candidates);
1474 self.assemble_candidates_from_projected_tys(obligation, &mut candidates);
1475 self.assemble_candidates_from_caller_bounds(stack, &mut candidates)?;
1476 // Auto implementations have lower priority, so we only
1477 // consider triggering a default if there is no other impl that can apply.
1478 if candidates.vec.is_empty() {
1479 self.assemble_candidates_from_auto_impls(obligation, &mut candidates)?;
1481 debug!("candidate list size: {}", candidates.vec.len());
1485 fn assemble_candidates_from_projected_tys(&mut self,
1486 obligation: &TraitObligation<'tcx>,
1487 candidates: &mut SelectionCandidateSet<'tcx>)
1489 debug!("assemble_candidates_for_projected_tys({:?})", obligation);
1491 // before we go into the whole skolemization thing, just
1492 // quickly check if the self-type is a projection at all.
1493 match obligation.predicate.skip_binder().trait_ref.self_ty().sty {
1494 ty::TyProjection(_) | ty::TyAnon(..) => {}
1495 ty::TyInfer(ty::TyVar(_)) => {
1496 span_bug!(obligation.cause.span,
1497 "Self=_ should have been handled by assemble_candidates");
1502 let result = self.probe(|this, snapshot| {
1503 this.match_projection_obligation_against_definition_bounds(obligation,
1508 candidates.vec.push(ProjectionCandidate);
1512 fn match_projection_obligation_against_definition_bounds(
1514 obligation: &TraitObligation<'tcx>,
1515 snapshot: &infer::CombinedSnapshot<'cx, 'tcx>)
1518 let poly_trait_predicate =
1519 self.infcx().resolve_type_vars_if_possible(&obligation.predicate);
1520 let (skol_trait_predicate, skol_map) =
1521 self.infcx().skolemize_late_bound_regions(&poly_trait_predicate);
1522 debug!("match_projection_obligation_against_definition_bounds: \
1523 skol_trait_predicate={:?} skol_map={:?}",
1524 skol_trait_predicate,
1527 let (def_id, substs) = match skol_trait_predicate.trait_ref.self_ty().sty {
1528 ty::TyProjection(ref data) =>
1529 (data.trait_ref(self.tcx()).def_id, data.substs),
1530 ty::TyAnon(def_id, substs) => (def_id, substs),
1533 obligation.cause.span,
1534 "match_projection_obligation_against_definition_bounds() called \
1535 but self-ty not a projection: {:?}",
1536 skol_trait_predicate.trait_ref.self_ty());
1539 debug!("match_projection_obligation_against_definition_bounds: \
1540 def_id={:?}, substs={:?}",
1543 let predicates_of = self.tcx().predicates_of(def_id);
1544 let bounds = predicates_of.instantiate(self.tcx(), substs);
1545 debug!("match_projection_obligation_against_definition_bounds: \
1549 let matching_bound =
1550 util::elaborate_predicates(self.tcx(), bounds.predicates)
1554 |this, _| this.match_projection(obligation,
1556 skol_trait_predicate.trait_ref.clone(),
1560 debug!("match_projection_obligation_against_definition_bounds: \
1561 matching_bound={:?}",
1563 match matching_bound {
1566 // Repeat the successful match, if any, this time outside of a probe.
1567 let result = self.match_projection(obligation,
1569 skol_trait_predicate.trait_ref.clone(),
1573 self.infcx.pop_skolemized(skol_map, snapshot);
1581 fn match_projection(&mut self,
1582 obligation: &TraitObligation<'tcx>,
1583 trait_bound: ty::PolyTraitRef<'tcx>,
1584 skol_trait_ref: ty::TraitRef<'tcx>,
1585 skol_map: &infer::SkolemizationMap<'tcx>,
1586 snapshot: &infer::CombinedSnapshot<'cx, 'tcx>)
1589 assert!(!skol_trait_ref.has_escaping_regions());
1590 if let Err(_) = self.infcx.at(&obligation.cause, obligation.param_env)
1591 .sup(ty::Binder::dummy(skol_trait_ref), trait_bound) {
1595 self.infcx.leak_check(false, obligation.cause.span, skol_map, snapshot).is_ok()
1598 /// Given an obligation like `<SomeTrait for T>`, search the obligations that the caller
1599 /// supplied to find out whether it is listed among them.
1601 /// Never affects inference environment.
1602 fn assemble_candidates_from_caller_bounds<'o>(&mut self,
1603 stack: &TraitObligationStack<'o, 'tcx>,
1604 candidates: &mut SelectionCandidateSet<'tcx>)
1605 -> Result<(),SelectionError<'tcx>>
1607 debug!("assemble_candidates_from_caller_bounds({:?})",
1611 stack.obligation.param_env.caller_bounds
1613 .filter_map(|o| o.to_opt_poly_trait_ref());
1615 // micro-optimization: filter out predicates relating to different
1617 let matching_bounds =
1618 all_bounds.filter(|p| p.def_id() == stack.obligation.predicate.def_id());
1620 // keep only those bounds which may apply, and propagate overflow if it occurs
1621 let mut param_candidates = vec![];
1622 for bound in matching_bounds {
1623 let wc = self.evaluate_where_clause(stack, bound.clone())?;
1625 param_candidates.push(ParamCandidate(bound));
1629 candidates.vec.extend(param_candidates);
1634 fn evaluate_where_clause<'o>(&mut self,
1635 stack: &TraitObligationStack<'o, 'tcx>,
1636 where_clause_trait_ref: ty::PolyTraitRef<'tcx>)
1637 -> Result<EvaluationResult, OverflowError>
1639 self.probe(move |this, _| {
1640 match this.match_where_clause_trait_ref(stack.obligation, where_clause_trait_ref) {
1641 Ok(obligations) => {
1642 this.evaluate_predicates_recursively(stack.list(), obligations.iter())
1644 Err(()) => Ok(EvaluatedToErr)
1649 fn assemble_generator_candidates(&mut self,
1650 obligation: &TraitObligation<'tcx>,
1651 candidates: &mut SelectionCandidateSet<'tcx>)
1652 -> Result<(),SelectionError<'tcx>>
1654 if self.tcx().lang_items().gen_trait() != Some(obligation.predicate.def_id()) {
1658 // ok to skip binder because the substs on generator types never
1659 // touch bound regions, they just capture the in-scope
1660 // type/region parameters
1661 let self_ty = *obligation.self_ty().skip_binder();
1663 ty::TyGenerator(..) => {
1664 debug!("assemble_generator_candidates: self_ty={:?} obligation={:?}",
1668 candidates.vec.push(GeneratorCandidate);
1671 ty::TyInfer(ty::TyVar(_)) => {
1672 debug!("assemble_generator_candidates: ambiguous self-type");
1673 candidates.ambiguous = true;
1676 _ => { return Ok(()); }
1680 /// Check for the artificial impl that the compiler will create for an obligation like `X :
1681 /// FnMut<..>` where `X` is a closure type.
1683 /// Note: the type parameters on a closure candidate are modeled as *output* type
1684 /// parameters and hence do not affect whether this trait is a match or not. They will be
1685 /// unified during the confirmation step.
1686 fn assemble_closure_candidates(&mut self,
1687 obligation: &TraitObligation<'tcx>,
1688 candidates: &mut SelectionCandidateSet<'tcx>)
1689 -> Result<(),SelectionError<'tcx>>
1691 let kind = match self.tcx().lang_items().fn_trait_kind(obligation.predicate.def_id()) {
1693 None => { return Ok(()); }
1696 // ok to skip binder because the substs on closure types never
1697 // touch bound regions, they just capture the in-scope
1698 // type/region parameters
1699 match obligation.self_ty().skip_binder().sty {
1700 ty::TyClosure(closure_def_id, closure_substs) => {
1701 debug!("assemble_unboxed_candidates: kind={:?} obligation={:?}",
1703 match self.infcx.closure_kind(closure_def_id, closure_substs) {
1704 Some(closure_kind) => {
1705 debug!("assemble_unboxed_candidates: closure_kind = {:?}", closure_kind);
1706 if closure_kind.extends(kind) {
1707 candidates.vec.push(ClosureCandidate);
1711 debug!("assemble_unboxed_candidates: closure_kind not yet known");
1712 candidates.vec.push(ClosureCandidate);
1717 ty::TyInfer(ty::TyVar(_)) => {
1718 debug!("assemble_unboxed_closure_candidates: ambiguous self-type");
1719 candidates.ambiguous = true;
1722 _ => { return Ok(()); }
1726 /// Implement one of the `Fn()` family for a fn pointer.
1727 fn assemble_fn_pointer_candidates(&mut self,
1728 obligation: &TraitObligation<'tcx>,
1729 candidates: &mut SelectionCandidateSet<'tcx>)
1730 -> Result<(),SelectionError<'tcx>>
1732 // We provide impl of all fn traits for fn pointers.
