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 `README.md` for high-level documentation
13 use self::SelectionCandidate::*;
14 use self::EvaluationResult::*;
16 use super::coherence::{self, Conflict};
17 use super::DerivedObligationCause;
18 use super::IntercrateMode;
20 use super::project::{normalize_with_depth, Normalized, ProjectionCacheKey};
21 use super::{PredicateObligation, TraitObligation, ObligationCause};
22 use super::{ObligationCauseCode, BuiltinDerivedObligation, ImplDerivedObligation};
23 use super::{SelectionError, Unimplemented, OutputTypeParameterMismatch};
24 use super::{ObjectCastObligation, Obligation};
25 use super::TraitNotObjectSafe;
27 use super::SelectionResult;
28 use super::{VtableBuiltin, VtableImpl, VtableParam, VtableClosure, VtableGenerator,
29 VtableFnPointer, VtableObject, VtableAutoImpl};
30 use super::{VtableImplData, VtableObjectData, VtableBuiltinData, VtableGeneratorData,
31 VtableClosureData, VtableAutoImplData, VtableFnPointerData};
34 use dep_graph::{DepNodeIndex, DepKind};
35 use hir::def_id::DefId;
37 use infer::{InferCtxt, InferOk, TypeFreshener};
38 use ty::subst::{Kind, Subst, Substs};
39 use ty::{self, ToPredicate, ToPolyTraitRef, Ty, TyCtxt, TypeFoldable};
41 use ty::relate::TypeRelation;
42 use middle::lang_items;
44 use rustc_data_structures::bitvec::BitVector;
45 use rustc_data_structures::snapshot_vec::{SnapshotVecDelegate, SnapshotVec};
47 use std::cell::RefCell;
50 use std::marker::PhantomData;
56 use util::nodemap::FxHashMap;
58 struct InferredObligationsSnapshotVecDelegate<'tcx> {
59 phantom: PhantomData<&'tcx i32>,
61 impl<'tcx> SnapshotVecDelegate for InferredObligationsSnapshotVecDelegate<'tcx> {
62 type Value = PredicateObligation<'tcx>;
64 fn reverse(_: &mut Vec<Self::Value>, _: Self::Undo) {}
67 pub struct SelectionContext<'cx, 'gcx: 'cx+'tcx, 'tcx: 'cx> {
68 infcx: &'cx InferCtxt<'cx, 'gcx, 'tcx>,
70 /// Freshener used specifically for skolemizing entries on the
71 /// obligation stack. This ensures that all entries on the stack
72 /// at one time will have the same set of skolemized entries,
73 /// which is important for checking for trait bounds that
74 /// recursively require themselves.
75 freshener: TypeFreshener<'cx, 'gcx, 'tcx>,
77 /// If true, indicates that the evaluation should be conservative
78 /// and consider the possibility of types outside this crate.
79 /// This comes up primarily when resolving ambiguity. Imagine
80 /// there is some trait reference `$0 : Bar` where `$0` is an
81 /// inference variable. If `intercrate` is true, then we can never
82 /// say for sure that this reference is not implemented, even if
83 /// there are *no impls at all for `Bar`*, because `$0` could be
84 /// bound to some type that in a downstream crate that implements
85 /// `Bar`. This is the suitable mode for coherence. Elsewhere,
86 /// though, we set this to false, because we are only interested
87 /// in types that the user could actually have written --- in
88 /// other words, we consider `$0 : Bar` to be unimplemented if
89 /// there is no type that the user could *actually name* that
90 /// would satisfy it. This avoids crippling inference, basically.
91 intercrate: Option<IntercrateMode>,
93 inferred_obligations: SnapshotVec<InferredObligationsSnapshotVecDelegate<'tcx>>,
95 intercrate_ambiguity_causes: Option<Vec<IntercrateAmbiguityCause>>,
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: RefCell<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 /// There is *no* impl for this, builtin or not. Ignore
309 /// all where-clauses.
311 /// It is unknown whether there is an impl.
315 #[derive(Copy, Clone, Debug, PartialOrd, Ord, PartialEq, Eq)]
316 /// The result of trait evaluation. The order is important
317 /// here as the evaluation of a list is the maximum of the
320 /// The evaluation results are ordered:
321 /// - `EvaluatedToOk` implies `EvaluatedToAmbig` implies `EvaluatedToUnknown`
322 /// - `EvaluatedToErr` implies `EvaluatedToRecur`
323 /// - the "union" of evaluation results is equal to their maximum -
324 /// all the "potential success" candidates can potentially succeed,
325 /// so they are no-ops when unioned with a definite error, and within
326 /// the categories it's easy to see that the unions are correct.
327 enum EvaluationResult {
328 /// Evaluation successful
330 /// Evaluation is known to be ambiguous - it *might* hold for some
331 /// assignment of inference variables, but it might not.
333 /// While this has the same meaning as `EvaluatedToUnknown` - we can't
334 /// know whether this obligation holds or not - it is the result we
335 /// would get with an empty stack, and therefore is cacheable.
337 /// Evaluation failed because of recursion involving inference
338 /// variables. We are somewhat imprecise there, so we don't actually
339 /// know the real result.
341 /// This can't be trivially cached for the same reason as `EvaluatedToRecur`.
343 /// Evaluation failed because we encountered an obligation we are already
344 /// trying to prove on this branch.
346 /// We know this branch can't be a part of a minimal proof-tree for
347 /// the "root" of our cycle, because then we could cut out the recursion
348 /// and maintain a valid proof tree. However, this does not mean
349 /// that all the obligations on this branch do not hold - it's possible
350 /// that we entered this branch "speculatively", and that there
351 /// might be some other way to prove this obligation that does not
352 /// go through this cycle - so we can't cache this as a failure.
354 /// For example, suppose we have this:
356 /// ```rust,ignore (pseudo-Rust)
357 /// pub trait Trait { fn xyz(); }
358 /// // This impl is "useless", but we can still have
359 /// // an `impl Trait for SomeUnsizedType` somewhere.
360 /// impl<T: Trait + Sized> Trait for T { fn xyz() {} }
362 /// pub fn foo<T: Trait + ?Sized>() {
363 /// <T as Trait>::xyz();
367 /// When checking `foo`, we have to prove `T: Trait`. This basically
368 /// translates into this:
370 /// (T: Trait + Sized →_\impl T: Trait), T: Trait ⊢ T: Trait
372 /// When we try to prove it, we first go the first option, which
373 /// recurses. This shows us that the impl is "useless" - it won't
374 /// tell us that `T: Trait` unless it already implemented `Trait`
375 /// by some other means. However, that does not prevent `T: Trait`
376 /// does not hold, because of the bound (which can indeed be satisfied
377 /// by `SomeUnsizedType` from another crate).
379 /// FIXME: when an `EvaluatedToRecur` goes past its parent root, we
380 /// ought to convert it to an `EvaluatedToErr`, because we know
381 /// there definitely isn't a proof tree for that obligation. Not
382 /// doing so is still sound - there isn't any proof tree, so the
383 /// branch still can't be a part of a minimal one - but does not
384 /// re-enable caching.
386 /// Evaluation failed
390 impl EvaluationResult {
391 fn may_apply(self) -> bool {
395 EvaluatedToUnknown => true,
398 EvaluatedToRecur => false
402 fn is_stack_dependent(self) -> bool {
405 EvaluatedToRecur => true,
409 EvaluatedToErr => false,
415 pub struct EvaluationCache<'tcx> {
416 hashmap: RefCell<FxHashMap<ty::PolyTraitRef<'tcx>, WithDepNode<EvaluationResult>>>
419 impl<'cx, 'gcx, 'tcx> SelectionContext<'cx, 'gcx, 'tcx> {
420 pub fn new(infcx: &'cx InferCtxt<'cx, 'gcx, 'tcx>) -> SelectionContext<'cx, 'gcx, 'tcx> {
423 freshener: infcx.freshener(),
425 inferred_obligations: SnapshotVec::new(),
426 intercrate_ambiguity_causes: None,
430 pub fn intercrate(infcx: &'cx InferCtxt<'cx, 'gcx, 'tcx>,
431 mode: IntercrateMode) -> SelectionContext<'cx, 'gcx, 'tcx> {
432 debug!("intercrate({:?})", mode);
435 freshener: infcx.freshener(),
436 intercrate: Some(mode),
437 inferred_obligations: SnapshotVec::new(),
438 intercrate_ambiguity_causes: None,
442 /// Enables tracking of intercrate ambiguity causes. These are
443 /// used in coherence to give improved diagnostics. We don't do
444 /// this until we detect a coherence error because it can lead to
445 /// false overflow results (#47139) and because it costs
446 /// computation time.
447 pub fn enable_tracking_intercrate_ambiguity_causes(&mut self) {
448 assert!(self.intercrate.is_some());
449 assert!(self.intercrate_ambiguity_causes.is_none());
450 self.intercrate_ambiguity_causes = Some(vec![]);
451 debug!("selcx: enable_tracking_intercrate_ambiguity_causes");
454 /// Gets the intercrate ambiguity causes collected since tracking
455 /// was enabled and disables tracking at the same time. If
456 /// tracking is not enabled, just returns an empty vector.
457 pub fn take_intercrate_ambiguity_causes(&mut self) -> Vec<IntercrateAmbiguityCause> {
458 assert!(self.intercrate.is_some());
459 self.intercrate_ambiguity_causes.take().unwrap_or(vec![])
462 pub fn infcx(&self) -> &'cx InferCtxt<'cx, 'gcx, 'tcx> {
466 pub fn tcx(&self) -> TyCtxt<'cx, 'gcx, 'tcx> {
470 pub fn closure_typer(&self) -> &'cx InferCtxt<'cx, 'gcx, 'tcx> {
474 /// Wraps the inference context's in_snapshot s.t. snapshot handling is only from the selection
476 fn in_snapshot<R, F>(&mut self, f: F) -> R
477 where F: FnOnce(&mut Self, &infer::CombinedSnapshot) -> R
479 // The irrefutable nature of the operation means we don't need to snapshot the
480 // inferred_obligations vector.
481 self.infcx.in_snapshot(|snapshot| f(self, snapshot))
484 /// Wraps a probe s.t. obligations collected during it are ignored and old obligations are
486 fn probe<R, F>(&mut self, f: F) -> R
487 where F: FnOnce(&mut Self, &infer::CombinedSnapshot) -> R
489 let inferred_obligations_snapshot = self.inferred_obligations.start_snapshot();
490 let result = self.infcx.probe(|snapshot| f(self, snapshot));
491 self.inferred_obligations.rollback_to(inferred_obligations_snapshot);
495 /// Wraps a commit_if_ok s.t. obligations collected during it are not returned in selection if
496 /// the transaction fails and s.t. old obligations are retained.
497 fn commit_if_ok<T, E, F>(&mut self, f: F) -> Result<T, E> where
498 F: FnOnce(&mut Self, &infer::CombinedSnapshot) -> Result<T, E>
500 let inferred_obligations_snapshot = self.inferred_obligations.start_snapshot();
501 match self.infcx.commit_if_ok(|snapshot| f(self, snapshot)) {
503 self.inferred_obligations.commit(inferred_obligations_snapshot);
507 self.inferred_obligations.rollback_to(inferred_obligations_snapshot);
514 ///////////////////////////////////////////////////////////////////////////
517 // The selection phase tries to identify *how* an obligation will
518 // be resolved. For example, it will identify which impl or
519 // parameter bound is to be used. The process can be inconclusive
520 // if the self type in the obligation is not fully inferred. Selection
521 // can result in an error in one of two ways:
523 // 1. If no applicable impl or parameter bound can be found.
524 // 2. If the output type parameters in the obligation do not match
525 // those specified by the impl/bound. For example, if the obligation
526 // is `Vec<Foo>:Iterable<Bar>`, but the impl specifies
527 // `impl<T> Iterable<T> for Vec<T>`, than an error would result.
529 /// Attempts to satisfy the obligation. If successful, this will affect the surrounding
530 /// type environment by performing unification.
531 pub fn select(&mut self, obligation: &TraitObligation<'tcx>)
532 -> SelectionResult<'tcx, Selection<'tcx>> {
533 debug!("select({:?})", obligation);
534 assert!(!obligation.predicate.has_escaping_regions());
536 let tcx = self.tcx();
538 let stack = self.push_stack(TraitObligationStackList::empty(), obligation);
539 let ret = match self.candidate_from_obligation(&stack)? {
542 let mut candidate = self.confirm_candidate(obligation, candidate)?;
543 let inferred_obligations = (*self.inferred_obligations).into_iter().cloned();
544 candidate.nested_obligations_mut().extend(inferred_obligations);
549 // Test whether this is a `()` which was produced by defaulting a
550 // diverging type variable with `!` disabled. If so, we may need
551 // to raise a warning.
552 if obligation.predicate.skip_binder().self_ty().is_defaulted_unit() {
553 let mut raise_warning = true;
554 // Don't raise a warning if the trait is implemented for ! and only
555 // permits a trivial implementation for !. This stops us warning
556 // about (for example) `(): Clone` becoming `!: Clone` because such
557 // a switch can't cause code to stop compiling or execute
559 let mut never_obligation = obligation.clone();
560 let def_id = never_obligation.predicate.skip_binder().trait_ref.def_id;
561 never_obligation.predicate = never_obligation.predicate.map_bound(|mut trait_pred| {
562 // Swap out () with ! so we can check if the trait is impld for !
564 let trait_ref = &mut trait_pred.trait_ref;
565 let unit_substs = trait_ref.substs;
566 let mut never_substs = Vec::with_capacity(unit_substs.len());
567 never_substs.push(From::from(tcx.types.never));
568 never_substs.extend(&unit_substs[1..]);
569 trait_ref.substs = tcx.intern_substs(&never_substs);
573 if let Ok(Some(..)) = self.select(&never_obligation) {
574 if !tcx.trait_relevant_for_never(def_id) {
575 // The trait is also implemented for ! and the resulting
576 // implementation cannot actually be invoked in any way.