1733 if self.tcx().lang_items().fn_trait_kind(obligation.predicate.def_id()).is_none() {
1737 // ok to skip binder because what we are inspecting doesn't involve bound regions
1738 let self_ty = *obligation.self_ty().skip_binder();
1740 ty::TyInfer(ty::TyVar(_)) => {
1741 debug!("assemble_fn_pointer_candidates: ambiguous self-type");
1742 candidates.ambiguous = true; // could wind up being a fn() type
1745 // provide an impl, but only for suitable `fn` pointers
1746 ty::TyFnDef(..) | ty::TyFnPtr(_) => {
1748 unsafety: hir::Unsafety::Normal,
1752 } = self_ty.fn_sig(self.tcx()).skip_binder() {
1753 candidates.vec.push(FnPointerCandidate);
1763 /// Search for impls that might apply to `obligation`.
1764 fn assemble_candidates_from_impls(&mut self,
1765 obligation: &TraitObligation<'tcx>,
1766 candidates: &mut SelectionCandidateSet<'tcx>)
1767 -> Result<(), SelectionError<'tcx>>
1769 debug!("assemble_candidates_from_impls(obligation={:?})", obligation);
1771 self.tcx().for_each_relevant_impl(
1772 obligation.predicate.def_id(),
1773 obligation.predicate.skip_binder().trait_ref.self_ty(),
1775 self.probe(|this, snapshot| { /* [1] */
1776 match this.match_impl(impl_def_id, obligation, snapshot) {
1778 candidates.vec.push(ImplCandidate(impl_def_id));
1780 // NB: we can safely drop the skol map
1781 // since we are in a probe [1]
1782 mem::drop(skol_map);
1793 fn assemble_candidates_from_auto_impls(&mut self,
1794 obligation: &TraitObligation<'tcx>,
1795 candidates: &mut SelectionCandidateSet<'tcx>)
1796 -> Result<(), SelectionError<'tcx>>
1798 // OK to skip binder here because the tests we do below do not involve bound regions
1799 let self_ty = *obligation.self_ty().skip_binder();
1800 debug!("assemble_candidates_from_auto_impls(self_ty={:?})", self_ty);
1802 let def_id = obligation.predicate.def_id();
1804 if self.tcx().trait_is_auto(def_id) {
1806 ty::TyDynamic(..) => {
1807 // For object types, we don't know what the closed
1808 // over types are. This means we conservatively
1809 // say nothing; a candidate may be added by
1810 // `assemble_candidates_from_object_ty`.
1812 ty::TyForeign(..) => {
1813 // Since the contents of foreign types is unknown,
1814 // we don't add any `..` impl. Default traits could
1815 // still be provided by a manual implementation for
1816 // this trait and type.
1819 ty::TyProjection(..) => {
1820 // In these cases, we don't know what the actual
1821 // type is. Therefore, we cannot break it down
1822 // into its constituent types. So we don't
1823 // consider the `..` impl but instead just add no
1824 // candidates: this means that typeck will only
1825 // succeed if there is another reason to believe
1826 // that this obligation holds. That could be a
1827 // where-clause or, in the case of an object type,
1828 // it could be that the object type lists the
1829 // trait (e.g. `Foo+Send : Send`). See
1830 // `compile-fail/typeck-default-trait-impl-send-param.rs`
1831 // for an example of a test case that exercises
1834 ty::TyInfer(ty::TyVar(_)) => {
1835 // the auto impl might apply, we don't know
1836 candidates.ambiguous = true;
1839 candidates.vec.push(AutoImplCandidate(def_id.clone()))
1847 /// Search for impls that might apply to `obligation`.
1848 fn assemble_candidates_from_object_ty(&mut self,
1849 obligation: &TraitObligation<'tcx>,
1850 candidates: &mut SelectionCandidateSet<'tcx>)
1852 debug!("assemble_candidates_from_object_ty(self_ty={:?})",
1853 obligation.self_ty().skip_binder());
1855 // Object-safety candidates are only applicable to object-safe
1856 // traits. Including this check is useful because it helps
1857 // inference in cases of traits like `BorrowFrom`, which are
1858 // not object-safe, and which rely on being able to infer the
1859 // self-type from one of the other inputs. Without this check,
1860 // these cases wind up being considered ambiguous due to a
1861 // (spurious) ambiguity introduced here.
1862 let predicate_trait_ref = obligation.predicate.to_poly_trait_ref();
1863 if !self.tcx().is_object_safe(predicate_trait_ref.def_id()) {
1867 self.probe(|this, _snapshot| {
1868 // the code below doesn't care about regions, and the
1869 // self-ty here doesn't escape this probe, so just erase
1871 let self_ty = this.tcx().erase_late_bound_regions(&obligation.self_ty());
1872 let poly_trait_ref = match self_ty.sty {
1873 ty::TyDynamic(ref data, ..) => {
1874 if data.auto_traits().any(|did| did == obligation.predicate.def_id()) {
1875 debug!("assemble_candidates_from_object_ty: matched builtin bound, \
1876 pushing candidate");
1877 candidates.vec.push(BuiltinObjectCandidate);
1881 match data.principal() {
1882 Some(p) => p.with_self_ty(this.tcx(), self_ty),
1886 ty::TyInfer(ty::TyVar(_)) => {
1887 debug!("assemble_candidates_from_object_ty: ambiguous");
1888 candidates.ambiguous = true; // could wind up being an object type
1896 debug!("assemble_candidates_from_object_ty: poly_trait_ref={:?}",
1899 // Count only those upcast versions that match the trait-ref
1900 // we are looking for. Specifically, do not only check for the
1901 // correct trait, but also the correct type parameters.
1902 // For example, we may be trying to upcast `Foo` to `Bar<i32>`,
1903 // but `Foo` is declared as `trait Foo : Bar<u32>`.
1904 let upcast_trait_refs =
1905 util::supertraits(this.tcx(), poly_trait_ref)
1906 .filter(|upcast_trait_ref| {
1907 this.probe(|this, _| {
1908 let upcast_trait_ref = upcast_trait_ref.clone();
1909 this.match_poly_trait_ref(obligation, upcast_trait_ref).is_ok()
1914 if upcast_trait_refs > 1 {
1915 // can be upcast in many ways; need more type information
1916 candidates.ambiguous = true;
1917 } else if upcast_trait_refs == 1 {
1918 candidates.vec.push(ObjectCandidate);
1923 /// Search for unsizing that might apply to `obligation`.
1924 fn assemble_candidates_for_unsizing(&mut self,
1925 obligation: &TraitObligation<'tcx>,
1926 candidates: &mut SelectionCandidateSet<'tcx>) {
1927 // We currently never consider higher-ranked obligations e.g.
1928 // `for<'a> &'a T: Unsize<Trait+'a>` to be implemented. This is not
1929 // because they are a priori invalid, and we could potentially add support
1930 // for them later, it's just that there isn't really a strong need for it.
1931 // A `T: Unsize<U>` obligation is always used as part of a `T: CoerceUnsize<U>`
1932 // impl, and those are generally applied to concrete types.
1934 // That said, one might try to write a fn with a where clause like
1935 // for<'a> Foo<'a, T>: Unsize<Foo<'a, Trait>>
1936 // where the `'a` is kind of orthogonal to the relevant part of the `Unsize`.
1937 // Still, you'd be more likely to write that where clause as
1939 // so it seems ok if we (conservatively) fail to accept that `Unsize`
1940 // obligation above. Should be possible to extend this in the future.
1941 let source = match obligation.self_ty().no_late_bound_regions() {
1944 // Don't add any candidates if there are bound regions.
1948 let target = obligation.predicate.skip_binder().trait_ref.substs.type_at(1);
1950 debug!("assemble_candidates_for_unsizing(source={:?}, target={:?})",
1953 let may_apply = match (&source.sty, &target.sty) {
1954 // Trait+Kx+'a -> Trait+Ky+'b (upcasts).
1955 (&ty::TyDynamic(ref data_a, ..), &ty::TyDynamic(ref data_b, ..)) => {
1956 // Upcasts permit two things:
1958 // 1. Dropping builtin bounds, e.g. `Foo+Send` to `Foo`
1959 // 2. Tightening the region bound, e.g. `Foo+'a` to `Foo+'b` if `'a : 'b`
1961 // Note that neither of these changes requires any
1962 // change at runtime. Eventually this will be
1965 // We always upcast when we can because of reason
1966 // #2 (region bounds).
1967 match (data_a.principal(), data_b.principal()) {
1968 (Some(a), Some(b)) => a.def_id() == b.def_id() &&
1969 data_b.auto_traits()
1970 // All of a's auto traits need to be in b's auto traits.
1971 .all(|b| data_a.auto_traits().any(|a| a == b)),
1977 (_, &ty::TyDynamic(..)) => true,
1979 // Ambiguous handling is below T -> Trait, because inference
1980 // variables can still implement Unsize<Trait> and nested
1981 // obligations will have the final say (likely deferred).