577 raise_warning = false;
582 tcx.lint_node(lint::builtin::RESOLVE_TRAIT_ON_DEFAULTED_UNIT,
583 obligation.cause.body_id,
584 obligation.cause.span,
585 &format!("code relies on type inference rules which are likely \
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 evaluate_obligation(&mut self,
604 obligation: &PredicateObligation<'tcx>)
607 debug!("evaluate_obligation({:?})",
610 self.probe(|this, _| {
611 this.evaluate_predicate_recursively(TraitObligationStackList::empty(), obligation)
616 /// Evaluates whether the obligation `obligation` can be satisfied,
617 /// and returns `false` if not certain. However, this is not entirely
618 /// accurate if inference variables are involved.
619 pub fn evaluate_obligation_conservatively(&mut self,
620 obligation: &PredicateObligation<'tcx>)
623 debug!("evaluate_obligation_conservatively({:?})",
626 self.probe(|this, _| {
627 this.evaluate_predicate_recursively(TraitObligationStackList::empty(), obligation)
632 /// Evaluates the predicates in `predicates` recursively. Note that
633 /// this applies projections in the predicates, and therefore
634 /// is run within an inference probe.
635 fn evaluate_predicates_recursively<'a,'o,I>(&mut self,
636 stack: TraitObligationStackList<'o, 'tcx>,
639 where I : Iterator<Item=&'a PredicateObligation<'tcx>>, 'tcx:'a
641 let mut result = EvaluatedToOk;
642 for obligation in predicates {
643 let eval = self.evaluate_predicate_recursively(stack, obligation);
644 debug!("evaluate_predicate_recursively({:?}) = {:?}",
646 if let EvaluatedToErr = eval {
647 // fast-path - EvaluatedToErr is the top of the lattice,
648 // so we don't need to look on the other predicates.
649 return EvaluatedToErr;
651 result = cmp::max(result, eval);
657 fn evaluate_predicate_recursively<'o>(&mut self,
658 previous_stack: TraitObligationStackList<'o, 'tcx>,
659 obligation: &PredicateObligation<'tcx>)
662 debug!("evaluate_predicate_recursively({:?})",
665 match obligation.predicate {
666 ty::Predicate::Trait(ref t) => {
667 assert!(!t.has_escaping_regions());
668 let obligation = obligation.with(t.clone());
669 self.evaluate_trait_predicate_recursively(previous_stack, obligation)
672 ty::Predicate::Equate(ref p) => {
673 // does this code ever run?
674 match self.infcx.equality_predicate(&obligation.cause, obligation.param_env, p) {
675 Ok(InferOk { obligations, .. }) => {
676 self.inferred_obligations.extend(obligations);
679 Err(_) => EvaluatedToErr
683 ty::Predicate::Subtype(ref p) => {
684 // does this code ever run?
685 match self.infcx.subtype_predicate(&obligation.cause, obligation.param_env, p) {
686 Some(Ok(InferOk { obligations, .. })) => {
687 self.inferred_obligations.extend(obligations);
690 Some(Err(_)) => EvaluatedToErr,
691 None => EvaluatedToAmbig,
695 ty::Predicate::WellFormed(ty) => {
696 match ty::wf::obligations(self.infcx,
697 obligation.param_env,
698 obligation.cause.body_id,
699 ty, obligation.cause.span) {
701 self.evaluate_predicates_recursively(previous_stack, obligations.iter()),
707 ty::Predicate::TypeOutlives(..) | ty::Predicate::RegionOutlives(..) => {
708 // we do not consider region relationships when
709 // evaluating trait matches
713 ty::Predicate::ObjectSafe(trait_def_id) => {
714 if self.tcx().is_object_safe(trait_def_id) {
721 ty::Predicate::Projection(ref data) => {
722 let project_obligation = obligation.with(data.clone());
723 match project::poly_project_and_unify_type(self, &project_obligation) {
724 Ok(Some(subobligations)) => {
725 let result = self.evaluate_predicates_recursively(previous_stack,
726 subobligations.iter());
728 ProjectionCacheKey::from_poly_projection_predicate(self, data)
730 self.infcx.projection_cache.borrow_mut().complete(key);
743 ty::Predicate::ClosureKind(closure_def_id, closure_substs, kind) => {
744 match self.infcx.closure_kind(closure_def_id, closure_substs) {
745 Some(closure_kind) => {
746 if closure_kind.extends(kind) {
758 ty::Predicate::ConstEvaluatable(def_id, substs) => {
759 match self.tcx().lift_to_global(&(obligation.param_env, substs)) {
760 Some((param_env, substs)) => {
761 match self.tcx().const_eval(param_env.and((def_id, substs))) {
762 Ok(_) => EvaluatedToOk,
763 Err(_) => EvaluatedToErr
767 // Inference variables still left in param_env or substs.
775 fn evaluate_trait_predicate_recursively<'o>(&mut self,
776 previous_stack: TraitObligationStackList<'o, 'tcx>,
777 mut obligation: TraitObligation<'tcx>)
780 debug!("evaluate_trait_predicate_recursively({:?})",
783 if !self.intercrate.is_some() && obligation.is_global() {
784 // If a param env is consistent, global obligations do not depend on its particular
785 // value in order to work, so we can clear out the param env and get better
786 // caching. (If the current param env is inconsistent, we don't care what happens).
787 debug!("evaluate_trait_predicate_recursively({:?}) - in global", obligation);
788 obligation.param_env = ty::ParamEnv::empty(obligation.param_env.reveal);
791 let stack = self.push_stack(previous_stack, &obligation);
792 let fresh_trait_ref = stack.fresh_trait_ref;
793 if let Some(result) = self.check_evaluation_cache(obligation.param_env, fresh_trait_ref) {
794 debug!("CACHE HIT: EVAL({:?})={:?}",
800 let (result, dep_node) = self.in_task(|this| this.evaluate_stack(&stack));
802 debug!("CACHE MISS: EVAL({:?})={:?}",
805 self.insert_evaluation_cache(obligation.param_env, fresh_trait_ref, dep_node, result);
810 fn evaluate_stack<'o>(&mut self,
811 stack: &TraitObligationStack<'o, 'tcx>)
814 // In intercrate mode, whenever any of the types are unbound,
815 // there can always be an impl. Even if there are no impls in
816 // this crate, perhaps the type would be unified with
817 // something from another crate that does provide an impl.
819 // In intra mode, we must still be conservative. The reason is
820 // that we want to avoid cycles. Imagine an impl like:
822 // impl<T:Eq> Eq for Vec<T>
824 // and a trait reference like `$0 : Eq` where `$0` is an
825 // unbound variable. When we evaluate this trait-reference, we
826 // will unify `$0` with `Vec<$1>` (for some fresh variable
827 // `$1`), on the condition that `$1 : Eq`. We will then wind
828 // up with many candidates (since that are other `Eq` impls
829 // that apply) and try to winnow things down. This results in
830 // a recursive evaluation that `$1 : Eq` -- as you can
831 // imagine, this is just where we started. To avoid that, we
832 // check for unbound variables and return an ambiguous (hence possible)
833 // match if we've seen this trait before.
835 // This suffices to allow chains like `FnMut` implemented in
836 // terms of `Fn` etc, but we could probably make this more
838 let unbound_input_types = stack.fresh_trait_ref.input_types().any(|ty| ty.is_fresh());
839 // this check was an imperfect workaround for a bug n the old
840 // intercrate mode, it should be removed when that goes away.
841 if unbound_input_types &&
842 self.intercrate == Some(IntercrateMode::Issue43355)
844 debug!("evaluate_stack({:?}) --> unbound argument, intercrate --> ambiguous",
845 stack.fresh_trait_ref);
846 // Heuristics: show the diagnostics when there are no candidates in crate.
847 if self.intercrate_ambiguity_causes.is_some() {
848 debug!("evaluate_stack: intercrate_ambiguity_causes is some");
849 if let Ok(candidate_set) = self.assemble_candidates(stack) {
850 if !candidate_set.ambiguous && candidate_set.vec.is_empty() {
851 let trait_ref = stack.obligation.predicate.skip_binder().trait_ref;
852 let self_ty = trait_ref.self_ty();
853 let cause = IntercrateAmbiguityCause::DownstreamCrate {
854 trait_desc: trait_ref.to_string(),
855 self_desc: if self_ty.has_concrete_skeleton() {
856 Some(self_ty.to_string())
861 debug!("evaluate_stack: pushing cause = {:?}", cause);
862 self.intercrate_ambiguity_causes.as_mut().unwrap().push(cause);
866 return EvaluatedToAmbig;
868 if unbound_input_types &&
869 stack.iter().skip(1).any(
870 |prev| stack.obligation.param_env == prev.obligation.param_env &&
871 self.match_fresh_trait_refs(&stack.fresh_trait_ref,
872 &prev.fresh_trait_ref))
874 debug!("evaluate_stack({:?}) --> unbound argument, recursive --> giving up",
875 stack.fresh_trait_ref);
876 return EvaluatedToUnknown;
879 // If there is any previous entry on the stack that precisely
880 // matches this obligation, then we can assume that the
881 // obligation is satisfied for now (still all other conditions
882 // must be met of course). One obvious case this comes up is
883 // marker traits like `Send`. Think of a linked list:
885 // struct List<T> { data: T, next: Option<Box<List<T>>> {
887 // `Box<List<T>>` will be `Send` if `T` is `Send` and
888 // `Option<Box<List<T>>>` is `Send`, and in turn
889 // `Option<Box<List<T>>>` is `Send` if `Box<List<T>>` is
892 // Note that we do this comparison using the `fresh_trait_ref`
893 // fields. Because these have all been skolemized using
894 // `self.freshener`, we can be sure that (a) this will not
895 // affect the inferencer state and (b) that if we see two
896 // skolemized types with the same index, they refer to the
897 // same unbound type variable.
898 if let Some(rec_index) =
900 .skip(1) // skip top-most frame
901 .position(|prev| stack.obligation.param_env == prev.obligation.param_env &&
902 stack.fresh_trait_ref == prev.fresh_trait_ref)
904 debug!("evaluate_stack({:?}) --> recursive",
905 stack.fresh_trait_ref);
906 let cycle = stack.iter().skip(1).take(rec_index+1);
907 let cycle = cycle.map(|stack| ty::Predicate::Trait(stack.obligation.predicate));
908 if self.coinductive_match(cycle) {
909 debug!("evaluate_stack({:?}) --> recursive, coinductive",
910 stack.fresh_trait_ref);
911 return EvaluatedToOk;
913 debug!("evaluate_stack({:?}) --> recursive, inductive",
914 stack.fresh_trait_ref);
915 return EvaluatedToRecur;
919 match self.candidate_from_obligation(stack) {
920 Ok(Some(c)) => self.evaluate_candidate(stack, &c),
921 Ok(None) => EvaluatedToAmbig,
922 Err(..) => EvaluatedToErr
926 /// For defaulted traits, we use a co-inductive strategy to solve, so
927 /// that recursion is ok. This routine returns true if the top of the
928 /// stack (`cycle[0]`):
930 /// - is a defaulted trait, and
931 /// - it also appears in the backtrace at some position `X`; and,
932 /// - all the predicates at positions `X..` between `X` an the top are
933 /// also defaulted traits.
934 pub fn coinductive_match<I>(&mut self, cycle: I) -> bool
935 where I: Iterator<Item=ty::Predicate<'tcx>>
937 let mut cycle = cycle;
938 cycle.all(|predicate| self.coinductive_predicate(predicate))
941 fn coinductive_predicate(&self, predicate: ty::Predicate<'tcx>) -> bool {
942 let result = match predicate {
943 ty::Predicate::Trait(ref data) => {
944 self.tcx().trait_is_auto(data.def_id())
950 debug!("coinductive_predicate({:?}) = {:?}", predicate, result);
954 /// Further evaluate `candidate` to decide whether all type parameters match and whether nested
955 /// obligations are met. Returns true if `candidate` remains viable after this further
957 fn evaluate_candidate<'o>(&mut self,
958 stack: &TraitObligationStack<'o, 'tcx>,
959 candidate: &SelectionCandidate<'tcx>)
962 debug!("evaluate_candidate: depth={} candidate={:?}",
963 stack.obligation.recursion_depth, candidate);
964 let result = self.probe(|this, _| {
965 let candidate = (*candidate).clone();
966 match this.confirm_candidate(stack.obligation, candidate) {
968 this.evaluate_predicates_recursively(
970 selection.nested_obligations().iter())
972 Err(..) => EvaluatedToErr
975 debug!("evaluate_candidate: depth={} result={:?}",
976 stack.obligation.recursion_depth, result);
980 fn check_evaluation_cache(&self,
981 param_env: ty::ParamEnv<'tcx>,
982 trait_ref: ty::PolyTraitRef<'tcx>)
983 -> Option<EvaluationResult>
985 let tcx = self.tcx();
986 if self.can_use_global_caches(param_env) {
987 let cache = tcx.evaluation_cache.hashmap.borrow();
988 if let Some(cached) = cache.get(&trait_ref) {
989 return Some(cached.get(tcx));
992 self.infcx.evaluation_cache.hashmap
998 fn insert_evaluation_cache(&mut self,
999 param_env: ty::ParamEnv<'tcx>,
1000 trait_ref: ty::PolyTraitRef<'tcx>,
1001 dep_node: DepNodeIndex,
1002 result: EvaluationResult)
1004 // Avoid caching results that depend on more than just the trait-ref
1005 // - the stack can create recursion.