1982 (&ty::TyInfer(ty::TyVar(_)), _) |
1983 (_, &ty::TyInfer(ty::TyVar(_))) => {
1984 debug!("assemble_candidates_for_unsizing: ambiguous");
1985 candidates.ambiguous = true;
1990 (&ty::TyArray(..), &ty::TySlice(_)) => true,
1992 // Struct<T> -> Struct<U>.
1993 (&ty::TyAdt(def_id_a, _), &ty::TyAdt(def_id_b, _)) if def_id_a.is_struct() => {
1994 def_id_a == def_id_b
1997 // (.., T) -> (.., U).
1998 (&ty::TyTuple(tys_a), &ty::TyTuple(tys_b)) => {
1999 tys_a.len() == tys_b.len()
2006 candidates.vec.push(BuiltinUnsizeCandidate);
2010 ///////////////////////////////////////////////////////////////////////////
2013 // Winnowing is the process of attempting to resolve ambiguity by
2014 // probing further. During the winnowing process, we unify all
2015 // type variables (ignoring skolemization) and then we also
2016 // attempt to evaluate recursive bounds to see if they are
2019 /// Returns true if `victim` should be dropped in favor of
2020 /// `other`. Generally speaking we will drop duplicate
2021 /// candidates and prefer where-clause candidates.
2023 /// See the comment for "SelectionCandidate" for more details.
2024 fn candidate_should_be_dropped_in_favor_of<'o>(
2026 victim: &EvaluatedCandidate<'tcx>,
2027 other: &EvaluatedCandidate<'tcx>)
2030 // Check if a bound would previously have been removed when normalizing
2031 // the param_env so that it can be given the lowest priority. See
2032 // #50825 for the motivation for this.
2033 let is_global = |cand: &ty::PolyTraitRef<'_>| {
2034 cand.is_global() && !cand.has_late_bound_regions()
2037 if victim.candidate == other.candidate {
2041 match other.candidate {
2042 ParamCandidate(ref cand) => match victim.candidate {
2043 AutoImplCandidate(..) => {
2045 "default implementations shouldn't be recorded \
2046 when there are other valid candidates");
2050 GeneratorCandidate |
2051 FnPointerCandidate |
2052 BuiltinObjectCandidate |
2053 BuiltinUnsizeCandidate |
2054 BuiltinCandidate { .. } => {
2055 // Global bounds from the where clause should be ignored
2056 // here (see issue #50825). Otherwise, we have a where
2057 // clause so don't go around looking for impls.
2061 ProjectionCandidate => {
2062 // Arbitrarily give param candidates priority
2063 // over projection and object candidates.
2066 ParamCandidate(..) => false,
2069 ProjectionCandidate => match victim.candidate {
2070 AutoImplCandidate(..) => {
2072 "default implementations shouldn't be recorded \
2073 when there are other valid candidates");
2077 GeneratorCandidate |
2078 FnPointerCandidate |
2079 BuiltinObjectCandidate |
2080 BuiltinUnsizeCandidate |
2081 BuiltinCandidate { .. } => {
2085 ProjectionCandidate => {
2086 // Arbitrarily give param candidates priority
2087 // over projection and object candidates.
2090 ParamCandidate(ref cand) => is_global(cand),
2092 ImplCandidate(other_def) => {
2093 // See if we can toss out `victim` based on specialization.
2094 // This requires us to know *for sure* that the `other` impl applies
2095 // i.e. EvaluatedToOk:
2096 if other.evaluation == EvaluatedToOk {
2097 match victim.candidate {
2098 ImplCandidate(victim_def) => {
2099 let tcx = self.tcx().global_tcx();
2100 return tcx.specializes((other_def, victim_def)) ||
2101 tcx.impls_are_allowed_to_overlap(other_def, victim_def);
2103 ParamCandidate(ref cand) => {
2104 // Prefer the impl to a global where clause candidate.
2105 return is_global(cand);
2114 GeneratorCandidate |
2115 FnPointerCandidate |
2116 BuiltinObjectCandidate |
2117 BuiltinUnsizeCandidate |
2118 BuiltinCandidate { .. } => {
2119 match victim.candidate {
2120 ParamCandidate(ref cand) => {
2121 // Prefer these to a global where-clause bound
2122 // (see issue #50825)
2123 is_global(cand) && other.evaluation == EvaluatedToOk
2132 ///////////////////////////////////////////////////////////////////////////
2135 // These cover the traits that are built-in to the language
2136 // itself: `Copy`, `Clone` and `Sized`.
2138 fn assemble_builtin_bound_candidates<'o>(&mut self,
2139 conditions: BuiltinImplConditions<'tcx>,
2140 candidates: &mut SelectionCandidateSet<'tcx>)
2141 -> Result<(),SelectionError<'tcx>>
2144 BuiltinImplConditions::Where(nested) => {
2145 debug!("builtin_bound: nested={:?}", nested);
2146 candidates.vec.push(BuiltinCandidate {
2147 has_nested: nested.skip_binder().len() > 0
2151 BuiltinImplConditions::None => { Ok(()) }
2152 BuiltinImplConditions::Ambiguous => {
2153 debug!("assemble_builtin_bound_candidates: ambiguous builtin");
2154 Ok(candidates.ambiguous = true)
2159 fn sized_conditions(&mut self, obligation: &TraitObligation<'tcx>)
2160 -> BuiltinImplConditions<'tcx>
2162 use self::BuiltinImplConditions::{Ambiguous, None, Where};
2164 // NOTE: binder moved to (*)
2165 let self_ty = self.infcx.shallow_resolve(
2166 obligation.predicate.skip_binder().self_ty());
2169 ty::TyInfer(ty::IntVar(_)) | ty::TyInfer(ty::FloatVar(_)) |
2170 ty::TyUint(_) | ty::TyInt(_) | ty::TyBool | ty::TyFloat(_) |
2171 ty::TyFnDef(..) | ty::TyFnPtr(_) | ty::TyRawPtr(..) |
2172 ty::TyChar | ty::TyRef(..) | ty::TyGenerator(..) |
2173 ty::TyGeneratorWitness(..) | ty::TyArray(..) | ty::TyClosure(..) |
2174 ty::TyNever | ty::TyError => {
2175 // safe for everything
2176 Where(ty::Binder::dummy(Vec::new()))
2179 ty::TyStr | ty::TySlice(_) | ty::TyDynamic(..) | ty::TyForeign(..) => None,
2181 ty::TyTuple(tys) => {
2182 Where(ty::Binder::bind(tys.last().into_iter().cloned().collect()))
2185 ty::TyAdt(def, substs) => {
2186 let sized_crit = def.sized_constraint(self.tcx());
2187 // (*) binder moved here
2188 Where(ty::Binder::bind(
2189 sized_crit.iter().map(|ty| ty.subst(self.tcx(), substs)).collect()
2193 ty::TyProjection(_) | ty::TyParam(_) | ty::TyAnon(..) => None,
2194 ty::TyInfer(ty::TyVar(_)) => Ambiguous,
2196 ty::TyInfer(ty::CanonicalTy(_)) |
2197 ty::TyInfer(ty::FreshTy(_)) |
2198 ty::TyInfer(ty::FreshIntTy(_)) |
2199 ty::TyInfer(ty::FreshFloatTy(_)) => {
2200 bug!("asked to assemble builtin bounds of unexpected type: {:?}",
2206 fn copy_clone_conditions(&mut self, obligation: &TraitObligation<'tcx>)
2207 -> BuiltinImplConditions<'tcx>
2209 // NOTE: binder moved to (*)
2210 let self_ty = self.infcx.shallow_resolve(
2211 obligation.predicate.skip_binder().self_ty());
2213 use self::BuiltinImplConditions::{Ambiguous, None, Where};
2216 ty::TyInfer(ty::IntVar(_)) | ty::TyInfer(ty::FloatVar(_)) |
2217 ty::TyFnDef(..) | ty::TyFnPtr(_) | ty::TyError => {
2218 Where(ty::Binder::dummy(Vec::new()))
2221 ty::TyUint(_) | ty::TyInt(_) | ty::TyBool | ty::TyFloat(_) |
2222 ty::TyChar | ty::TyRawPtr(..) | ty::TyNever |
2223 ty::TyRef(_, _, hir::MutImmutable) => {
2224 // Implementations provided in libcore
2228 ty::TyDynamic(..) | ty::TyStr | ty::TySlice(..) |
2229 ty::TyGenerator(..) | ty::TyGeneratorWitness(..) | ty::TyForeign(..) |
2230 ty::TyRef(_, _, hir::MutMutable) => {
2234 ty::TyArray(element_ty, _) => {
2235 // (*) binder moved here
2236 Where(ty::Binder::bind(vec![element_ty]))
2239 ty::TyTuple(tys) => {
2240 // (*) binder moved here
2241 Where(ty::Binder::bind(tys.to_vec()))
2244 ty::TyClosure(def_id, substs) => {
2245 let trait_id = obligation.predicate.def_id();
2246 let is_copy_trait = Some(trait_id) == self.tcx().lang_items().copy_trait();
2247 let is_clone_trait = Some(trait_id) == self.tcx().lang_items().clone_trait();
2248 if is_copy_trait || is_clone_trait {
2249 Where(ty::Binder::bind(substs.upvar_tys(def_id, self.tcx()).collect()))
2255 ty::TyAdt(..) | ty::TyProjection(..) | ty::TyParam(..) | ty::TyAnon(..) => {
2256 // Fallback to whatever user-defined impls exist in this case.
2260 ty::TyInfer(ty::TyVar(_)) => {
2261 // Unbound type variable. Might or might not have
2262 // applicable impls and so forth, depending on what
2263 // those type variables wind up being bound to.
2267 ty::TyInfer(ty::CanonicalTy(_)) |
2268 ty::TyInfer(ty::FreshTy(_)) |
2269 ty::TyInfer(ty::FreshIntTy(_)) |
2270 ty::TyInfer(ty::FreshFloatTy(_)) => {
2271 bug!("asked to assemble builtin bounds of unexpected type: {:?}",
2277 /// For default impls, we need to break apart a type into its
2278 /// "constituent types" -- meaning, the types that it contains.
2280 /// Here are some (simple) examples:
2283 /// (i32, u32) -> [i32, u32]
2284 /// Foo where struct Foo { x: i32, y: u32 } -> [i32, u32]
2285 /// Bar<i32> where struct Bar<T> { x: T, y: u32 } -> [i32, u32]
2286 /// Zed<i32> where enum Zed { A(T), B(u32) } -> [i32, u32]
2288 fn constituent_types_for_ty(&self, t: Ty<'tcx>) -> Vec<Ty<'tcx>> {
2298 ty::TyInfer(ty::IntVar(_)) |
2299 ty::TyInfer(ty::FloatVar(_)) |
2308 ty::TyProjection(..) |
2309 ty::TyInfer(ty::CanonicalTy(_)) |
2310 ty::TyInfer(ty::TyVar(_)) |
2311 ty::TyInfer(ty::FreshTy(_)) |
2312 ty::TyInfer(ty::FreshIntTy(_)) |
2313 ty::TyInfer(ty::FreshFloatTy(_)) => {
2314 bug!("asked to assemble constituent types of unexpected type: {:?}",
2318 ty::TyRawPtr(ty::TypeAndMut { ty: element_ty, ..}) |
2319 ty::TyRef(_, element_ty, _) => {
2323 ty::TyArray(element_ty, _) | ty::TySlice(element_ty) => {
2327 ty::TyTuple(ref tys) => {
2328 // (T1, ..., Tn) -- meets any bound that all of T1...Tn meet
2332 ty::TyClosure(def_id, ref substs) => {
2333 substs.upvar_tys(def_id, self.tcx()).collect()
2336 ty::TyGenerator(def_id, ref substs, _) => {
2337 let witness = substs.witness(def_id, self.tcx());
2338 substs.upvar_tys(def_id, self.tcx()).chain(iter::once(witness)).collect()
2341 ty::TyGeneratorWitness(types) => {
2342 // This is sound because no regions in the witness can refer to
2343 // the binder outside the witness. So we'll effectivly reuse
2344 // the implicit binder around the witness.
2345 types.skip_binder().to_vec()
2348 // for `PhantomData<T>`, we pass `T`
2349 ty::TyAdt(def, substs) if def.is_phantom_data() => {
2350 substs.types().collect()
2353 ty::TyAdt(def, substs) => {
2355 .map(|f| f.ty(self.tcx(), substs))
2359 ty::TyAnon(def_id, substs) => {
2360 // We can resolve the `impl Trait` to its concrete type,
2361 // which enforces a DAG between the functions requiring
2362 // the auto trait bounds in question.
2363 vec![self.tcx().type_of(def_id).subst(self.tcx(), substs)]
2368 fn collect_predicates_for_types(&mut self,
2369 param_env: ty::ParamEnv<'tcx>,
2370 cause: ObligationCause<'tcx>,
2371 recursion_depth: usize,
2372 trait_def_id: DefId,
2373 types: ty::Binder<Vec<Ty<'tcx>>>)
2374 -> Vec<PredicateObligation<'tcx>>
2376 // Because the types were potentially derived from
2377 // higher-ranked obligations they may reference late-bound
2378 // regions. For example, `for<'a> Foo<&'a int> : Copy` would
2379 // yield a type like `for<'a> &'a int`. In general, we
2380 // maintain the invariant that we never manipulate bound
2381 // regions, so we have to process these bound regions somehow.
2383 // The strategy is to:
2385 // 1. Instantiate those regions to skolemized regions (e.g.,
2386 // `for<'a> &'a int` becomes `&0 int`.
2387 // 2. Produce something like `&'0 int : Copy`
2388 // 3. Re-bind the regions back to `for<'a> &'a int : Copy`
2390 types.skip_binder().into_iter().flat_map(|ty| { // binder moved -\
2391 let ty: ty::Binder<Ty<'tcx>> = ty::Binder::bind(ty); // <----/
2393 self.in_snapshot(|this, snapshot| {
2394 let (skol_ty, skol_map) =
2395 this.infcx().skolemize_late_bound_regions(&ty);
2396 let Normalized { value: normalized_ty, mut obligations } =
2397 project::normalize_with_depth(this,
2402 let skol_obligation =
2403 this.tcx().predicate_for_trait_def(param_env,
2409 obligations.push(skol_obligation);
2410 this.infcx().plug_leaks(skol_map, snapshot, obligations)
2415 ///////////////////////////////////////////////////////////////////////////
2418 // Confirmation unifies the output type parameters of the trait
2419 // with the values found in the obligation, possibly yielding a
2420 // type error. See [rustc guide] for more details.
2423 // https://rust-lang-nursery.github.io/rustc-guide/traits/resolution.html#confirmation
2425 fn confirm_candidate(&mut self,
2426 obligation: &TraitObligation<'tcx>,
2427 candidate: SelectionCandidate<'tcx>)
2428 -> Result<Selection<'tcx>,SelectionError<'tcx>>
2430 debug!("confirm_candidate({:?}, {:?})",
2435 BuiltinCandidate { has_nested } => {
2436 let data = self.confirm_builtin_candidate(obligation, has_nested);
2437 Ok(VtableBuiltin(data))
2440 ParamCandidate(param) => {
2441 let obligations = self.confirm_param_candidate(obligation, param);
2442 Ok(VtableParam(obligations))
2445 AutoImplCandidate(trait_def_id) => {
2446 let data = self.confirm_auto_impl_candidate(obligation, trait_def_id);
2447 Ok(VtableAutoImpl(data))
2450 ImplCandidate(impl_def_id) => {
2451 Ok(VtableImpl(self.confirm_impl_candidate(obligation, impl_def_id)))
2454 ClosureCandidate => {
2455 let vtable_closure = self.confirm_closure_candidate(obligation)?;
2456 Ok(VtableClosure(vtable_closure))
2459 GeneratorCandidate => {
2460 let vtable_generator = self.confirm_generator_candidate(obligation)?;
2461 Ok(VtableGenerator(vtable_generator))
2464 BuiltinObjectCandidate => {
2465 // This indicates something like `(Trait+Send) :
2466 // Send`. In this case, we know that this holds
2467 // because that's what the object type is telling us,
2468 // and there's really no additional obligations to
2469 // prove and no types in particular to unify etc.
2470 Ok(VtableParam(Vec::new()))
2473 ObjectCandidate => {
2474 let data = self.confirm_object_candidate(obligation);
2475 Ok(VtableObject(data))
2478 FnPointerCandidate => {
2480 self.confirm_fn_pointer_candidate(obligation)?;
2481 Ok(VtableFnPointer(data))
2484 ProjectionCandidate => {
2485 self.confirm_projection_candidate(obligation);
2486 Ok(VtableParam(Vec::new()))
2489 BuiltinUnsizeCandidate => {
2490 let data = self.confirm_builtin_unsize_candidate(obligation)?;
2491 Ok(VtableBuiltin(data))
2496 fn confirm_projection_candidate(&mut self,
2497 obligation: &TraitObligation<'tcx>)
2499 self.in_snapshot(|this, snapshot| {
2501 this.match_projection_obligation_against_definition_bounds(obligation,
2507 fn confirm_param_candidate(&mut self,
2508 obligation: &TraitObligation<'tcx>,
2509 param: ty::PolyTraitRef<'tcx>)
2510 -> Vec<PredicateObligation<'tcx>>
2512 debug!("confirm_param_candidate({:?},{:?})",
2516 // During evaluation, we already checked that this
2517 // where-clause trait-ref could be unified with the obligation
2518 // trait-ref. Repeat that unification now without any
2519 // transactional boundary; it should not fail.