1006 if result.is_stack_dependent() {
1010 if self.can_use_global_caches(param_env) {
1011 let mut cache = self.tcx().evaluation_cache.hashmap.borrow_mut();
1012 if let Some(trait_ref) = self.tcx().lift_to_global(&trait_ref) {
1013 cache.insert(trait_ref, WithDepNode::new(dep_node, result));
1018 self.infcx.evaluation_cache.hashmap
1020 .insert(trait_ref, WithDepNode::new(dep_node, result));
1023 ///////////////////////////////////////////////////////////////////////////
1024 // CANDIDATE ASSEMBLY
1026 // The selection process begins by examining all in-scope impls,
1027 // caller obligations, and so forth and assembling a list of
1028 // candidates. See `README.md` and the `Candidate` type for more
1031 fn candidate_from_obligation<'o>(&mut self,
1032 stack: &TraitObligationStack<'o, 'tcx>)
1033 -> SelectionResult<'tcx, SelectionCandidate<'tcx>>
1035 // Watch out for overflow. This intentionally bypasses (and does
1036 // not update) the cache.
1037 let recursion_limit = self.infcx.tcx.sess.recursion_limit.get();
1038 if stack.obligation.recursion_depth >= recursion_limit {
1039 self.infcx().report_overflow_error(&stack.obligation, true);
1042 // Check the cache. Note that we skolemize the trait-ref
1043 // separately rather than using `stack.fresh_trait_ref` -- this
1044 // is because we want the unbound variables to be replaced
1045 // with fresh skolemized types starting from index 0.
1046 let cache_fresh_trait_pred =
1047 self.infcx.freshen(stack.obligation.predicate.clone());
1048 debug!("candidate_from_obligation(cache_fresh_trait_pred={:?}, obligation={:?})",
1049 cache_fresh_trait_pred,
1051 assert!(!stack.obligation.predicate.has_escaping_regions());
1053 if let Some(c) = self.check_candidate_cache(stack.obligation.param_env,
1054 &cache_fresh_trait_pred) {
1055 debug!("CACHE HIT: SELECT({:?})={:?}",
1056 cache_fresh_trait_pred,
1061 // If no match, compute result and insert into cache.
1062 let (candidate, dep_node) = self.in_task(|this| {
1063 this.candidate_from_obligation_no_cache(stack)
1066 debug!("CACHE MISS: SELECT({:?})={:?}",
1067 cache_fresh_trait_pred, candidate);
1068 self.insert_candidate_cache(stack.obligation.param_env,
1069 cache_fresh_trait_pred,
1075 fn in_task<OP, R>(&mut self, op: OP) -> (R, DepNodeIndex)
1076 where OP: FnOnce(&mut Self) -> R
1078 let (result, dep_node) = self.tcx().dep_graph.with_anon_task(DepKind::TraitSelect, || {
1081 self.tcx().dep_graph.read_index(dep_node);
1085 // Treat negative impls as unimplemented
1086 fn filter_negative_impls(&self, candidate: SelectionCandidate<'tcx>)
1087 -> SelectionResult<'tcx, SelectionCandidate<'tcx>> {
1088 if let ImplCandidate(def_id) = candidate {
1089 if self.tcx().impl_polarity(def_id) == hir::ImplPolarity::Negative {
1090 return Err(Unimplemented)
1096 fn candidate_from_obligation_no_cache<'o>(&mut self,
1097 stack: &TraitObligationStack<'o, 'tcx>)
1098 -> SelectionResult<'tcx, SelectionCandidate<'tcx>>
1100 if stack.obligation.predicate.references_error() {
1101 // If we encounter a `TyError`, we generally prefer the
1102 // most "optimistic" result in response -- that is, the
1103 // one least likely to report downstream errors. But
1104 // because this routine is shared by coherence and by
1105 // trait selection, there isn't an obvious "right" choice
1106 // here in that respect, so we opt to just return
1107 // ambiguity and let the upstream clients sort it out.
1111 match self.is_knowable(stack) {
1114 debug!("coherence stage: not knowable");
1115 if self.intercrate_ambiguity_causes.is_some() {
1116 debug!("evaluate_stack: intercrate_ambiguity_causes is some");
1117 // Heuristics: show the diagnostics when there are no candidates in crate.
1118 let candidate_set = self.assemble_candidates(stack)?;
1119 if !candidate_set.ambiguous && candidate_set.vec.iter().all(|c| {
1120 !self.evaluate_candidate(stack, &c).may_apply()
1122 let trait_ref = stack.obligation.predicate.skip_binder().trait_ref;
1123 let self_ty = trait_ref.self_ty();
1124 let trait_desc = trait_ref.to_string();
1125 let self_desc = if self_ty.has_concrete_skeleton() {
1126 Some(self_ty.to_string())
1130 let cause = if let Conflict::Upstream = conflict {
1131 IntercrateAmbiguityCause::UpstreamCrateUpdate { trait_desc, self_desc }
1133 IntercrateAmbiguityCause::DownstreamCrate { trait_desc, self_desc }
1135 debug!("evaluate_stack: pushing cause = {:?}", cause);
1136 self.intercrate_ambiguity_causes.as_mut().unwrap().push(cause);
1143 let candidate_set = self.assemble_candidates(stack)?;
1145 if candidate_set.ambiguous {
1146 debug!("candidate set contains ambig");
1150 let mut candidates = candidate_set.vec;
1152 debug!("assembled {} candidates for {:?}: {:?}",
1157 // At this point, we know that each of the entries in the
1158 // candidate set is *individually* applicable. Now we have to
1159 // figure out if they contain mutual incompatibilities. This
1160 // frequently arises if we have an unconstrained input type --
1161 // for example, we are looking for $0:Eq where $0 is some
1162 // unconstrained type variable. In that case, we'll get a
1163 // candidate which assumes $0 == int, one that assumes $0 ==
1164 // usize, etc. This spells an ambiguity.
1166 // If there is more than one candidate, first winnow them down
1167 // by considering extra conditions (nested obligations and so
1168 // forth). We don't winnow if there is exactly one
1169 // candidate. This is a relatively minor distinction but it
1170 // can lead to better inference and error-reporting. An
1171 // example would be if there was an impl:
1173 // impl<T:Clone> Vec<T> { fn push_clone(...) { ... } }
1175 // and we were to see some code `foo.push_clone()` where `boo`
1176 // is a `Vec<Bar>` and `Bar` does not implement `Clone`. If
1177 // we were to winnow, we'd wind up with zero candidates.
1178 // Instead, we select the right impl now but report `Bar does
1179 // not implement Clone`.
1180 if candidates.len() == 1 {
1181 return self.filter_negative_impls(candidates.pop().unwrap());
1184 // Winnow, but record the exact outcome of evaluation, which
1185 // is needed for specialization.
1186 let mut candidates: Vec<_> = candidates.into_iter().filter_map(|c| {
1187 let eval = self.evaluate_candidate(stack, &c);
1188 if eval.may_apply() {
1189 Some(EvaluatedCandidate {
1198 // If there are STILL multiple candidate, we can further
1199 // reduce the list by dropping duplicates -- including
1200 // resolving specializations.
1201 if candidates.len() > 1 {
1203 while i < candidates.len() {
1205 (0..candidates.len())
1206 .filter(|&j| i != j)
1207 .any(|j| self.candidate_should_be_dropped_in_favor_of(&candidates[i],
1210 debug!("Dropping candidate #{}/{}: {:?}",
1211 i, candidates.len(), candidates[i]);
1212 candidates.swap_remove(i);
1214 debug!("Retaining candidate #{}/{}: {:?}",
1215 i, candidates.len(), candidates[i]);
1218 // If there are *STILL* multiple candidates, give up
1219 // and report ambiguity.
1221 debug!("multiple matches, ambig");
1228 // If there are *NO* candidates, then there are no impls --
1229 // that we know of, anyway. Note that in the case where there
1230 // are unbound type variables within the obligation, it might
1231 // be the case that you could still satisfy the obligation
1232 // from another crate by instantiating the type variables with
1233 // a type from another crate that does have an impl. This case
1234 // is checked for in `evaluate_stack` (and hence users
1235 // who might care about this case, like coherence, should use
1237 if candidates.is_empty() {
1238 return Err(Unimplemented);
1241 // Just one candidate left.
1242 self.filter_negative_impls(candidates.pop().unwrap().candidate)
1245 fn is_knowable<'o>(&mut self,
1246 stack: &TraitObligationStack<'o, 'tcx>)
1249 debug!("is_knowable(intercrate={:?})", self.intercrate);
1251 if !self.intercrate.is_some() {
1255 let obligation = &stack.obligation;
1256 let predicate = self.infcx().resolve_type_vars_if_possible(&obligation.predicate);
1258 // ok to skip binder because of the nature of the
1259 // trait-ref-is-knowable check, which does not care about
1261 let trait_ref = predicate.skip_binder().trait_ref;
1263 let result = coherence::trait_ref_is_knowable(self.tcx(), trait_ref);
1264 if let (Some(Conflict::Downstream { used_to_be_broken: true }),
1265 Some(IntercrateMode::Issue43355)) = (result, self.intercrate) {
1266 debug!("is_knowable: IGNORING conflict to be bug-compatible with #43355");
1273 /// Returns true if the global caches can be used.
1274 /// Do note that if the type itself is not in the
1275 /// global tcx, the local caches will be used.
1276 fn can_use_global_caches(&self, param_env: ty::ParamEnv<'tcx>) -> bool {
1277 // If there are any where-clauses in scope, then we always use
1278 // a cache local to this particular scope. Otherwise, we
1279 // switch to a global cache. We used to try and draw
1280 // finer-grained distinctions, but that led to a serious of
1281 // annoying and weird bugs like #22019 and #18290. This simple
1282 // rule seems to be pretty clearly safe and also still retains
1283 // a very high hit rate (~95% when compiling rustc).
1284 if !param_env.caller_bounds.is_empty() {
1288 // Avoid using the master cache during coherence and just rely
1289 // on the local cache. This effectively disables caching
1290 // during coherence. It is really just a simplification to
1291 // avoid us having to fear that coherence results "pollute"
1292 // the master cache. Since coherence executes pretty quickly,
1293 // it's not worth going to more trouble to increase the
1294 // hit-rate I don't think.
1295 if self.intercrate.is_some() {
1299 // Otherwise, we can use the global cache.
1303 fn check_candidate_cache(&mut self,
1304 param_env: ty::ParamEnv<'tcx>,
1305 cache_fresh_trait_pred: &ty::PolyTraitPredicate<'tcx>)
1306 -> Option<SelectionResult<'tcx, SelectionCandidate<'tcx>>>
1308 let tcx = self.tcx();
1309 let trait_ref = &cache_fresh_trait_pred.0.trait_ref;
1310 if self.can_use_global_caches(param_env) {
1311 let cache = tcx.selection_cache.hashmap.borrow();
1312 if let Some(cached) = cache.get(&trait_ref) {
1313 return Some(cached.get(tcx));
1316 self.infcx.selection_cache.hashmap
1319 .map(|v| v.get(tcx))
1322 fn insert_candidate_cache(&mut self,
1323 param_env: ty::ParamEnv<'tcx>,
1324 cache_fresh_trait_pred: ty::PolyTraitPredicate<'tcx>,
1325 dep_node: DepNodeIndex,
1326 candidate: SelectionResult<'tcx, SelectionCandidate<'tcx>>)
1328 let tcx = self.tcx();
1329 let trait_ref = cache_fresh_trait_pred.0.trait_ref;
1330 if self.can_use_global_caches(param_env) {
1331 let mut cache = tcx.selection_cache.hashmap.borrow_mut();
1332 if let Some(trait_ref) = tcx.lift_to_global(&trait_ref) {
1333 if let Some(candidate) = tcx.lift_to_global(&candidate) {
1334 cache.insert(trait_ref, WithDepNode::new(dep_node, candidate));
1340 self.infcx.selection_cache.hashmap
1342 .insert(trait_ref, WithDepNode::new(dep_node, candidate));
1345 fn assemble_candidates<'o>(&mut self,
1346 stack: &TraitObligationStack<'o, 'tcx>)
1347 -> Result<SelectionCandidateSet<'tcx>, SelectionError<'tcx>>
1349 let TraitObligationStack { obligation, .. } = *stack;
1350 let ref obligation = Obligation {
1351 param_env: obligation.param_env,
1352 cause: obligation.cause.clone(),
1353 recursion_depth: obligation.recursion_depth,
1354 predicate: self.infcx().resolve_type_vars_if_possible(&obligation.predicate)
1357 if obligation.predicate.skip_binder().self_ty().is_ty_var() {
1358 // Self is a type variable (e.g. `_: AsRef<str>`).
1360 // This is somewhat problematic, as the current scheme can't really
1361 // handle it turning to be a projection. This does end up as truly
1362 // ambiguous in most cases anyway.
1364 // Take the fast path out - this also improves
1365 // performance by preventing assemble_candidates_from_impls from
1366 // matching every impl for this trait.
1367 return Ok(SelectionCandidateSet { vec: vec![], ambiguous: true });
1370 let mut candidates = SelectionCandidateSet {
1375 // Other bounds. Consider both in-scope bounds from fn decl
1376 // and applicable impls. There is a certain set of precedence rules here.
1378 let def_id = obligation.predicate.def_id();
1379 let lang_items = self.tcx().lang_items();
1380 if lang_items.copy_trait() == Some(def_id) {
1381 debug!("obligation self ty is {:?}",
1382 obligation.predicate.0.self_ty());
1384 // User-defined copy impls are permitted, but only for
1385 // structs and enums.
1386 self.assemble_candidates_from_impls(obligation, &mut candidates)?;
1388 // For other types, we'll use the builtin rules.
1389 let copy_conditions = self.copy_clone_conditions(obligation);
1390 self.assemble_builtin_bound_candidates(copy_conditions, &mut candidates)?;
1391 } else if lang_items.sized_trait() == Some(def_id) {
1392 // Sized is never implementable by end-users, it is
1393 // always automatically computed.