2520 match self.match_where_clause_trait_ref(obligation, param.clone()) {
2521 Ok(obligations) => obligations,
2523 bug!("Where clause `{:?}` was applicable to `{:?}` but now is not",
2530 fn confirm_builtin_candidate(&mut self,
2531 obligation: &TraitObligation<'tcx>,
2533 -> VtableBuiltinData<PredicateObligation<'tcx>>
2535 debug!("confirm_builtin_candidate({:?}, {:?})",
2536 obligation, has_nested);
2538 let lang_items = self.tcx().lang_items();
2539 let obligations = if has_nested {
2540 let trait_def = obligation.predicate.def_id();
2541 let conditions = match trait_def {
2542 _ if Some(trait_def) == lang_items.sized_trait() => {
2543 self.sized_conditions(obligation)
2545 _ if Some(trait_def) == lang_items.copy_trait() => {
2546 self.copy_clone_conditions(obligation)
2548 _ if Some(trait_def) == lang_items.clone_trait() => {
2549 self.copy_clone_conditions(obligation)
2551 _ => bug!("unexpected builtin trait {:?}", trait_def)
2553 let nested = match conditions {
2554 BuiltinImplConditions::Where(nested) => nested,
2555 _ => bug!("obligation {:?} had matched a builtin impl but now doesn't",
2559 let cause = obligation.derived_cause(BuiltinDerivedObligation);
2560 self.collect_predicates_for_types(obligation.param_env,
2562 obligation.recursion_depth+1,
2569 debug!("confirm_builtin_candidate: obligations={:?}",
2572 VtableBuiltinData { nested: obligations }
2575 /// This handles the case where a `auto trait Foo` impl is being used.
2576 /// The idea is that the impl applies to `X : Foo` if the following conditions are met:
2578 /// 1. For each constituent type `Y` in `X`, `Y : Foo` holds
2579 /// 2. For each where-clause `C` declared on `Foo`, `[Self => X] C` holds.
2580 fn confirm_auto_impl_candidate(&mut self,
2581 obligation: &TraitObligation<'tcx>,
2582 trait_def_id: DefId)
2583 -> VtableAutoImplData<PredicateObligation<'tcx>>
2585 debug!("confirm_auto_impl_candidate({:?}, {:?})",
2589 let types = obligation.predicate.map_bound(|inner| {
2590 let self_ty = self.infcx.shallow_resolve(inner.self_ty());
2591 self.constituent_types_for_ty(self_ty)
2593 self.vtable_auto_impl(obligation, trait_def_id, types)
2596 /// See `confirm_auto_impl_candidate`
2597 fn vtable_auto_impl(&mut self,
2598 obligation: &TraitObligation<'tcx>,
2599 trait_def_id: DefId,
2600 nested: ty::Binder<Vec<Ty<'tcx>>>)
2601 -> VtableAutoImplData<PredicateObligation<'tcx>>
2603 debug!("vtable_auto_impl: nested={:?}", nested);
2605 let cause = obligation.derived_cause(BuiltinDerivedObligation);
2606 let mut obligations = self.collect_predicates_for_types(
2607 obligation.param_env,
2609 obligation.recursion_depth+1,
2613 let trait_obligations = self.in_snapshot(|this, snapshot| {
2614 let poly_trait_ref = obligation.predicate.to_poly_trait_ref();
2615 let (trait_ref, skol_map) =
2616 this.infcx().skolemize_late_bound_regions(&poly_trait_ref);
2617 let cause = obligation.derived_cause(ImplDerivedObligation);
2618 this.impl_or_trait_obligations(cause,
2619 obligation.recursion_depth + 1,
2620 obligation.param_env,
2627 obligations.extend(trait_obligations);
2629 debug!("vtable_auto_impl: obligations={:?}", obligations);
2631 VtableAutoImplData {
2637 fn confirm_impl_candidate(&mut self,
2638 obligation: &TraitObligation<'tcx>,
2640 -> VtableImplData<'tcx, PredicateObligation<'tcx>>
2642 debug!("confirm_impl_candidate({:?},{:?})",
2646 // First, create the substitutions by matching the impl again,
2647 // this time not in a probe.
2648 self.in_snapshot(|this, snapshot| {
2649 let (substs, skol_map) =
2650 this.rematch_impl(impl_def_id, obligation,
2652 debug!("confirm_impl_candidate substs={:?}", substs);
2653 let cause = obligation.derived_cause(ImplDerivedObligation);
2654 this.vtable_impl(impl_def_id,
2657 obligation.recursion_depth + 1,
2658 obligation.param_env,
2664 fn vtable_impl(&mut self,
2666 mut substs: Normalized<'tcx, &'tcx Substs<'tcx>>,
2667 cause: ObligationCause<'tcx>,
2668 recursion_depth: usize,
2669 param_env: ty::ParamEnv<'tcx>,
2670 skol_map: infer::SkolemizationMap<'tcx>,
2671 snapshot: &infer::CombinedSnapshot<'cx, 'tcx>)
2672 -> VtableImplData<'tcx, PredicateObligation<'tcx>>
2674 debug!("vtable_impl(impl_def_id={:?}, substs={:?}, recursion_depth={}, skol_map={:?})",
2680 let mut impl_obligations =
2681 self.impl_or_trait_obligations(cause,
2689 debug!("vtable_impl: impl_def_id={:?} impl_obligations={:?}",
2693 // Because of RFC447, the impl-trait-ref and obligations
2694 // are sufficient to determine the impl substs, without
2695 // relying on projections in the impl-trait-ref.
2697 // e.g. `impl<U: Tr, V: Iterator<Item=U>> Foo<<U as Tr>::T> for V`
2698 impl_obligations.append(&mut substs.obligations);
2700 VtableImplData { impl_def_id,
2701 substs: substs.value,
2702 nested: impl_obligations }
2705 fn confirm_object_candidate(&mut self,
2706 obligation: &TraitObligation<'tcx>)
2707 -> VtableObjectData<'tcx, PredicateObligation<'tcx>>
2709 debug!("confirm_object_candidate({:?})",
2712 // FIXME skipping binder here seems wrong -- we should
2713 // probably flatten the binder from the obligation and the
2714 // binder from the object. Have to try to make a broken test
2715 // case that results. -nmatsakis
2716 let self_ty = self.infcx.shallow_resolve(*obligation.self_ty().skip_binder());
2717 let poly_trait_ref = match self_ty.sty {
2718 ty::TyDynamic(ref data, ..) => {
2719 data.principal().unwrap().with_self_ty(self.tcx(), self_ty)
2722 span_bug!(obligation.cause.span,
2723 "object candidate with non-object");
2727 let mut upcast_trait_ref = None;
2728 let mut nested = vec![];
2732 let tcx = self.tcx();
2734 // We want to find the first supertrait in the list of
2735 // supertraits that we can unify with, and do that
2736 // unification. We know that there is exactly one in the list
2737 // where we can unify because otherwise select would have
2738 // reported an ambiguity. (When we do find a match, also
2739 // record it for later.)