1394 let sized_conditions = self.sized_conditions(obligation);
1395 self.assemble_builtin_bound_candidates(sized_conditions,
1397 } else if lang_items.unsize_trait() == Some(def_id) {
1398 self.assemble_candidates_for_unsizing(obligation, &mut candidates);
1400 if lang_items.clone_trait() == Some(def_id) {
1401 // Same builtin conditions as `Copy`, i.e. every type which has builtin support
1402 // for `Copy` also has builtin support for `Clone`, + tuples and arrays of `Clone`
1403 // types have builtin support for `Clone`.
1404 let clone_conditions = self.copy_clone_conditions(obligation);
1405 self.assemble_builtin_bound_candidates(clone_conditions, &mut candidates)?;
1408 self.assemble_generator_candidates(obligation, &mut candidates)?;
1409 self.assemble_closure_candidates(obligation, &mut candidates)?;
1410 self.assemble_fn_pointer_candidates(obligation, &mut candidates)?;
1411 self.assemble_candidates_from_impls(obligation, &mut candidates)?;
1412 self.assemble_candidates_from_object_ty(obligation, &mut candidates);
1415 self.assemble_candidates_from_projected_tys(obligation, &mut candidates);
1416 self.assemble_candidates_from_caller_bounds(stack, &mut candidates)?;
1417 // Auto implementations have lower priority, so we only
1418 // consider triggering a default if there is no other impl that can apply.
1419 if candidates.vec.is_empty() {
1420 self.assemble_candidates_from_auto_impls(obligation, &mut candidates)?;
1422 debug!("candidate list size: {}", candidates.vec.len());
1426 fn assemble_candidates_from_projected_tys(&mut self,
1427 obligation: &TraitObligation<'tcx>,
1428 candidates: &mut SelectionCandidateSet<'tcx>)
1430 debug!("assemble_candidates_for_projected_tys({:?})", obligation);
1432 // before we go into the whole skolemization thing, just
1433 // quickly check if the self-type is a projection at all.
1434 match obligation.predicate.0.trait_ref.self_ty().sty {
1435 ty::TyProjection(_) | ty::TyAnon(..) => {}
1436 ty::TyInfer(ty::TyVar(_)) => {
1437 span_bug!(obligation.cause.span,
1438 "Self=_ should have been handled by assemble_candidates");
1443 let result = self.probe(|this, snapshot| {
1444 this.match_projection_obligation_against_definition_bounds(obligation,
1449 candidates.vec.push(ProjectionCandidate);
1453 fn match_projection_obligation_against_definition_bounds(
1455 obligation: &TraitObligation<'tcx>,
1456 snapshot: &infer::CombinedSnapshot)
1459 let poly_trait_predicate =
1460 self.infcx().resolve_type_vars_if_possible(&obligation.predicate);
1461 let (skol_trait_predicate, skol_map) =
1462 self.infcx().skolemize_late_bound_regions(&poly_trait_predicate, snapshot);
1463 debug!("match_projection_obligation_against_definition_bounds: \
1464 skol_trait_predicate={:?} skol_map={:?}",
1465 skol_trait_predicate,
1468 let (def_id, substs) = match skol_trait_predicate.trait_ref.self_ty().sty {
1469 ty::TyProjection(ref data) =>
1470 (data.trait_ref(self.tcx()).def_id, data.substs),
1471 ty::TyAnon(def_id, substs) => (def_id, substs),
1474 obligation.cause.span,
1475 "match_projection_obligation_against_definition_bounds() called \
1476 but self-ty not a projection: {:?}",
1477 skol_trait_predicate.trait_ref.self_ty());
1480 debug!("match_projection_obligation_against_definition_bounds: \
1481 def_id={:?}, substs={:?}",
1484 let predicates_of = self.tcx().predicates_of(def_id);
1485 let bounds = predicates_of.instantiate(self.tcx(), substs);
1486 debug!("match_projection_obligation_against_definition_bounds: \
1490 let matching_bound =
1491 util::elaborate_predicates(self.tcx(), bounds.predicates)
1495 |this, _| this.match_projection(obligation,
1497 skol_trait_predicate.trait_ref.clone(),
1501 debug!("match_projection_obligation_against_definition_bounds: \
1502 matching_bound={:?}",
1504 match matching_bound {
1507 // Repeat the successful match, if any, this time outside of a probe.
1508 let result = self.match_projection(obligation,
1510 skol_trait_predicate.trait_ref.clone(),
1514 self.infcx.pop_skolemized(skol_map, snapshot);
1522 fn match_projection(&mut self,
1523 obligation: &TraitObligation<'tcx>,
1524 trait_bound: ty::PolyTraitRef<'tcx>,
1525 skol_trait_ref: ty::TraitRef<'tcx>,
1526 skol_map: &infer::SkolemizationMap<'tcx>,
1527 snapshot: &infer::CombinedSnapshot)
1530 assert!(!skol_trait_ref.has_escaping_regions());
1531 match self.infcx.at(&obligation.cause, obligation.param_env)
1532 .sup(ty::Binder(skol_trait_ref), trait_bound) {
1533 Ok(InferOk { obligations, .. }) => {
1534 self.inferred_obligations.extend(obligations);
1536 Err(_) => { return false; }
1539 self.infcx.leak_check(false, obligation.cause.span, skol_map, snapshot).is_ok()
1542 /// Given an obligation like `<SomeTrait for T>`, search the obligations that the caller
1543 /// supplied to find out whether it is listed among them.
1545 /// Never affects inference environment.
1546 fn assemble_candidates_from_caller_bounds<'o>(&mut self,
1547 stack: &TraitObligationStack<'o, 'tcx>,
1548 candidates: &mut SelectionCandidateSet<'tcx>)
1549 -> Result<(),SelectionError<'tcx>>
1551 debug!("assemble_candidates_from_caller_bounds({:?})",
1555 stack.obligation.param_env.caller_bounds
1557 .filter_map(|o| o.to_opt_poly_trait_ref());
1559 // micro-optimization: filter out predicates relating to different
1561 let matching_bounds =
1562 all_bounds.filter(|p| p.def_id() == stack.obligation.predicate.def_id());
1564 let matching_bounds =
1565 matching_bounds.filter(
1566 |bound| self.evaluate_where_clause(stack, bound.clone()).may_apply());
1568 let param_candidates =
1569 matching_bounds.map(|bound| ParamCandidate(bound));
1571 candidates.vec.extend(param_candidates);
1576 fn evaluate_where_clause<'o>(&mut self,
1577 stack: &TraitObligationStack<'o, 'tcx>,
1578 where_clause_trait_ref: ty::PolyTraitRef<'tcx>)
1581 self.probe(move |this, _| {
1582 match this.match_where_clause_trait_ref(stack.obligation, where_clause_trait_ref) {
1583 Ok(obligations) => {
1584 this.evaluate_predicates_recursively(stack.list(), obligations.iter())
1586 Err(()) => EvaluatedToErr
1591 fn assemble_generator_candidates(&mut self,
1592 obligation: &TraitObligation<'tcx>,
1593 candidates: &mut SelectionCandidateSet<'tcx>)
1594 -> Result<(),SelectionError<'tcx>>
1596 if self.tcx().lang_items().gen_trait() != Some(obligation.predicate.def_id()) {
1600 // ok to skip binder because the substs on generator types never
1601 // touch bound regions, they just capture the in-scope
1602 // type/region parameters
1603 let self_ty = *obligation.self_ty().skip_binder();
1605 ty::TyGenerator(..) => {
1606 debug!("assemble_generator_candidates: self_ty={:?} obligation={:?}",
1610 candidates.vec.push(GeneratorCandidate);
1613 ty::TyInfer(ty::TyVar(_)) => {
1614 debug!("assemble_generator_candidates: ambiguous self-type");
1615 candidates.ambiguous = true;
1618 _ => { return Ok(()); }
1622 /// Check for the artificial impl that the compiler will create for an obligation like `X :
1623 /// FnMut<..>` where `X` is a closure type.
1625 /// Note: the type parameters on a closure candidate are modeled as *output* type
1626 /// parameters and hence do not affect whether this trait is a match or not. They will be
1627 /// unified during the confirmation step.
1628 fn assemble_closure_candidates(&mut self,
1629 obligation: &TraitObligation<'tcx>,
1630 candidates: &mut SelectionCandidateSet<'tcx>)
1631 -> Result<(),SelectionError<'tcx>>
1633 let kind = match self.tcx().lang_items().fn_trait_kind(obligation.predicate.0.def_id()) {
1635 None => { return Ok(()); }
1638 // ok to skip binder because the substs on closure types never
1639 // touch bound regions, they just capture the in-scope
1640 // type/region parameters
1641 match obligation.self_ty().skip_binder().sty {
1642 ty::TyClosure(closure_def_id, closure_substs) => {
1643 debug!("assemble_unboxed_candidates: kind={:?} obligation={:?}",
1645 match self.infcx.closure_kind(closure_def_id, closure_substs) {
1646 Some(closure_kind) => {
1647 debug!("assemble_unboxed_candidates: closure_kind = {:?}", closure_kind);
1648 if closure_kind.extends(kind) {
1649 candidates.vec.push(ClosureCandidate);
1653 debug!("assemble_unboxed_candidates: closure_kind not yet known");
1654 candidates.vec.push(ClosureCandidate);
1659 ty::TyInfer(ty::TyVar(_)) => {
1660 debug!("assemble_unboxed_closure_candidates: ambiguous self-type");
1661 candidates.ambiguous = true;
1664 _ => { return Ok(()); }
1668 /// Implement one of the `Fn()` family for a fn pointer.
1669 fn assemble_fn_pointer_candidates(&mut self,
1670 obligation: &TraitObligation<'tcx>,
1671 candidates: &mut SelectionCandidateSet<'tcx>)
1672 -> Result<(),SelectionError<'tcx>>
1674 // We provide impl of all fn traits for fn pointers.
1675 if self.tcx().lang_items().fn_trait_kind(obligation.predicate.def_id()).is_none() {
1679 // ok to skip binder because what we are inspecting doesn't involve bound regions
1680 let self_ty = *obligation.self_ty().skip_binder();
1682 ty::TyInfer(ty::TyVar(_)) => {
1683 debug!("assemble_fn_pointer_candidates: ambiguous self-type");
1684 candidates.ambiguous = true; // could wind up being a fn() type
1687 // provide an impl, but only for suitable `fn` pointers
1688 ty::TyFnDef(..) | ty::TyFnPtr(_) => {
1689 if let ty::Binder(ty::FnSig {
1690 unsafety: hir::Unsafety::Normal,
1694 }) = self_ty.fn_sig(self.tcx()) {
1695 candidates.vec.push(FnPointerCandidate);
1705 /// Search for impls that might apply to `obligation`.
1706 fn assemble_candidates_from_impls(&mut self,
1707 obligation: &TraitObligation<'tcx>,
1708 candidates: &mut SelectionCandidateSet<'tcx>)
1709 -> Result<(), SelectionError<'tcx>>
1711 debug!("assemble_candidates_from_impls(obligation={:?})", obligation);
1713 self.tcx().for_each_relevant_impl(
1714 obligation.predicate.def_id(),
1715 obligation.predicate.0.trait_ref.self_ty(),
1717 self.probe(|this, snapshot| { /* [1] */
1718 match this.match_impl(impl_def_id, obligation, snapshot) {
1720 candidates.vec.push(ImplCandidate(impl_def_id));
1722 // NB: we can safely drop the skol map
1723 // since we are in a probe [1]
1724 mem::drop(skol_map);
1735 fn assemble_candidates_from_auto_impls(&mut self,
1736 obligation: &TraitObligation<'tcx>,
1737 candidates: &mut SelectionCandidateSet<'tcx>)
1738 -> Result<(), SelectionError<'tcx>>
1740 // OK to skip binder here because the tests we do below do not involve bound regions
1741 let self_ty = *obligation.self_ty().skip_binder();
1742 debug!("assemble_candidates_from_auto_impls(self_ty={:?})", self_ty);
1744 let def_id = obligation.predicate.def_id();
1746 if self.tcx().trait_is_auto(def_id) {
1748 ty::TyDynamic(..) => {
1749 // For object types, we don't know what the closed
1750 // over types are. This means we conservatively
1751 // say nothing; a candidate may be added by
1752 // `assemble_candidates_from_object_ty`.
1754 ty::TyForeign(..) => {
1755 // Since the contents of foreign types is unknown,
1756 // we don't add any `..` impl. Default traits could
1757 // still be provided by a manual implementation for
1758 // this trait and type.
1761 ty::TyProjection(..) => {
1762 // In these cases, we don't know what the actual
1763 // type is. Therefore, we cannot break it down
1764 // into its constituent types. So we don't
1765 // consider the `..` impl but instead just add no
1766 // candidates: this means that typeck will only
1767 // succeed if there is another reason to believe
1768 // that this obligation holds. That could be a
1769 // where-clause or, in the case of an object type,
1770 // it could be that the object type lists the
1771 // trait (e.g. `Foo+Send : Send`). See
1772 // `compile-fail/typeck-default-trait-impl-send-param.rs`
1773 // for an example of a test case that exercises
1776 ty::TyInfer(ty::TyVar(_)) => {
1777 // the auto impl might apply, we don't know
1778 candidates.ambiguous = true;
1781 candidates.vec.push(AutoImplCandidate(def_id.clone()))
1789 /// Search for impls that might apply to `obligation`.