2741 util::supertraits(tcx, poly_trait_ref)
2745 |this, _| this.match_poly_trait_ref(obligation, t))
2747 Ok(obligations) => {
2748 upcast_trait_ref = Some(t);
2749 nested.extend(obligations);
2756 // Additionally, for each of the nonmatching predicates that
2757 // we pass over, we sum up the set of number of vtable
2758 // entries, so that we can compute the offset for the selected
2761 nonmatching.map(|t| tcx.count_own_vtable_entries(t))
2767 upcast_trait_ref: upcast_trait_ref.unwrap(),
2773 fn confirm_fn_pointer_candidate(&mut self, obligation: &TraitObligation<'tcx>)
2774 -> Result<VtableFnPointerData<'tcx, PredicateObligation<'tcx>>, SelectionError<'tcx>>
2776 debug!("confirm_fn_pointer_candidate({:?})",
2779 // ok to skip binder; it is reintroduced below
2780 let self_ty = self.infcx.shallow_resolve(*obligation.self_ty().skip_binder());
2781 let sig = self_ty.fn_sig(self.tcx());
2783 self.tcx().closure_trait_ref_and_return_type(obligation.predicate.def_id(),
2786 util::TupleArgumentsFlag::Yes)
2787 .map_bound(|(trait_ref, _)| trait_ref);
2789 let Normalized { value: trait_ref, obligations } =
2790 project::normalize_with_depth(self,
2791 obligation.param_env,
2792 obligation.cause.clone(),
2793 obligation.recursion_depth + 1,
2796 self.confirm_poly_trait_refs(obligation.cause.clone(),
2797 obligation.param_env,
2798 obligation.predicate.to_poly_trait_ref(),
2800 Ok(VtableFnPointerData { fn_ty: self_ty, nested: obligations })
2803 fn confirm_generator_candidate(&mut self,
2804 obligation: &TraitObligation<'tcx>)
2805 -> Result<VtableGeneratorData<'tcx, PredicateObligation<'tcx>>,
2806 SelectionError<'tcx>>
2808 // ok to skip binder because the substs on generator types never
2809 // touch bound regions, they just capture the in-scope
2810 // type/region parameters
2811 let self_ty = self.infcx.shallow_resolve(obligation.self_ty().skip_binder());
2812 let (generator_def_id, substs) = match self_ty.sty {
2813 ty::TyGenerator(id, substs, _) => (id, substs),
2814 _ => bug!("closure candidate for non-closure {:?}", obligation)
2817 debug!("confirm_generator_candidate({:?},{:?},{:?})",
2823 self.generator_trait_ref_unnormalized(obligation, generator_def_id, substs);
2827 } = normalize_with_depth(self,
2828 obligation.param_env,
2829 obligation.cause.clone(),
2830 obligation.recursion_depth+1,
2833 debug!("confirm_generator_candidate(generator_def_id={:?}, \
2834 trait_ref={:?}, obligations={:?})",
2840 self.confirm_poly_trait_refs(obligation.cause.clone(),
2841 obligation.param_env,
2842 obligation.predicate.to_poly_trait_ref(),
2845 Ok(VtableGeneratorData {
2846 generator_def_id: generator_def_id,
2847 substs: substs.clone(),
2852 fn confirm_closure_candidate(&mut self,
2853 obligation: &TraitObligation<'tcx>)
2854 -> Result<VtableClosureData<'tcx, PredicateObligation<'tcx>>,
2855 SelectionError<'tcx>>
2857 debug!("confirm_closure_candidate({:?})", obligation);
2859 let kind = match self.tcx().lang_items().fn_trait_kind(obligation.predicate.def_id()) {
2861 None => bug!("closure candidate for non-fn trait {:?}", obligation)
2864 // ok to skip binder because the substs on closure types never
2865 // touch bound regions, they just capture the in-scope
2866 // type/region parameters
2867 let self_ty = self.infcx.shallow_resolve(obligation.self_ty().skip_binder());
2868 let (closure_def_id, substs) = match self_ty.sty {
2869 ty::TyClosure(id, substs) => (id, substs),
2870 _ => bug!("closure candidate for non-closure {:?}", obligation)
2874 self.closure_trait_ref_unnormalized(obligation, closure_def_id, substs);
2878 } = normalize_with_depth(self,
2879 obligation.param_env,
2880 obligation.cause.clone(),
2881 obligation.recursion_depth+1,
2884 debug!("confirm_closure_candidate(closure_def_id={:?}, trait_ref={:?}, obligations={:?})",
2890 self.confirm_poly_trait_refs(obligation.cause.clone(),
2891 obligation.param_env,
2892 obligation.predicate.to_poly_trait_ref(),
2895 obligations.push(Obligation::new(
2896 obligation.cause.clone(),
2897 obligation.param_env,
2898 ty::Predicate::ClosureKind(closure_def_id, substs, kind)));
2900 Ok(VtableClosureData {
2902 substs: substs.clone(),
2907 /// In the case of closure types and fn pointers,
2908 /// we currently treat the input type parameters on the trait as
2909 /// outputs. This means that when we have a match we have only
2910 /// considered the self type, so we have to go back and make sure
2911 /// to relate the argument types too. This is kind of wrong, but
2912 /// since we control the full set of impls, also not that wrong,
2913 /// and it DOES yield better error messages (since we don't report
2914 /// errors as if there is no applicable impl, but rather report
2915 /// errors are about mismatched argument types.
2917 /// Here is an example. Imagine we have a closure expression
2918 /// and we desugared it so that the type of the expression is
2919 /// `Closure`, and `Closure` expects an int as argument. Then it
2920 /// is "as if" the compiler generated this impl:
2922 /// impl Fn(int) for Closure { ... }
2924 /// Now imagine our obligation is `Fn(usize) for Closure`. So far
2925 /// we have matched the self-type `Closure`. At this point we'll
2926 /// compare the `int` to `usize` and generate an error.
2928 /// Note that this checking occurs *after* the impl has selected,
2929 /// because these output type parameters should not affect the
2930 /// selection of the impl. Therefore, if there is a mismatch, we
2931 /// report an error to the user.
2932 fn confirm_poly_trait_refs(&mut self,
2933 obligation_cause: ObligationCause<'tcx>,
2934 obligation_param_env: ty::ParamEnv<'tcx>,
2935 obligation_trait_ref: ty::PolyTraitRef<'tcx>,
2936 expected_trait_ref: ty::PolyTraitRef<'tcx>)
2937 -> Result<Vec<PredicateObligation<'tcx>>, SelectionError<'tcx>>
2939 let obligation_trait_ref = obligation_trait_ref.clone();
2941 .at(&obligation_cause, obligation_param_env)
2942 .sup(obligation_trait_ref, expected_trait_ref)
2943 .map(|InferOk { obligations, .. }| obligations)
2944 .map_err(|e| OutputTypeParameterMismatch(expected_trait_ref, obligation_trait_ref, e))
2947 fn confirm_builtin_unsize_candidate(&mut self,
2948 obligation: &TraitObligation<'tcx>,)
2949 -> Result<VtableBuiltinData<PredicateObligation<'tcx>>, SelectionError<'tcx>>
2951 let tcx = self.tcx();
2953 // assemble_candidates_for_unsizing should ensure there are no late bound
2954 // regions here. See the comment there for more details.
2955 let source = self.infcx.shallow_resolve(
2956 obligation.self_ty().no_late_bound_regions().unwrap());
2957 let target = obligation.predicate.skip_binder().trait_ref.substs.type_at(1);
2958 let target = self.infcx.shallow_resolve(target);
2960 debug!("confirm_builtin_unsize_candidate(source={:?}, target={:?})",
2963 let mut nested = vec![];
2964 match (&source.sty, &target.sty) {
2965 // Trait+Kx+'a -> Trait+Ky+'b (upcasts).
2966 (&ty::TyDynamic(ref data_a, r_a), &ty::TyDynamic(ref data_b, r_b)) => {
2967 // See assemble_candidates_for_unsizing for more info.
2968 let existential_predicates = data_a.map_bound(|data_a| {
2969 let principal = data_a.principal();
2970 let iter = principal.into_iter().map(ty::ExistentialPredicate::Trait)
2971 .chain(data_a.projection_bounds()
2972 .map(|x| ty::ExistentialPredicate::Projection(x)))
2973 .chain(data_b.auto_traits().map(ty::ExistentialPredicate::AutoTrait));
2974 tcx.mk_existential_predicates(iter)
2976 let new_trait = tcx.mk_dynamic(existential_predicates, r_b);
2977 let InferOk { obligations, .. } =
2978 self.infcx.at(&obligation.cause, obligation.param_env)
2979 .eq(target, new_trait)
2980 .map_err(|_| Unimplemented)?;
2981 nested.extend(obligations);
2983 // Register one obligation for 'a: 'b.
2984 let cause = ObligationCause::new(obligation.cause.span,
2985 obligation.cause.body_id,
2986 ObjectCastObligation(target));
2987 let outlives = ty::OutlivesPredicate(r_a, r_b);
2988 nested.push(Obligation::with_depth(cause,
2989 obligation.recursion_depth + 1,
2990 obligation.param_env,
2991 ty::Binder::bind(outlives).to_predicate()));
2995 (_, &ty::TyDynamic(ref data, r)) => {
2996 let mut object_dids =
2997 data.auto_traits().chain(data.principal().map(|p| p.def_id()));
2998 if let Some(did) = object_dids.find(|did| {
2999 !tcx.is_object_safe(*did)
3001 return Err(TraitNotObjectSafe(did))
3004 let cause = ObligationCause::new(obligation.cause.span,
3005 obligation.cause.body_id,
3006 ObjectCastObligation(target));
3007 let mut push = |predicate| {
3008 nested.push(Obligation::with_depth(cause.clone(),
3009 obligation.recursion_depth + 1,
3010 obligation.param_env,
3014 // Create obligations:
3015 // - Casting T to Trait
3016 // - For all the various builtin bounds attached to the object cast. (In other
3017 // words, if the object type is Foo+Send, this would create an obligation for the
3019 // - Projection predicates
3020 for predicate in data.iter() {
3021 push(predicate.with_self_ty(tcx, source));
3024 // We can only make objects from sized types.
3025 let tr = ty::TraitRef {
3026 def_id: tcx.require_lang_item(lang_items::SizedTraitLangItem),
3027 substs: tcx.mk_substs_trait(source, &[]),
3029 push(tr.to_predicate());
3031 // If the type is `Foo+'a`, ensures that the type
3032 // being cast to `Foo+'a` outlives `'a`:
3033 let outlives = ty::OutlivesPredicate(source, r);
3034 push(ty::Binder::dummy(outlives).to_predicate());
3038 (&ty::TyArray(a, _), &ty::TySlice(b)) => {
3039 let InferOk { obligations, .. } =
3040 self.infcx.at(&obligation.cause, obligation.param_env)
3042 .map_err(|_| Unimplemented)?;
3043 nested.extend(obligations);
3046 // Struct<T> -> Struct<U>.
3047 (&ty::TyAdt(def, substs_a), &ty::TyAdt(_, substs_b)) => {
3050 .map(|f| tcx.type_of(f.did))
3051 .collect::<Vec<_>>();
3053 // The last field of the structure has to exist and contain type parameters.
3054 let field = if let Some(&field) = fields.last() {
3057 return Err(Unimplemented);
3059 let mut ty_params = BitArray::new(substs_a.types().count());
3060 let mut found = false;
3061 for ty in field.walk() {
3062 if let ty::TyParam(p) = ty.sty {
3063 ty_params.insert(p.idx as usize);
3068 return Err(Unimplemented);
3071 // Replace type parameters used in unsizing with
3072 // TyError and ensure they do not affect any other fields.
3073 // This could be checked after type collection for any struct
3074 // with a potentially unsized trailing field.
3075 let params = substs_a.iter().enumerate().map(|(i, &k)| {
3076 if ty_params.contains(i) {
3077 tcx.types.err.into()
3082 let substs = tcx.mk_substs(params);
3083 for &ty in fields.split_last().unwrap().1 {
3084 if ty.subst(tcx, substs).references_error() {
3085 return Err(Unimplemented);
3089 // Extract Field<T> and Field<U> from Struct<T> and Struct<U>.
3090 let inner_source = field.subst(tcx, substs_a);
3091 let inner_target = field.subst(tcx, substs_b);
3093 // Check that the source struct with the target's
3094 // unsized parameters is equal to the target.
3095 let params = substs_a.iter().enumerate().map(|(i, &k)| {
3096 if ty_params.contains(i) {
3097 substs_b.type_at(i).into()
3102 let new_struct = tcx.mk_adt(def, tcx.mk_substs(params));
3103 let InferOk { obligations, .. } =
3104 self.infcx.at(&obligation.cause, obligation.param_env)
3105 .eq(target, new_struct)
3106 .map_err(|_| Unimplemented)?;
3107 nested.extend(obligations);
3109 // Construct the nested Field<T>: Unsize<Field<U>> predicate.
3110 nested.push(tcx.predicate_for_trait_def(
3111 obligation.param_env,
3112 obligation.cause.clone(),
3113 obligation.predicate.def_id(),
3114 obligation.recursion_depth + 1,
3116 &[inner_target.into()]));
3119 // (.., T) -> (.., U).
3120 (&ty::TyTuple(tys_a), &ty::TyTuple(tys_b)) => {
3121 assert_eq!(tys_a.len(), tys_b.len());
3123 // The last field of the tuple has to exist.
3124 let (&a_last, a_mid) = if let Some(x) = tys_a.split_last() {
3127 return Err(Unimplemented);
3129 let &b_last = tys_b.last().unwrap();
3131 // Check that the source tuple with the target's
3132 // last element is equal to the target.
3133 let new_tuple = tcx.mk_tup(a_mid.iter().cloned().chain(iter::once(b_last)));
3134 let InferOk { obligations, .. } =
3135 self.infcx.at(&obligation.cause, obligation.param_env)
3136 .eq(target, new_tuple)
3137 .map_err(|_| Unimplemented)?;
3138 nested.extend(obligations);
3140 // Construct the nested T: Unsize<U> predicate.
3141 nested.push(tcx.predicate_for_trait_def(
3142 obligation.param_env,
3143 obligation.cause.clone(),
3144 obligation.predicate.def_id(),
3145 obligation.recursion_depth + 1,
3153 Ok(VtableBuiltinData { nested: nested })
3156 ///////////////////////////////////////////////////////////////////////////
3159 // Matching is a common path used for both evaluation and
3160 // confirmation. It basically unifies types that appear in impls
3161 // and traits. This does affect the surrounding environment;
3162 // therefore, when used during evaluation, match routines must be
3163 // run inside of a `probe()` so that their side-effects are
3166 fn rematch_impl(&mut self,
3168 obligation: &TraitObligation<'tcx>,
3169 snapshot: &infer::CombinedSnapshot<'cx, 'tcx>)
3170 -> (Normalized<'tcx, &'tcx Substs<'tcx>>,
3171 infer::SkolemizationMap<'tcx>)
3173 match self.match_impl(impl_def_id, obligation, snapshot) {
3174 Ok((substs, skol_map)) => (substs, skol_map),
3176 bug!("Impl {:?} was matchable against {:?} but now is not",
3183 fn match_impl(&mut self,
3185 obligation: &TraitObligation<'tcx>,
3186 snapshot: &infer::CombinedSnapshot<'cx, 'tcx>)
3187 -> Result<(Normalized<'tcx, &'tcx Substs<'tcx>>,
3188 infer::SkolemizationMap<'tcx>), ()>
3190 let impl_trait_ref = self.tcx().impl_trait_ref(impl_def_id).unwrap();
3192 // Before we create the substitutions and everything, first
3193 // consider a "quick reject". This avoids creating more types
3194 // and so forth that we need to.
3195 if self.fast_reject_trait_refs(obligation, &impl_trait_ref) {
3199 let (skol_obligation, skol_map) = self.infcx().skolemize_late_bound_regions(
3200 &obligation.predicate);
3201 let skol_obligation_trait_ref = skol_obligation.trait_ref;
3203 let impl_substs = self.infcx.fresh_substs_for_item(obligation.cause.span,
3206 let impl_trait_ref = impl_trait_ref.subst(self.tcx(),
3209 let Normalized { value: impl_trait_ref, obligations: mut nested_obligations } =
3210 project::normalize_with_depth(self,
3211 obligation.param_env,
3212 obligation.cause.clone(),
3213 obligation.recursion_depth + 1,
3216 debug!("match_impl(impl_def_id={:?}, obligation={:?}, \
3217 impl_trait_ref={:?}, skol_obligation_trait_ref={:?})",
3221 skol_obligation_trait_ref);
3223 let InferOk { obligations, .. } =
3224 self.infcx.at(&obligation.cause, obligation.param_env)
3225 .eq(skol_obligation_trait_ref, impl_trait_ref)
3227 debug!("match_impl: failed eq_trait_refs due to `{}`", e);
3230 nested_obligations.extend(obligations);
3232 if let Err(e) = self.infcx.leak_check(false,
3233 obligation.cause.span,
3236 debug!("match_impl: failed leak check due to `{}`", e);
3240 debug!("match_impl: success impl_substs={:?}", impl_substs);
3243 obligations: nested_obligations
3247 fn fast_reject_trait_refs(&mut self,
3248 obligation: &TraitObligation,
3249 impl_trait_ref: &ty::TraitRef)
3252 // We can avoid creating type variables and doing the full
3253 // substitution if we find that any of the input types, when
3254 // simplified, do not match.
3256 obligation.predicate.skip_binder().input_types()
3257 .zip(impl_trait_ref.input_types())
3258 .any(|(obligation_ty, impl_ty)| {
3259 let simplified_obligation_ty =
3260 fast_reject::simplify_type(self.tcx(), obligation_ty, true);
3261 let simplified_impl_ty =
3262 fast_reject::simplify_type(self.tcx(), impl_ty, false);
3264 simplified_obligation_ty.is_some() &&
3265 simplified_impl_ty.is_some() &&
3266 simplified_obligation_ty != simplified_impl_ty
3270 /// Normalize `where_clause_trait_ref` and try to match it against
3271 /// `obligation`. If successful, return any predicates that
3272 /// result from the normalization. Normalization is necessary
3273 /// because where-clauses are stored in the parameter environment
3275 fn match_where_clause_trait_ref(&mut self,
3276 obligation: &TraitObligation<'tcx>,
3277 where_clause_trait_ref: ty::PolyTraitRef<'tcx>)
3278 -> Result<Vec<PredicateObligation<'tcx>>,()>
3280 self.match_poly_trait_ref(obligation, where_clause_trait_ref)
3283 /// Returns `Ok` if `poly_trait_ref` being true implies that the
3284 /// obligation is satisfied.