1790 fn assemble_candidates_from_object_ty(&mut self,
1791 obligation: &TraitObligation<'tcx>,
1792 candidates: &mut SelectionCandidateSet<'tcx>)
1794 debug!("assemble_candidates_from_object_ty(self_ty={:?})",
1795 obligation.self_ty().skip_binder());
1797 // Object-safety candidates are only applicable to object-safe
1798 // traits. Including this check is useful because it helps
1799 // inference in cases of traits like `BorrowFrom`, which are
1800 // not object-safe, and which rely on being able to infer the
1801 // self-type from one of the other inputs. Without this check,
1802 // these cases wind up being considered ambiguous due to a
1803 // (spurious) ambiguity introduced here.
1804 let predicate_trait_ref = obligation.predicate.to_poly_trait_ref();
1805 if !self.tcx().is_object_safe(predicate_trait_ref.def_id()) {
1809 self.probe(|this, _snapshot| {
1810 // the code below doesn't care about regions, and the
1811 // self-ty here doesn't escape this probe, so just erase
1813 let self_ty = this.tcx().erase_late_bound_regions(&obligation.self_ty());
1814 let poly_trait_ref = match self_ty.sty {
1815 ty::TyDynamic(ref data, ..) => {
1816 if data.auto_traits().any(|did| did == obligation.predicate.def_id()) {
1817 debug!("assemble_candidates_from_object_ty: matched builtin bound, \
1818 pushing candidate");
1819 candidates.vec.push(BuiltinObjectCandidate);
1823 match data.principal() {
1824 Some(p) => p.with_self_ty(this.tcx(), self_ty),
1828 ty::TyInfer(ty::TyVar(_)) => {
1829 debug!("assemble_candidates_from_object_ty: ambiguous");
1830 candidates.ambiguous = true; // could wind up being an object type
1838 debug!("assemble_candidates_from_object_ty: poly_trait_ref={:?}",
1841 // Count only those upcast versions that match the trait-ref
1842 // we are looking for. Specifically, do not only check for the
1843 // correct trait, but also the correct type parameters.
1844 // For example, we may be trying to upcast `Foo` to `Bar<i32>`,
1845 // but `Foo` is declared as `trait Foo : Bar<u32>`.
1846 let upcast_trait_refs =
1847 util::supertraits(this.tcx(), poly_trait_ref)
1848 .filter(|upcast_trait_ref| {
1849 this.probe(|this, _| {
1850 let upcast_trait_ref = upcast_trait_ref.clone();
1851 this.match_poly_trait_ref(obligation, upcast_trait_ref).is_ok()
1856 if upcast_trait_refs > 1 {
1857 // can be upcast in many ways; need more type information
1858 candidates.ambiguous = true;
1859 } else if upcast_trait_refs == 1 {
1860 candidates.vec.push(ObjectCandidate);
1865 /// Search for unsizing that might apply to `obligation`.
1866 fn assemble_candidates_for_unsizing(&mut self,
1867 obligation: &TraitObligation<'tcx>,
1868 candidates: &mut SelectionCandidateSet<'tcx>) {
1869 // We currently never consider higher-ranked obligations e.g.
1870 // `for<'a> &'a T: Unsize<Trait+'a>` to be implemented. This is not
1871 // because they are a priori invalid, and we could potentially add support
1872 // for them later, it's just that there isn't really a strong need for it.
1873 // A `T: Unsize<U>` obligation is always used as part of a `T: CoerceUnsize<U>`
1874 // impl, and those are generally applied to concrete types.
1876 // That said, one might try to write a fn with a where clause like
1877 // for<'a> Foo<'a, T>: Unsize<Foo<'a, Trait>>
1878 // where the `'a` is kind of orthogonal to the relevant part of the `Unsize`.
1879 // Still, you'd be more likely to write that where clause as
1881 // so it seems ok if we (conservatively) fail to accept that `Unsize`
1882 // obligation above. Should be possible to extend this in the future.
1883 let source = match obligation.self_ty().no_late_bound_regions() {
1886 // Don't add any candidates if there are bound regions.
1890 let target = obligation.predicate.skip_binder().trait_ref.substs.type_at(1);
1892 debug!("assemble_candidates_for_unsizing(source={:?}, target={:?})",
1895 let may_apply = match (&source.sty, &target.sty) {
1896 // Trait+Kx+'a -> Trait+Ky+'b (upcasts).
1897 (&ty::TyDynamic(ref data_a, ..), &ty::TyDynamic(ref data_b, ..)) => {
1898 // Upcasts permit two things:
1900 // 1. Dropping builtin bounds, e.g. `Foo+Send` to `Foo`
1901 // 2. Tightening the region bound, e.g. `Foo+'a` to `Foo+'b` if `'a : 'b`
1903 // Note that neither of these changes requires any
1904 // change at runtime. Eventually this will be
1907 // We always upcast when we can because of reason
1908 // #2 (region bounds).
1909 match (data_a.principal(), data_b.principal()) {
1910 (Some(a), Some(b)) => a.def_id() == b.def_id() &&
1911 data_b.auto_traits()
1912 // All of a's auto traits need to be in b's auto traits.
1913 .all(|b| data_a.auto_traits().any(|a| a == b)),
1919 (_, &ty::TyDynamic(..)) => true,
1921 // Ambiguous handling is below T -> Trait, because inference
1922 // variables can still implement Unsize<Trait> and nested
1923 // obligations will have the final say (likely deferred).
1924 (&ty::TyInfer(ty::TyVar(_)), _) |
1925 (_, &ty::TyInfer(ty::TyVar(_))) => {
1926 debug!("assemble_candidates_for_unsizing: ambiguous");
1927 candidates.ambiguous = true;
1932 (&ty::TyArray(..), &ty::TySlice(_)) => true,
1934 // Struct<T> -> Struct<U>.
1935 (&ty::TyAdt(def_id_a, _), &ty::TyAdt(def_id_b, _)) if def_id_a.is_struct() => {
1936 def_id_a == def_id_b
1939 // (.., T) -> (.., U).
1940 (&ty::TyTuple(tys_a, _), &ty::TyTuple(tys_b, _)) => {
1941 tys_a.len() == tys_b.len()
1948 candidates.vec.push(BuiltinUnsizeCandidate);
1952 ///////////////////////////////////////////////////////////////////////////
1955 // Winnowing is the process of attempting to resolve ambiguity by
1956 // probing further. During the winnowing process, we unify all
1957 // type variables (ignoring skolemization) and then we also
1958 // attempt to evaluate recursive bounds to see if they are
1961 /// Returns true if `candidate_i` should be dropped in favor of
1962 /// `candidate_j`. Generally speaking we will drop duplicate
1963 /// candidates and prefer where-clause candidates.
1964 /// Returns true if `victim` should be dropped in favor of
1965 /// `other`. Generally speaking we will drop duplicate
1966 /// candidates and prefer where-clause candidates.
1968 /// See the comment for "SelectionCandidate" for more details.
1969 fn candidate_should_be_dropped_in_favor_of<'o>(
1971 victim: &EvaluatedCandidate<'tcx>,
1972 other: &EvaluatedCandidate<'tcx>)
1975 if victim.candidate == other.candidate {
1979 match other.candidate {
1981 ParamCandidate(_) | ProjectionCandidate => match victim.candidate {
1982 AutoImplCandidate(..) => {
1984 "default implementations shouldn't be recorded \
1985 when there are other valid candidates");
1989 GeneratorCandidate |
1990 FnPointerCandidate |
1991 BuiltinObjectCandidate |
1992 BuiltinUnsizeCandidate |
1993 BuiltinCandidate { .. } => {
1994 // We have a where-clause so don't go around looking
1999 ProjectionCandidate => {
2000 // Arbitrarily give param candidates priority
2001 // over projection and object candidates.
2004 ParamCandidate(..) => false,
2006 ImplCandidate(other_def) => {
2007 // See if we can toss out `victim` based on specialization.
2008 // This requires us to know *for sure* that the `other` impl applies
2009 // i.e. EvaluatedToOk:
2010 if other.evaluation == EvaluatedToOk {
2011 if let ImplCandidate(victim_def) = victim.candidate {
2012 let tcx = self.tcx().global_tcx();
2013 return tcx.specializes((other_def, victim_def)) ||
2014 tcx.impls_are_allowed_to_overlap(other_def, victim_def);
2024 ///////////////////////////////////////////////////////////////////////////
2027 // These cover the traits that are built-in to the language
2028 // itself. This includes `Copy` and `Sized` for sure. For the
2029 // moment, it also includes `Send` / `Sync` and a few others, but
2030 // those will hopefully change to library-defined traits in the
2033 // HACK: if this returns an error, selection exits without considering
2035 fn assemble_builtin_bound_candidates<'o>(&mut self,
2036 conditions: BuiltinImplConditions<'tcx>,
2037 candidates: &mut SelectionCandidateSet<'tcx>)
2038 -> Result<(),SelectionError<'tcx>>
2041 BuiltinImplConditions::Where(nested) => {
2042 debug!("builtin_bound: nested={:?}", nested);
2043 candidates.vec.push(BuiltinCandidate {
2044 has_nested: nested.skip_binder().len() > 0
2048 BuiltinImplConditions::None => { Ok(()) }
2049 BuiltinImplConditions::Ambiguous => {
2050 debug!("assemble_builtin_bound_candidates: ambiguous builtin");
2051 Ok(candidates.ambiguous = true)
2053 BuiltinImplConditions::Never => { Err(Unimplemented) }
2057 fn sized_conditions(&mut self, obligation: &TraitObligation<'tcx>)
2058 -> BuiltinImplConditions<'tcx>
2060 use self::BuiltinImplConditions::{Ambiguous, None, Never, Where};
2062 // NOTE: binder moved to (*)
2063 let self_ty = self.infcx.shallow_resolve(
2064 obligation.predicate.skip_binder().self_ty());
2067 ty::TyInfer(ty::IntVar(_)) | ty::TyInfer(ty::FloatVar(_)) |
2068 ty::TyUint(_) | ty::TyInt(_) | ty::TyBool | ty::TyFloat(_) |
2069 ty::TyFnDef(..) | ty::TyFnPtr(_) | ty::TyRawPtr(..) |
2070 ty::TyChar | ty::TyRef(..) | ty::TyGenerator(..) |
2071 ty::TyArray(..) | ty::TyClosure(..) | ty::TyNever |
2073 // safe for everything
2074 Where(ty::Binder(Vec::new()))
2077 ty::TyStr | ty::TySlice(_) | ty::TyDynamic(..) | ty::TyForeign(..) => Never,
2079 ty::TyTuple(tys, _) => {
2080 Where(ty::Binder(tys.last().into_iter().cloned().collect()))
2083 ty::TyAdt(def, substs) => {
2084 let sized_crit = def.sized_constraint(self.tcx());
2085 // (*) binder moved here
2087 sized_crit.iter().map(|ty| ty.subst(self.tcx(), substs)).collect()
2091 ty::TyProjection(_) | ty::TyParam(_) | ty::TyAnon(..) => None,
2092 ty::TyInfer(ty::TyVar(_)) => Ambiguous,
2094 ty::TyInfer(ty::FreshTy(_))
2095 | ty::TyInfer(ty::FreshIntTy(_))
2096 | ty::TyInfer(ty::FreshFloatTy(_)) => {
2097 bug!("asked to assemble builtin bounds of unexpected type: {:?}",
2103 fn copy_clone_conditions(&mut self, obligation: &TraitObligation<'tcx>)
2104 -> BuiltinImplConditions<'tcx>
2106 // NOTE: binder moved to (*)
2107 let self_ty = self.infcx.shallow_resolve(
2108 obligation.predicate.skip_binder().self_ty());
2110 use self::BuiltinImplConditions::{Ambiguous, None, Never, Where};
2113 ty::TyInfer(ty::IntVar(_)) | ty::TyInfer(ty::FloatVar(_)) |
2114 ty::TyUint(_) | ty::TyInt(_) | ty::TyBool | ty::TyFloat(_) |
2115 ty::TyFnDef(..) | ty::TyFnPtr(_) | ty::TyChar |
2116 ty::TyRawPtr(..) | ty::TyError | ty::TyNever |
2117 ty::TyRef(_, ty::TypeAndMut { ty: _, mutbl: hir::MutImmutable }) => {
2118 Where(ty::Binder(Vec::new()))
2121 ty::TyDynamic(..) | ty::TyStr | ty::TySlice(..) |
2122 ty::TyGenerator(..) | ty::TyForeign(..) |
2123 ty::TyRef(_, ty::TypeAndMut { ty: _, mutbl: hir::MutMutable }) => {
2127 ty::TyArray(element_ty, _) => {
2128 // (*) binder moved here
2129 Where(ty::Binder(vec![element_ty]))
2132 ty::TyTuple(tys, _) => {
2133 // (*) binder moved here
2134 Where(ty::Binder(tys.to_vec()))
2137 ty::TyClosure(def_id, substs) => {
2138 let trait_id = obligation.predicate.def_id();
2140 Some(trait_id) == self.tcx().lang_items().copy_trait() &&
2141 self.tcx().has_copy_closures(def_id.krate);
2142 let clone_closures =
2143 Some(trait_id) == self.tcx().lang_items().clone_trait() &&
2144 self.tcx().has_clone_closures(def_id.krate);
2146 if copy_closures || clone_closures {
2147 Where(ty::Binder(substs.upvar_tys(def_id, self.tcx()).collect()))
2153 ty::TyAdt(..) | ty::TyProjection(..) | ty::TyParam(..) | ty::TyAnon(..) => {
2154 // Fallback to whatever user-defined impls exist in this case.
2158 ty::TyInfer(ty::TyVar(_)) => {
2159 // Unbound type variable. Might or might not have
2160 // applicable impls and so forth, depending on what
2161 // those type variables wind up being bound to.
2165 ty::TyInfer(ty::FreshTy(_))
2166 | ty::TyInfer(ty::FreshIntTy(_))
2167 | ty::TyInfer(ty::FreshFloatTy(_)) => {
2168 bug!("asked to assemble builtin bounds of unexpected type: {:?}",
2174 /// For default impls, we need to break apart a type into its
2175 /// "constituent types" -- meaning, the types that it contains.