3285 fn match_poly_trait_ref(&mut self,
3286 obligation: &TraitObligation<'tcx>,
3287 poly_trait_ref: ty::PolyTraitRef<'tcx>)
3288 -> Result<Vec<PredicateObligation<'tcx>>,()>
3290 debug!("match_poly_trait_ref: obligation={:?} poly_trait_ref={:?}",
3294 self.infcx.at(&obligation.cause, obligation.param_env)
3295 .sup(obligation.predicate.to_poly_trait_ref(), poly_trait_ref)
3296 .map(|InferOk { obligations, .. }| obligations)
3300 ///////////////////////////////////////////////////////////////////////////
3303 fn match_fresh_trait_refs(&self,
3304 previous: &ty::PolyTraitRef<'tcx>,
3305 current: &ty::PolyTraitRef<'tcx>)
3308 let mut matcher = ty::_match::Match::new(self.tcx());
3309 matcher.relate(previous, current).is_ok()
3312 fn push_stack<'o,'s:'o>(&mut self,
3313 previous_stack: TraitObligationStackList<'s, 'tcx>,
3314 obligation: &'o TraitObligation<'tcx>)
3315 -> TraitObligationStack<'o, 'tcx>
3317 let fresh_trait_ref =
3318 obligation.predicate.to_poly_trait_ref().fold_with(&mut self.freshener);
3320 TraitObligationStack {
3323 previous: previous_stack,
3327 fn closure_trait_ref_unnormalized(&mut self,
3328 obligation: &TraitObligation<'tcx>,
3329 closure_def_id: DefId,
3330 substs: ty::ClosureSubsts<'tcx>)
3331 -> ty::PolyTraitRef<'tcx>
3333 let closure_type = self.infcx.closure_sig(closure_def_id, substs);
3335 // (1) Feels icky to skip the binder here, but OTOH we know
3336 // that the self-type is an unboxed closure type and hence is
3337 // in fact unparameterized (or at least does not reference any
3338 // regions bound in the obligation). Still probably some
3339 // refactoring could make this nicer.
3341 self.tcx().closure_trait_ref_and_return_type(obligation.predicate.def_id(),
3342 obligation.predicate
3343 .skip_binder().self_ty(), // (1)
3345 util::TupleArgumentsFlag::No)
3346 .map_bound(|(trait_ref, _)| trait_ref)
3349 fn generator_trait_ref_unnormalized(&mut self,
3350 obligation: &TraitObligation<'tcx>,
3351 closure_def_id: DefId,
3352 substs: ty::GeneratorSubsts<'tcx>)
3353 -> ty::PolyTraitRef<'tcx>
3355 let gen_sig = substs.poly_sig(closure_def_id, self.tcx());
3357 // (1) Feels icky to skip the binder here, but OTOH we know
3358 // that the self-type is an generator type and hence is
3359 // in fact unparameterized (or at least does not reference any
3360 // regions bound in the obligation). Still probably some
3361 // refactoring could make this nicer.
3363 self.tcx().generator_trait_ref_and_outputs(obligation.predicate.def_id(),
3364 obligation.predicate
3365 .skip_binder().self_ty(), // (1)
3367 .map_bound(|(trait_ref, ..)| trait_ref)
3370 /// Returns the obligations that are implied by instantiating an
3371 /// impl or trait. The obligations are substituted and fully
3372 /// normalized. This is used when confirming an impl or default
3374 fn impl_or_trait_obligations(&mut self,
3375 cause: ObligationCause<'tcx>,
3376 recursion_depth: usize,
3377 param_env: ty::ParamEnv<'tcx>,
3378 def_id: DefId, // of impl or trait
3379 substs: &Substs<'tcx>, // for impl or trait
3380 skol_map: infer::SkolemizationMap<'tcx>,
3381 snapshot: &infer::CombinedSnapshot<'cx, 'tcx>)
3382 -> Vec<PredicateObligation<'tcx>>
3384 debug!("impl_or_trait_obligations(def_id={:?})", def_id);
3385 let tcx = self.tcx();
3387 // To allow for one-pass evaluation of the nested obligation,
3388 // each predicate must be preceded by the obligations required
3390 // for example, if we have:
3391 // impl<U: Iterator, V: Iterator<Item=U>> Foo for V where U::Item: Copy
3392 // the impl will have the following predicates:
3393 // <V as Iterator>::Item = U,
3394 // U: Iterator, U: Sized,
3395 // V: Iterator, V: Sized,
3396 // <U as Iterator>::Item: Copy
3397 // When we substitute, say, `V => IntoIter<u32>, U => $0`, the last
3398 // obligation will normalize to `<$0 as Iterator>::Item = $1` and
3399 // `$1: Copy`, so we must ensure the obligations are emitted in
3401 let predicates = tcx.predicates_of(def_id);
3402 assert_eq!(predicates.parent, None);
3403 let mut predicates: Vec<_> = predicates.predicates.iter().flat_map(|predicate| {
3404 let predicate = normalize_with_depth(self, param_env, cause.clone(), recursion_depth,
3405 &predicate.subst(tcx, substs));
3406 predicate.obligations.into_iter().chain(
3408 cause: cause.clone(),
3411 predicate: predicate.value
3415 // We are performing deduplication here to avoid exponential blowups
3416 // (#38528) from happening, but the real cause of the duplication is
3417 // unknown. What we know is that the deduplication avoids exponential
3418 // amount of predicates being propagated when processing deeply nested
3421 // This code is hot enough that it's worth avoiding the allocation
3422 // required for the FxHashSet when possible. Special-casing lengths 0,
3423 // 1 and 2 covers roughly 75--80% of the cases.
3424 if predicates.len() <= 1 {
3425 // No possibility of duplicates.
3426 } else if predicates.len() == 2 {
3427 // Only two elements. Drop the second if they are equal.
3428 if predicates[0] == predicates[1] {
3429 predicates.truncate(1);
3432 // Three or more elements. Use a general deduplication process.
3433 let mut seen = FxHashSet();
3434 predicates.retain(|i| seen.insert(i.clone()));
3436 self.infcx().plug_leaks(skol_map, snapshot, predicates)
3440 impl<'tcx> TraitObligation<'tcx> {
3441 #[allow(unused_comparisons)]
3442 pub fn derived_cause(&self,
3443 variant: fn(DerivedObligationCause<'tcx>) -> ObligationCauseCode<'tcx>)
3444 -> ObligationCause<'tcx>
3447 * Creates a cause for obligations that are derived from
3448 * `obligation` by a recursive search (e.g., for a builtin
3449 * bound, or eventually a `auto trait Foo`). If `obligation`
3450 * is itself a derived obligation, this is just a clone, but
3451 * otherwise we create a "derived obligation" cause so as to
3452 * keep track of the original root obligation for error
3456 let obligation = self;
3458 // NOTE(flaper87): As of now, it keeps track of the whole error
3459 // chain. Ideally, we should have a way to configure this either
3460 // by using -Z verbose or just a CLI argument.
3461 if obligation.recursion_depth >= 0 {
3462 let derived_cause = DerivedObligationCause {
3463 parent_trait_ref: obligation.predicate.to_poly_trait_ref(),
3464 parent_code: Rc::new(obligation.cause.code.clone())
3466 let derived_code = variant(derived_cause);
3467 ObligationCause::new(obligation.cause.span, obligation.cause.body_id, derived_code)
3469 obligation.cause.clone()
3474 impl<'tcx> SelectionCache<'tcx> {
3475 pub fn new() -> SelectionCache<'tcx> {
3477 hashmap: Lock::new(FxHashMap())
3481 pub fn clear(&self) {
3482 *self.hashmap.borrow_mut() = FxHashMap()
3486 impl<'tcx> EvaluationCache<'tcx> {
3487 pub fn new() -> EvaluationCache<'tcx> {
3489 hashmap: Lock::new(FxHashMap())
3493 pub fn clear(&self) {
3494 *self.hashmap.borrow_mut() = FxHashMap()
3498 impl<'o,'tcx> TraitObligationStack<'o,'tcx> {
3499 fn list(&'o self) -> TraitObligationStackList<'o,'tcx> {
3500 TraitObligationStackList::with(self)
3503 fn iter(&'o self) -> TraitObligationStackList<'o,'tcx> {
3508 #[derive(Copy, Clone)]
3509 struct TraitObligationStackList<'o,'tcx:'o> {
3510 head: Option<&'o TraitObligationStack<'o,'tcx>>
3513 impl<'o,'tcx> TraitObligationStackList<'o,'tcx> {
3514 fn empty() -> TraitObligationStackList<'o,'tcx> {
3515 TraitObligationStackList { head: None }
3518 fn with(r: &'o TraitObligationStack<'o,'tcx>) -> TraitObligationStackList<'o,'tcx> {
3519 TraitObligationStackList { head: Some(r) }
3523 impl<'o,'tcx> Iterator for TraitObligationStackList<'o,'tcx>{
3524 type Item = &'o TraitObligationStack<'o,'tcx>;
3526 fn next(&mut self) -> Option<&'o TraitObligationStack<'o,'tcx>> {
3537 impl<'o,'tcx> fmt::Debug for TraitObligationStack<'o,'tcx> {
3538 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
3539 write!(f, "TraitObligationStack({:?})", self.obligation)
3543 #[derive(Clone, Eq, PartialEq)]
3544 pub struct WithDepNode<T> {
3545 dep_node: DepNodeIndex,
3549 impl<T: Clone> WithDepNode<T> {
3550 pub fn new(dep_node: DepNodeIndex, cached_value: T) -> Self {
3551 WithDepNode { dep_node, cached_value }
3554 pub fn get(&self, tcx: TyCtxt) -> T {
3555 tcx.dep_graph.read_index(self.dep_node);
3556 self.cached_value.clone()