2177 /// Here are some (simple) examples:
2180 /// (i32, u32) -> [i32, u32]
2181 /// Foo where struct Foo { x: i32, y: u32 } -> [i32, u32]
2182 /// Bar<i32> where struct Bar<T> { x: T, y: u32 } -> [i32, u32]
2183 /// Zed<i32> where enum Zed { A(T), B(u32) } -> [i32, u32]
2185 fn constituent_types_for_ty(&self, t: Ty<'tcx>) -> Vec<Ty<'tcx>> {
2195 ty::TyInfer(ty::IntVar(_)) |
2196 ty::TyInfer(ty::FloatVar(_)) |
2205 ty::TyProjection(..) |
2206 ty::TyInfer(ty::TyVar(_)) |
2207 ty::TyInfer(ty::FreshTy(_)) |
2208 ty::TyInfer(ty::FreshIntTy(_)) |
2209 ty::TyInfer(ty::FreshFloatTy(_)) => {
2210 bug!("asked to assemble constituent types of unexpected type: {:?}",
2214 ty::TyRawPtr(ty::TypeAndMut { ty: element_ty, ..}) |
2215 ty::TyRef(_, ty::TypeAndMut { ty: element_ty, ..}) => {
2219 ty::TyArray(element_ty, _) | ty::TySlice(element_ty) => {
2223 ty::TyTuple(ref tys, _) => {
2224 // (T1, ..., Tn) -- meets any bound that all of T1...Tn meet
2228 ty::TyClosure(def_id, ref substs) => {
2229 substs.upvar_tys(def_id, self.tcx()).collect()
2232 ty::TyGenerator(def_id, ref substs, interior) => {
2233 let witness = iter::once(interior.witness);
2234 substs.upvar_tys(def_id, self.tcx()).chain(witness).collect()
2237 // for `PhantomData<T>`, we pass `T`
2238 ty::TyAdt(def, substs) if def.is_phantom_data() => {
2239 substs.types().collect()
2242 ty::TyAdt(def, substs) => {
2244 .map(|f| f.ty(self.tcx(), substs))
2248 ty::TyAnon(def_id, substs) => {
2249 // We can resolve the `impl Trait` to its concrete type,
2250 // which enforces a DAG between the functions requiring
2251 // the auto trait bounds in question.
2252 vec![self.tcx().type_of(def_id).subst(self.tcx(), substs)]
2257 fn collect_predicates_for_types(&mut self,
2258 param_env: ty::ParamEnv<'tcx>,
2259 cause: ObligationCause<'tcx>,
2260 recursion_depth: usize,
2261 trait_def_id: DefId,
2262 types: ty::Binder<Vec<Ty<'tcx>>>)
2263 -> Vec<PredicateObligation<'tcx>>
2265 // Because the types were potentially derived from
2266 // higher-ranked obligations they may reference late-bound
2267 // regions. For example, `for<'a> Foo<&'a int> : Copy` would
2268 // yield a type like `for<'a> &'a int`. In general, we
2269 // maintain the invariant that we never manipulate bound
2270 // regions, so we have to process these bound regions somehow.
2272 // The strategy is to:
2274 // 1. Instantiate those regions to skolemized regions (e.g.,
2275 // `for<'a> &'a int` becomes `&0 int`.
2276 // 2. Produce something like `&'0 int : Copy`
2277 // 3. Re-bind the regions back to `for<'a> &'a int : Copy`
2279 types.skip_binder().into_iter().flat_map(|ty| { // binder moved -\
2280 let ty: ty::Binder<Ty<'tcx>> = ty::Binder(ty); // <----------/
2282 self.in_snapshot(|this, snapshot| {
2283 let (skol_ty, skol_map) =
2284 this.infcx().skolemize_late_bound_regions(&ty, snapshot);
2285 let Normalized { value: normalized_ty, mut obligations } =
2286 project::normalize_with_depth(this,
2291 let skol_obligation =
2292 this.tcx().predicate_for_trait_def(param_env,
2298 obligations.push(skol_obligation);
2299 this.infcx().plug_leaks(skol_map, snapshot, obligations)
2304 ///////////////////////////////////////////////////////////////////////////
2307 // Confirmation unifies the output type parameters of the trait
2308 // with the values found in the obligation, possibly yielding a
2309 // type error. See `README.md` for more details.
2311 fn confirm_candidate(&mut self,
2312 obligation: &TraitObligation<'tcx>,
2313 candidate: SelectionCandidate<'tcx>)
2314 -> Result<Selection<'tcx>,SelectionError<'tcx>>
2316 debug!("confirm_candidate({:?}, {:?})",
2321 BuiltinCandidate { has_nested } => {
2322 let data = self.confirm_builtin_candidate(obligation, has_nested);
2323 Ok(VtableBuiltin(data))
2326 ParamCandidate(param) => {
2327 let obligations = self.confirm_param_candidate(obligation, param);
2328 Ok(VtableParam(obligations))
2331 AutoImplCandidate(trait_def_id) => {
2332 let data = self.confirm_auto_impl_candidate(obligation, trait_def_id);
2333 Ok(VtableAutoImpl(data))
2336 ImplCandidate(impl_def_id) => {
2337 Ok(VtableImpl(self.confirm_impl_candidate(obligation, impl_def_id)))
2340 ClosureCandidate => {
2341 let vtable_closure = self.confirm_closure_candidate(obligation)?;
2342 Ok(VtableClosure(vtable_closure))
2345 GeneratorCandidate => {
2346 let vtable_generator = self.confirm_generator_candidate(obligation)?;
2347 Ok(VtableGenerator(vtable_generator))
2350 BuiltinObjectCandidate => {
2351 // This indicates something like `(Trait+Send) :
2352 // Send`. In this case, we know that this holds
2353 // because that's what the object type is telling us,
2354 // and there's really no additional obligations to
2355 // prove and no types in particular to unify etc.
2356 Ok(VtableParam(Vec::new()))
2359 ObjectCandidate => {
2360 let data = self.confirm_object_candidate(obligation);
2361 Ok(VtableObject(data))
2364 FnPointerCandidate => {
2366 self.confirm_fn_pointer_candidate(obligation)?;
2367 Ok(VtableFnPointer(data))
2370 ProjectionCandidate => {
2371 self.confirm_projection_candidate(obligation);
2372 Ok(VtableParam(Vec::new()))
2375 BuiltinUnsizeCandidate => {
2376 let data = self.confirm_builtin_unsize_candidate(obligation)?;
2377 Ok(VtableBuiltin(data))
2382 fn confirm_projection_candidate(&mut self,
2383 obligation: &TraitObligation<'tcx>)
2385 self.in_snapshot(|this, snapshot| {
2387 this.match_projection_obligation_against_definition_bounds(obligation,
2393 fn confirm_param_candidate(&mut self,
2394 obligation: &TraitObligation<'tcx>,
2395 param: ty::PolyTraitRef<'tcx>)
2396 -> Vec<PredicateObligation<'tcx>>
2398 debug!("confirm_param_candidate({:?},{:?})",
2402 // During evaluation, we already checked that this
2403 // where-clause trait-ref could be unified with the obligation
2404 // trait-ref. Repeat that unification now without any
2405 // transactional boundary; it should not fail.
2406 match self.match_where_clause_trait_ref(obligation, param.clone()) {
2407 Ok(obligations) => obligations,
2409 bug!("Where clause `{:?}` was applicable to `{:?}` but now is not",
2416 fn confirm_builtin_candidate(&mut self,
2417 obligation: &TraitObligation<'tcx>,
2419 -> VtableBuiltinData<PredicateObligation<'tcx>>
2421 debug!("confirm_builtin_candidate({:?}, {:?})",
2422 obligation, has_nested);
2424 let lang_items = self.tcx().lang_items();
2425 let obligations = if has_nested {
2426 let trait_def = obligation.predicate.def_id();
2427 let conditions = match trait_def {
2428 _ if Some(trait_def) == lang_items.sized_trait() => {
2429 self.sized_conditions(obligation)
2431 _ if Some(trait_def) == lang_items.copy_trait() => {
2432 self.copy_clone_conditions(obligation)
2434 _ if Some(trait_def) == lang_items.clone_trait() => {
2435 self.copy_clone_conditions(obligation)
2437 _ => bug!("unexpected builtin trait {:?}", trait_def)
2439 let nested = match conditions {
2440 BuiltinImplConditions::Where(nested) => nested,
2441 _ => bug!("obligation {:?} had matched a builtin impl but now doesn't",
2445 let cause = obligation.derived_cause(BuiltinDerivedObligation);
2446 self.collect_predicates_for_types(obligation.param_env,
2448 obligation.recursion_depth+1,
2455 debug!("confirm_builtin_candidate: obligations={:?}",
2458 VtableBuiltinData { nested: obligations }
2461 /// This handles the case where a `auto trait Foo` impl is being used.
2462 /// The idea is that the impl applies to `X : Foo` if the following conditions are met:
2464 /// 1. For each constituent type `Y` in `X`, `Y : Foo` holds
2465 /// 2. For each where-clause `C` declared on `Foo`, `[Self => X] C` holds.
2466 fn confirm_auto_impl_candidate(&mut self,
2467 obligation: &TraitObligation<'tcx>,
2468 trait_def_id: DefId)
2469 -> VtableAutoImplData<PredicateObligation<'tcx>>
2471 debug!("confirm_auto_impl_candidate({:?}, {:?})",
2475 // binder is moved below
2476 let self_ty = self.infcx.shallow_resolve(obligation.predicate.skip_binder().self_ty());
2477 let types = self.constituent_types_for_ty(self_ty);
2478 self.vtable_auto_impl(obligation, trait_def_id, ty::Binder(types))
2481 /// See `confirm_auto_impl_candidate`
2482 fn vtable_auto_impl(&mut self,
2483 obligation: &TraitObligation<'tcx>,
2484 trait_def_id: DefId,
2485 nested: ty::Binder<Vec<Ty<'tcx>>>)
2486 -> VtableAutoImplData<PredicateObligation<'tcx>>
2488 debug!("vtable_auto_impl: nested={:?}", nested);
2490 let cause = obligation.derived_cause(BuiltinDerivedObligation);
2491 let mut obligations = self.collect_predicates_for_types(
2492 obligation.param_env,
2494 obligation.recursion_depth+1,
2498 let trait_obligations = self.in_snapshot(|this, snapshot| {
2499 let poly_trait_ref = obligation.predicate.to_poly_trait_ref();
2500 let (trait_ref, skol_map) =
2501 this.infcx().skolemize_late_bound_regions(&poly_trait_ref, snapshot);
2502 let cause = obligation.derived_cause(ImplDerivedObligation);
2503 this.impl_or_trait_obligations(cause,
2504 obligation.recursion_depth + 1,
2505 obligation.param_env,
2512 obligations.extend(trait_obligations);
2514 debug!("vtable_auto_impl: obligations={:?}", obligations);
2516 VtableAutoImplData {
2522 fn confirm_impl_candidate(&mut self,
2523 obligation: &TraitObligation<'tcx>,
2525 -> VtableImplData<'tcx, PredicateObligation<'tcx>>
2527 debug!("confirm_impl_candidate({:?},{:?})",
2531 // First, create the substitutions by matching the impl again,
2532 // this time not in a probe.
2533 self.in_snapshot(|this, snapshot| {
2534 let (substs, skol_map) =
2535 this.rematch_impl(impl_def_id, obligation,
2537 debug!("confirm_impl_candidate substs={:?}", substs);
2538 let cause = obligation.derived_cause(ImplDerivedObligation);
2539 this.vtable_impl(impl_def_id,
2542 obligation.recursion_depth + 1,
2543 obligation.param_env,
2549 fn vtable_impl(&mut self,
2551 mut substs: Normalized<'tcx, &'tcx Substs<'tcx>>,
2552 cause: ObligationCause<'tcx>,
2553 recursion_depth: usize,
2554 param_env: ty::ParamEnv<'tcx>,
2555 skol_map: infer::SkolemizationMap<'tcx>,
2556 snapshot: &infer::CombinedSnapshot)
2557 -> VtableImplData<'tcx, PredicateObligation<'tcx>>
2559 debug!("vtable_impl(impl_def_id={:?}, substs={:?}, recursion_depth={}, skol_map={:?})",
2565 let mut impl_obligations =
2566 self.impl_or_trait_obligations(cause,
2574 debug!("vtable_impl: impl_def_id={:?} impl_obligations={:?}",
2578 // Because of RFC447, the impl-trait-ref and obligations
2579 // are sufficient to determine the impl substs, without
2580 // relying on projections in the impl-trait-ref.
2582 // e.g. `impl<U: Tr, V: Iterator<Item=U>> Foo<<U as Tr>::T> for V`
2583 impl_obligations.append(&mut substs.obligations);
2585 VtableImplData { impl_def_id,
2586 substs: substs.value,
2587 nested: impl_obligations }
2590 fn confirm_object_candidate(&mut self,
2591 obligation: &TraitObligation<'tcx>)
2592 -> VtableObjectData<'tcx, PredicateObligation<'tcx>>
2594 debug!("confirm_object_candidate({:?})",
2597 // FIXME skipping binder here seems wrong -- we should
2598 // probably flatten the binder from the obligation and the
2599 // binder from the object. Have to try to make a broken test
2600 // case that results. -nmatsakis
2601 let self_ty = self.infcx.shallow_resolve(*obligation.self_ty().skip_binder());
2602 let poly_trait_ref = match self_ty.sty {
2603 ty::TyDynamic(ref data, ..) => {
2604 data.principal().unwrap().with_self_ty(self.tcx(), self_ty)
2607 span_bug!(obligation.cause.span,
2608 "object candidate with non-object");
2612 let mut upcast_trait_ref = None;
2616 let tcx = self.tcx();
2618 // We want to find the first supertrait in the list of
2619 // supertraits that we can unify with, and do that
2620 // unification. We know that there is exactly one in the list
2621 // where we can unify because otherwise select would have
2622 // reported an ambiguity. (When we do find a match, also
2623 // record it for later.)
2625 util::supertraits(tcx, poly_trait_ref)
2629 |this, _| this.match_poly_trait_ref(obligation, t))
2631 Ok(_) => { upcast_trait_ref = Some(t); false }
2636 // Additionally, for each of the nonmatching predicates that
2637 // we pass over, we sum up the set of number of vtable
2638 // entries, so that we can compute the offset for the selected
2641 nonmatching.map(|t| tcx.count_own_vtable_entries(t))
2647 upcast_trait_ref: upcast_trait_ref.unwrap(),
2653 fn confirm_fn_pointer_candidate(&mut self, obligation: &TraitObligation<'tcx>)
2654 -> Result<VtableFnPointerData<'tcx, PredicateObligation<'tcx>>, SelectionError<'tcx>>
2656 debug!("confirm_fn_pointer_candidate({:?})",
2659 // ok to skip binder; it is reintroduced below
2660 let self_ty = self.infcx.shallow_resolve(*obligation.self_ty().skip_binder());
2661 let sig = self_ty.fn_sig(self.tcx());
2663 self.tcx().closure_trait_ref_and_return_type(obligation.predicate.def_id(),
2666 util::TupleArgumentsFlag::Yes)
2667 .map_bound(|(trait_ref, _)| trait_ref);
2669 let Normalized { value: trait_ref, obligations } =
2670 project::normalize_with_depth(self,
2671 obligation.param_env,
2672 obligation.cause.clone(),
2673 obligation.recursion_depth + 1,
2676 self.confirm_poly_trait_refs(obligation.cause.clone(),
2677 obligation.param_env,
2678 obligation.predicate.to_poly_trait_ref(),
2680 Ok(VtableFnPointerData { fn_ty: self_ty, nested: obligations })
2683 fn confirm_generator_candidate(&mut self,
2684 obligation: &TraitObligation<'tcx>)
2685 -> Result<VtableGeneratorData<'tcx, PredicateObligation<'tcx>>,
2686 SelectionError<'tcx>>
2688 // ok to skip binder because the substs on generator types never
2689 // touch bound regions, they just capture the in-scope
2690 // type/region parameters
2691 let self_ty = self.infcx.shallow_resolve(obligation.self_ty().skip_binder());
2692 let (closure_def_id, substs) = match self_ty.sty {
2693 ty::TyGenerator(id, substs, _) => (id, substs),
2694 _ => bug!("closure candidate for non-closure {:?}", obligation)
2697 debug!("confirm_generator_candidate({:?},{:?},{:?})",
2703 self.generator_trait_ref_unnormalized(obligation, closure_def_id, substs);
2707 } = normalize_with_depth(self,
2708 obligation.param_env,
2709 obligation.cause.clone(),
2710 obligation.recursion_depth+1,
2713 debug!("confirm_generator_candidate(closure_def_id={:?}, trait_ref={:?}, obligations={:?})",
2718 self.confirm_poly_trait_refs(obligation.cause.clone(),
2719 obligation.param_env,
2720 obligation.predicate.to_poly_trait_ref(),
2723 Ok(VtableGeneratorData {
2724 closure_def_id: closure_def_id,
2725 substs: substs.clone(),
2730 fn confirm_closure_candidate(&mut self,
2731 obligation: &TraitObligation<'tcx>)
2732 -> Result<VtableClosureData<'tcx, PredicateObligation<'tcx>>,
2733 SelectionError<'tcx>>
2735 debug!("confirm_closure_candidate({:?})", obligation);
2737 let kind = match self.tcx().lang_items().fn_trait_kind(obligation.predicate.0.def_id()) {
2739 None => bug!("closure candidate for non-fn trait {:?}", obligation)
2742 // ok to skip binder because the substs on closure types never
2743 // touch bound regions, they just capture the in-scope
2744 // type/region parameters
2745 let self_ty = self.infcx.shallow_resolve(obligation.self_ty().skip_binder());
2746 let (closure_def_id, substs) = match self_ty.sty {
2747 ty::TyClosure(id, substs) => (id, substs),
2748 _ => bug!("closure candidate for non-closure {:?}", obligation)
2752 self.closure_trait_ref_unnormalized(obligation, closure_def_id, substs);
2756 } = normalize_with_depth(self,
2757 obligation.param_env,
2758 obligation.cause.clone(),
2759 obligation.recursion_depth+1,
2762 debug!("confirm_closure_candidate(closure_def_id={:?}, trait_ref={:?}, obligations={:?})",
2767 self.confirm_poly_trait_refs(obligation.cause.clone(),
2768 obligation.param_env,
2769 obligation.predicate.to_poly_trait_ref(),
2772 obligations.push(Obligation::new(
2773 obligation.cause.clone(),
2774 obligation.param_env,
2775 ty::Predicate::ClosureKind(closure_def_id, substs, kind)));
2777 Ok(VtableClosureData {
2779 substs: substs.clone(),
2784 /// In the case of closure types and fn pointers,
2785 /// we currently treat the input type parameters on the trait as
2786 /// outputs. This means that when we have a match we have only
2787 /// considered the self type, so we have to go back and make sure
2788 /// to relate the argument types too. This is kind of wrong, but
2789 /// since we control the full set of impls, also not that wrong,
2790 /// and it DOES yield better error messages (since we don't report
2791 /// errors as if there is no applicable impl, but rather report
2792 /// errors are about mismatched argument types.
2794 /// Here is an example. Imagine we have a closure expression
2795 /// and we desugared it so that the type of the expression is
2796 /// `Closure`, and `Closure` expects an int as argument. Then it
2797 /// is "as if" the compiler generated this impl:
2799 /// impl Fn(int) for Closure { ... }
2801 /// Now imagine our obligation is `Fn(usize) for Closure`. So far
2802 /// we have matched the self-type `Closure`. At this point we'll
2803 /// compare the `int` to `usize` and generate an error.
2805 /// Note that this checking occurs *after* the impl has selected,
2806 /// because these output type parameters should not affect the
2807 /// selection of the impl. Therefore, if there is a mismatch, we
2808 /// report an error to the user.
2809 fn confirm_poly_trait_refs(&mut self,
2810 obligation_cause: ObligationCause<'tcx>,
2811 obligation_param_env: ty::ParamEnv<'tcx>,
2812 obligation_trait_ref: ty::PolyTraitRef<'tcx>,
2813 expected_trait_ref: ty::PolyTraitRef<'tcx>)
2814 -> Result<(), SelectionError<'tcx>>
2816 let obligation_trait_ref = obligation_trait_ref.clone();
2818 .at(&obligation_cause, obligation_param_env)
2819 .sup(obligation_trait_ref, expected_trait_ref)
2820 .map(|InferOk { obligations, .. }| self.inferred_obligations.extend(obligations))
2821 .map_err(|e| OutputTypeParameterMismatch(expected_trait_ref, obligation_trait_ref, e))
2824 fn confirm_builtin_unsize_candidate(&mut self,
2825 obligation: &TraitObligation<'tcx>,)
2826 -> Result<VtableBuiltinData<PredicateObligation<'tcx>>, SelectionError<'tcx>>
2828 let tcx = self.tcx();
2830 // assemble_candidates_for_unsizing should ensure there are no late bound
2831 // regions here. See the comment there for more details.
2832 let source = self.infcx.shallow_resolve(
2833 obligation.self_ty().no_late_bound_regions().unwrap());
2834 let target = obligation.predicate.skip_binder().trait_ref.substs.type_at(1);
2835 let target = self.infcx.shallow_resolve(target);
2837 debug!("confirm_builtin_unsize_candidate(source={:?}, target={:?})",
2840 let mut nested = vec![];
2841 match (&source.sty, &target.sty) {
2842 // Trait+Kx+'a -> Trait+Ky+'b (upcasts).
2843 (&ty::TyDynamic(ref data_a, r_a), &ty::TyDynamic(ref data_b, r_b)) => {
2844 // See assemble_candidates_for_unsizing for more info.
2845 // Binders reintroduced below in call to mk_existential_predicates.
2846 let principal = data_a.skip_binder().principal();
2847 let iter = principal.into_iter().map(ty::ExistentialPredicate::Trait)
2848 .chain(data_a.skip_binder().projection_bounds()
2849 .map(|x| ty::ExistentialPredicate::Projection(x)))
2850 .chain(data_b.auto_traits().map(ty::ExistentialPredicate::AutoTrait));
2851 let new_trait = tcx.mk_dynamic(
2852 ty::Binder(tcx.mk_existential_predicates(iter)), r_b);
2853 let InferOk { obligations, .. } =
2854 self.infcx.at(&obligation.cause, obligation.param_env)
2855 .eq(target, new_trait)
2856 .map_err(|_| Unimplemented)?;
2857 self.inferred_obligations.extend(obligations);
2859 // Register one obligation for 'a: 'b.
2860 let cause = ObligationCause::new(obligation.cause.span,
2861 obligation.cause.body_id,
2862 ObjectCastObligation(target));
2863 let outlives = ty::OutlivesPredicate(r_a, r_b);
2864 nested.push(Obligation::with_depth(cause,
2865 obligation.recursion_depth + 1,
2866 obligation.param_env,
2867 ty::Binder(outlives).to_predicate()));
2871 (_, &ty::TyDynamic(ref data, r)) => {
2872 let mut object_dids =
2873 data.auto_traits().chain(data.principal().map(|p| p.def_id()));
2874 if let Some(did) = object_dids.find(|did| {
2875 !tcx.is_object_safe(*did)
2877 return Err(TraitNotObjectSafe(did))
2880 let cause = ObligationCause::new(obligation.cause.span,
2881 obligation.cause.body_id,
2882 ObjectCastObligation(target));
2883 let mut push = |predicate| {
2884 nested.push(Obligation::with_depth(cause.clone(),
2885 obligation.recursion_depth + 1,
2886 obligation.param_env,
2890 // Create obligations:
2891 // - Casting T to Trait
2892 // - For all the various builtin bounds attached to the object cast. (In other
2893 // words, if the object type is Foo+Send, this would create an obligation for the
2895 // - Projection predicates
2896 for predicate in data.iter() {
2897 push(predicate.with_self_ty(tcx, source));
2900 // We can only make objects from sized types.
2901 let tr = ty::TraitRef {
2902 def_id: tcx.require_lang_item(lang_items::SizedTraitLangItem),
2903 substs: tcx.mk_substs_trait(source, &[]),
2905 push(tr.to_predicate());
2907 // If the type is `Foo+'a`, ensures that the type
2908 // being cast to `Foo+'a` outlives `'a`:
2909 let outlives = ty::OutlivesPredicate(source, r);
2910 push(ty::Binder(outlives).to_predicate());
2914 (&ty::TyArray(a, _), &ty::TySlice(b)) => {
2915 let InferOk { obligations, .. } =
2916 self.infcx.at(&obligation.cause, obligation.param_env)
2918 .map_err(|_| Unimplemented)?;
2919 self.inferred_obligations.extend(obligations);
2922 // Struct<T> -> Struct<U>.
2923 (&ty::TyAdt(def, substs_a), &ty::TyAdt(_, substs_b)) => {
2926 .map(|f| tcx.type_of(f.did))
2927 .collect::<Vec<_>>();
2929 // The last field of the structure has to exist and contain type parameters.
2930 let field = if let Some(&field) = fields.last() {
2933 return Err(Unimplemented);
2935 let mut ty_params = BitVector::new(substs_a.types().count());
2936 let mut found = false;
2937 for ty in field.walk() {
2938 if let ty::TyParam(p) = ty.sty {
2939 ty_params.insert(p.idx as usize);
2944 return Err(Unimplemented);
2947 // Replace type parameters used in unsizing with
2948 // TyError and ensure they do not affect any other fields.
2949 // This could be checked after type collection for any struct
2950 // with a potentially unsized trailing field.
2951 let params = substs_a.iter().enumerate().map(|(i, &k)| {
2952 if ty_params.contains(i) {
2953 Kind::from(tcx.types.err)
2958 let substs = tcx.mk_substs(params);
2959 for &ty in fields.split_last().unwrap().1 {
2960 if ty.subst(tcx, substs).references_error() {
2961 return Err(Unimplemented);
2965 // Extract Field<T> and Field<U> from Struct<T> and Struct<U>.
2966 let inner_source = field.subst(tcx, substs_a);
2967 let inner_target = field.subst(tcx, substs_b);
2969 // Check that the source struct with the target's
2970 // unsized parameters is equal to the target.
2971 let params = substs_a.iter().enumerate().map(|(i, &k)| {
2972 if ty_params.contains(i) {
2973 Kind::from(substs_b.type_at(i))
2978 let new_struct = tcx.mk_adt(def, tcx.mk_substs(params));
2979 let InferOk { obligations, .. } =
2980 self.infcx.at(&obligation.cause, obligation.param_env)
2981 .eq(target, new_struct)
2982 .map_err(|_| Unimplemented)?;
2983 self.inferred_obligations.extend(obligations);
2985 // Construct the nested Field<T>: Unsize<Field<U>> predicate.
2986 nested.push(tcx.predicate_for_trait_def(
2987 obligation.param_env,
2988 obligation.cause.clone(),
2989 obligation.predicate.def_id(),
2990 obligation.recursion_depth + 1,
2995 // (.., T) -> (.., U).
2996 (&ty::TyTuple(tys_a, _), &ty::TyTuple(tys_b, _)) => {
2997 assert_eq!(tys_a.len(), tys_b.len());
2999 // The last field of the tuple has to exist.
3000 let (a_last, a_mid) = if let Some(x) = tys_a.split_last() {
3003 return Err(Unimplemented);
3005 let b_last = tys_b.last().unwrap();
3007 // Check that the source tuple with the target's
3008 // last element is equal to the target.
3009 let new_tuple = tcx.mk_tup(a_mid.iter().chain(Some(b_last)), false);
3010 let InferOk { obligations, .. } =
3011 self.infcx.at(&obligation.cause, obligation.param_env)
3012 .eq(target, new_tuple)
3013 .map_err(|_| Unimplemented)?;
3014 self.inferred_obligations.extend(obligations);
3016 // Construct the nested T: Unsize<U> predicate.
3017 nested.push(tcx.predicate_for_trait_def(
3018 obligation.param_env,
3019 obligation.cause.clone(),
3020 obligation.predicate.def_id(),
3021 obligation.recursion_depth + 1,
3029 Ok(VtableBuiltinData { nested: nested })
3032 ///////////////////////////////////////////////////////////////////////////
3035 // Matching is a common path used for both evaluation and
3036 // confirmation. It basically unifies types that appear in impls
3037 // and traits. This does affect the surrounding environment;
3038 // therefore, when used during evaluation, match routines must be
3039 // run inside of a `probe()` so that their side-effects are
3042 fn rematch_impl(&mut self,
3044 obligation: &TraitObligation<'tcx>,
3045 snapshot: &infer::CombinedSnapshot)
3046 -> (Normalized<'tcx, &'tcx Substs<'tcx>>,
3047 infer::SkolemizationMap<'tcx>)
3049 match self.match_impl(impl_def_id, obligation, snapshot) {
3050 Ok((substs, skol_map)) => (substs, skol_map),
3052 bug!("Impl {:?} was matchable against {:?} but now is not",
3059 fn match_impl(&mut self,
3061 obligation: &TraitObligation<'tcx>,
3062 snapshot: &infer::CombinedSnapshot)
3063 -> Result<(Normalized<'tcx, &'tcx Substs<'tcx>>,
3064 infer::SkolemizationMap<'tcx>), ()>
3066 let impl_trait_ref = self.tcx().impl_trait_ref(impl_def_id).unwrap();
3068 // Before we create the substitutions and everything, first
3069 // consider a "quick reject". This avoids creating more types
3070 // and so forth that we need to.
3071 if self.fast_reject_trait_refs(obligation, &impl_trait_ref) {
3075 let (skol_obligation, skol_map) = self.infcx().skolemize_late_bound_regions(
3076 &obligation.predicate,
3078 let skol_obligation_trait_ref = skol_obligation.trait_ref;
3080 let impl_substs = self.infcx.fresh_substs_for_item(obligation.cause.span,
3083 let impl_trait_ref = impl_trait_ref.subst(self.tcx(),
3086 let impl_trait_ref =
3087 project::normalize_with_depth(self,
3088 obligation.param_env,
3089 obligation.cause.clone(),
3090 obligation.recursion_depth + 1,
3093 debug!("match_impl(impl_def_id={:?}, obligation={:?}, \
3094 impl_trait_ref={:?}, skol_obligation_trait_ref={:?})",
3098 skol_obligation_trait_ref);
3100 let InferOk { obligations, .. } =
3101 self.infcx.at(&obligation.cause, obligation.param_env)
3102 .eq(skol_obligation_trait_ref, impl_trait_ref.value)
3104 debug!("match_impl: failed eq_trait_refs due to `{}`", e);
3107 self.inferred_obligations.extend(obligations);
3109 if let Err(e) = self.infcx.leak_check(false,
3110 obligation.cause.span,
3113 debug!("match_impl: failed leak check due to `{}`", e);
3117 debug!("match_impl: success impl_substs={:?}", impl_substs);
3120 obligations: impl_trait_ref.obligations
3124 fn fast_reject_trait_refs(&mut self,
3125 obligation: &TraitObligation,
3126 impl_trait_ref: &ty::TraitRef)
3129 // We can avoid creating type variables and doing the full
3130 // substitution if we find that any of the input types, when
3131 // simplified, do not match.
3133 obligation.predicate.skip_binder().input_types()
3134 .zip(impl_trait_ref.input_types())
3135 .any(|(obligation_ty, impl_ty)| {
3136 let simplified_obligation_ty =
3137 fast_reject::simplify_type(self.tcx(), obligation_ty, true);
3138 let simplified_impl_ty =
3139 fast_reject::simplify_type(self.tcx(), impl_ty, false);
3141 simplified_obligation_ty.is_some() &&
3142 simplified_impl_ty.is_some() &&
3143 simplified_obligation_ty != simplified_impl_ty
3147 /// Normalize `where_clause_trait_ref` and try to match it against
3148 /// `obligation`. If successful, return any predicates that
3149 /// result from the normalization. Normalization is necessary
3150 /// because where-clauses are stored in the parameter environment
3152 fn match_where_clause_trait_ref(&mut self,
3153 obligation: &TraitObligation<'tcx>,
3154 where_clause_trait_ref: ty::PolyTraitRef<'tcx>)
3155 -> Result<Vec<PredicateObligation<'tcx>>,()>
3157 self.match_poly_trait_ref(obligation, where_clause_trait_ref)?;
3161 /// Returns `Ok` if `poly_trait_ref` being true implies that the
3162 /// obligation is satisfied.
3163 fn match_poly_trait_ref(&mut self,
3164 obligation: &TraitObligation<'tcx>,
3165 poly_trait_ref: ty::PolyTraitRef<'tcx>)
3168 debug!("match_poly_trait_ref: obligation={:?} poly_trait_ref={:?}",
3172 self.infcx.at(&obligation.cause, obligation.param_env)
3173 .sup(obligation.predicate.to_poly_trait_ref(), poly_trait_ref)
3174 .map(|InferOk { obligations, .. }| self.inferred_obligations.extend(obligations))
3178 ///////////////////////////////////////////////////////////////////////////
3181 fn match_fresh_trait_refs(&self,
3182 previous: &ty::PolyTraitRef<'tcx>,
3183 current: &ty::PolyTraitRef<'tcx>)
3186 let mut matcher = ty::_match::Match::new(self.tcx());
3187 matcher.relate(previous, current).is_ok()
3190 fn push_stack<'o,'s:'o>(&mut self,
3191 previous_stack: TraitObligationStackList<'s, 'tcx>,
3192 obligation: &'o TraitObligation<'tcx>)
3193 -> TraitObligationStack<'o, 'tcx>
3195 let fresh_trait_ref =
3196 obligation.predicate.to_poly_trait_ref().fold_with(&mut self.freshener);
3198 TraitObligationStack {
3201 previous: previous_stack,
3205 fn closure_trait_ref_unnormalized(&mut self,
3206 obligation: &TraitObligation<'tcx>,
3207 closure_def_id: DefId,
3208 substs: ty::ClosureSubsts<'tcx>)
3209 -> ty::PolyTraitRef<'tcx>
3211 let closure_type = self.infcx.closure_sig(closure_def_id, substs);
3212 let ty::Binder((trait_ref, _)) =
3213 self.tcx().closure_trait_ref_and_return_type(obligation.predicate.def_id(),
3214 obligation.predicate.0.self_ty(), // (1)
3216 util::TupleArgumentsFlag::No);
3217 // (1) Feels icky to skip the binder here, but OTOH we know
3218 // that the self-type is an unboxed closure type and hence is
3219 // in fact unparameterized (or at least does not reference any
3220 // regions bound in the obligation). Still probably some
3221 // refactoring could make this nicer.
3223 ty::Binder(trait_ref)
3226 fn generator_trait_ref_unnormalized(&mut self,
3227 obligation: &TraitObligation<'tcx>,
3228 closure_def_id: DefId,
3229 substs: ty::ClosureSubsts<'tcx>)
3230 -> ty::PolyTraitRef<'tcx>
3232 let gen_sig = substs.generator_poly_sig(closure_def_id, self.tcx());
3233 let ty::Binder((trait_ref, ..)) =
3234 self.tcx().generator_trait_ref_and_outputs(obligation.predicate.def_id(),
3235 obligation.predicate.0.self_ty(), // (1)
3237 // (1) Feels icky to skip the binder here, but OTOH we know
3238 // that the self-type is an generator type and hence is
3239 // in fact unparameterized (or at least does not reference any
3240 // regions bound in the obligation). Still probably some
3241 // refactoring could make this nicer.
3243 ty::Binder(trait_ref)
3246 /// Returns the obligations that are implied by instantiating an
3247 /// impl or trait. The obligations are substituted and fully
3248 /// normalized. This is used when confirming an impl or default
3250 fn impl_or_trait_obligations(&mut self,
3251 cause: ObligationCause<'tcx>,
3252 recursion_depth: usize,
3253 param_env: ty::ParamEnv<'tcx>,
3254 def_id: DefId, // of impl or trait
3255 substs: &Substs<'tcx>, // for impl or trait
3256 skol_map: infer::SkolemizationMap<'tcx>,
3257 snapshot: &infer::CombinedSnapshot)
3258 -> Vec<PredicateObligation<'tcx>>
3260 debug!("impl_or_trait_obligations(def_id={:?})", def_id);
3261 let tcx = self.tcx();
3263 // To allow for one-pass evaluation of the nested obligation,
3264 // each predicate must be preceded by the obligations required
3266 // for example, if we have:
3267 // impl<U: Iterator, V: Iterator<Item=U>> Foo for V where U::Item: Copy
3268 // the impl will have the following predicates:
3269 // <V as Iterator>::Item = U,
3270 // U: Iterator, U: Sized,
3271 // V: Iterator, V: Sized,
3272 // <U as Iterator>::Item: Copy
3273 // When we substitute, say, `V => IntoIter<u32>, U => $0`, the last
3274 // obligation will normalize to `<$0 as Iterator>::Item = $1` and
3275 // `$1: Copy`, so we must ensure the obligations are emitted in
3277 let predicates = tcx.predicates_of(def_id);
3278 assert_eq!(predicates.parent, None);
3279 let predicates = predicates.predicates.iter().flat_map(|predicate| {
3280 let predicate = normalize_with_depth(self, param_env, cause.clone(), recursion_depth,
3281 &predicate.subst(tcx, substs));
3282 predicate.obligations.into_iter().chain(
3284 cause: cause.clone(),
3287 predicate: predicate.value
3290 self.infcx().plug_leaks(skol_map, snapshot, predicates)
3294 impl<'tcx> TraitObligation<'tcx> {
3295 #[allow(unused_comparisons)]
3296 pub fn derived_cause(&self,
3297 variant: fn(DerivedObligationCause<'tcx>) -> ObligationCauseCode<'tcx>)
3298 -> ObligationCause<'tcx>
3301 * Creates a cause for obligations that are derived from
3302 * `obligation` by a recursive search (e.g., for a builtin
3303 * bound, or eventually a `auto trait Foo`). If `obligation`
3304 * is itself a derived obligation, this is just a clone, but
3305 * otherwise we create a "derived obligation" cause so as to
3306 * keep track of the original root obligation for error
3310 let obligation = self;
3312 // NOTE(flaper87): As of now, it keeps track of the whole error
3313 // chain. Ideally, we should have a way to configure this either
3314 // by using -Z verbose or just a CLI argument.
3315 if obligation.recursion_depth >= 0 {
3316 let derived_cause = DerivedObligationCause {
3317 parent_trait_ref: obligation.predicate.to_poly_trait_ref(),
3318 parent_code: Rc::new(obligation.cause.code.clone())
3320 let derived_code = variant(derived_cause);
3321 ObligationCause::new(obligation.cause.span, obligation.cause.body_id, derived_code)
3323 obligation.cause.clone()
3328 impl<'tcx> SelectionCache<'tcx> {
3329 pub fn new() -> SelectionCache<'tcx> {
3331 hashmap: RefCell::new(FxHashMap())
3336 impl<'tcx> EvaluationCache<'tcx> {
3337 pub fn new() -> EvaluationCache<'tcx> {
3339 hashmap: RefCell::new(FxHashMap())
3344 impl<'o,'tcx> TraitObligationStack<'o,'tcx> {
3345 fn list(&'o self) -> TraitObligationStackList<'o,'tcx> {
3346 TraitObligationStackList::with(self)
3349 fn iter(&'o self) -> TraitObligationStackList<'o,'tcx> {
3354 #[derive(Copy, Clone)]
3355 struct TraitObligationStackList<'o,'tcx:'o> {
3356 head: Option<&'o TraitObligationStack<'o,'tcx>>
3359 impl<'o,'tcx> TraitObligationStackList<'o,'tcx> {
3360 fn empty() -> TraitObligationStackList<'o,'tcx> {
3361 TraitObligationStackList { head: None }
3364 fn with(r: &'o TraitObligationStack<'o,'tcx>) -> TraitObligationStackList<'o,'tcx> {
3365 TraitObligationStackList { head: Some(r) }
3369 impl<'o,'tcx> Iterator for TraitObligationStackList<'o,'tcx>{
3370 type Item = &'o TraitObligationStack<'o,'tcx>;
3372 fn next(&mut self) -> Option<&'o TraitObligationStack<'o,'tcx>> {
3383 impl<'o,'tcx> fmt::Debug for TraitObligationStack<'o,'tcx> {
3384 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
3385 write!(f, "TraitObligationStack({:?})", self.obligation)
3390 pub struct WithDepNode<T> {
3391 dep_node: DepNodeIndex,
3395 impl<T: Clone> WithDepNode<T> {
3396 pub fn new(dep_node: DepNodeIndex, cached_value: T) -> Self {
3397 WithDepNode { dep_node, cached_value }
3400 pub fn get(&self, tcx: TyCtxt) -> T {
3401 tcx.dep_graph.read_index(self.dep_node);
3402 self.cached_value.clone()