1 //! Candidate assembly.
3 //! The selection process begins by examining all in-scope impls,
4 //! caller obligations, and so forth and assembling a list of
5 //! candidates. See the [rustc dev guide] for more details.
7 //! [rustc dev guide]:https://rustc-dev-guide.rust-lang.org/traits/resolution.html#candidate-assembly
9 use rustc_infer::traits::{Obligation, SelectionError, TraitObligation};
10 use rustc_middle::ty::print::with_no_trimmed_paths;
11 use rustc_middle::ty::{self, TypeFoldable};
12 use rustc_target::spec::abi::Abi;
14 use crate::traits::coherence::Conflict;
15 use crate::traits::{util, SelectionResult};
16 use crate::traits::{Overflow, Unimplemented};
18 use super::BuiltinImplConditions;
19 use super::IntercrateAmbiguityCause;
20 use super::OverflowError;
21 use super::SelectionCandidate::{self, *};
22 use super::{EvaluatedCandidate, SelectionCandidateSet, SelectionContext, TraitObligationStack};
24 impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
25 #[instrument(level = "debug", skip(self))]
26 pub(super) fn candidate_from_obligation<'o>(
28 stack: &TraitObligationStack<'o, 'tcx>,
29 ) -> SelectionResult<'tcx, SelectionCandidate<'tcx>> {
30 // Watch out for overflow. This intentionally bypasses (and does
31 // not update) the cache.
32 self.check_recursion_limit(&stack.obligation, &stack.obligation)?;
34 // Check the cache. Note that we freshen the trait-ref
35 // separately rather than using `stack.fresh_trait_ref` --
36 // this is because we want the unbound variables to be
37 // replaced with fresh types starting from index 0.
38 let cache_fresh_trait_pred = self.infcx.freshen(stack.obligation.predicate);
39 debug!(?cache_fresh_trait_pred);
40 debug_assert!(!stack.obligation.predicate.has_escaping_bound_vars());
43 self.check_candidate_cache(stack.obligation.param_env, cache_fresh_trait_pred)
45 debug!(candidate = ?c, "CACHE HIT");
49 // If no match, compute result and insert into cache.
51 // FIXME(nikomatsakis) -- this cache is not taking into
52 // account cycles that may have occurred in forming the
53 // candidate. I don't know of any specific problems that
54 // result but it seems awfully suspicious.
55 let (candidate, dep_node) =
56 self.in_task(|this| this.candidate_from_obligation_no_cache(stack));
58 debug!(?candidate, "CACHE MISS");
59 self.insert_candidate_cache(
60 stack.obligation.param_env,
61 cache_fresh_trait_pred,
68 fn candidate_from_obligation_no_cache<'o>(
70 stack: &TraitObligationStack<'o, 'tcx>,
71 ) -> SelectionResult<'tcx, SelectionCandidate<'tcx>> {
72 if let Some(conflict) = self.is_knowable(stack) {
73 debug!("coherence stage: not knowable");
74 if self.intercrate_ambiguity_causes.is_some() {
75 debug!("evaluate_stack: intercrate_ambiguity_causes is some");
76 // Heuristics: show the diagnostics when there are no candidates in crate.
77 if let Ok(candidate_set) = self.assemble_candidates(stack) {
78 let mut no_candidates_apply = true;
80 for c in candidate_set.vec.iter() {
81 if self.evaluate_candidate(stack, &c)?.may_apply() {
82 no_candidates_apply = false;
87 if !candidate_set.ambiguous && no_candidates_apply {
88 let trait_ref = stack.obligation.predicate.skip_binder().trait_ref;
89 let self_ty = trait_ref.self_ty();
90 let (trait_desc, self_desc) = with_no_trimmed_paths(|| {
91 let trait_desc = trait_ref.print_only_trait_path().to_string();
92 let self_desc = if self_ty.has_concrete_skeleton() {
93 Some(self_ty.to_string())
97 (trait_desc, self_desc)
99 let cause = if let Conflict::Upstream = conflict {
100 IntercrateAmbiguityCause::UpstreamCrateUpdate { trait_desc, self_desc }
102 IntercrateAmbiguityCause::DownstreamCrate { trait_desc, self_desc }
104 debug!(?cause, "evaluate_stack: pushing cause");
105 self.intercrate_ambiguity_causes.as_mut().unwrap().push(cause);
112 let candidate_set = self.assemble_candidates(stack)?;
114 if candidate_set.ambiguous {
115 debug!("candidate set contains ambig");
119 let mut candidates = candidate_set.vec;
121 debug!(?stack, ?candidates, "assembled {} candidates", candidates.len());
123 // At this point, we know that each of the entries in the
124 // candidate set is *individually* applicable. Now we have to
125 // figure out if they contain mutual incompatibilities. This
126 // frequently arises if we have an unconstrained input type --
127 // for example, we are looking for `$0: Eq` where `$0` is some
128 // unconstrained type variable. In that case, we'll get a
129 // candidate which assumes $0 == int, one that assumes `$0 ==
130 // usize`, etc. This spells an ambiguity.
132 // If there is more than one candidate, first winnow them down
133 // by considering extra conditions (nested obligations and so
134 // forth). We don't winnow if there is exactly one
135 // candidate. This is a relatively minor distinction but it
136 // can lead to better inference and error-reporting. An
137 // example would be if there was an impl:
139 // impl<T:Clone> Vec<T> { fn push_clone(...) { ... } }
141 // and we were to see some code `foo.push_clone()` where `boo`
142 // is a `Vec<Bar>` and `Bar` does not implement `Clone`. If
143 // we were to winnow, we'd wind up with zero candidates.
144 // Instead, we select the right impl now but report "`Bar` does
145 // not implement `Clone`".
146 if candidates.len() == 1 {
147 return self.filter_negative_and_reservation_impls(candidates.pop().unwrap());
150 // Winnow, but record the exact outcome of evaluation, which
151 // is needed for specialization. Propagate overflow if it occurs.
152 let mut candidates = candidates
154 .map(|c| match self.evaluate_candidate(stack, &c) {
155 Ok(eval) if eval.may_apply() => {
156 Ok(Some(EvaluatedCandidate { candidate: c, evaluation: eval }))
159 Err(OverflowError) => Err(Overflow),
161 .flat_map(Result::transpose)
162 .collect::<Result<Vec<_>, _>>()?;
164 debug!(?stack, ?candidates, "winnowed to {} candidates", candidates.len());
166 let needs_infer = stack.obligation.predicate.has_infer_types_or_consts();
168 // If there are STILL multiple candidates, we can further
169 // reduce the list by dropping duplicates -- including
170 // resolving specializations.
171 if candidates.len() > 1 {
173 while i < candidates.len() {
174 let is_dup = (0..candidates.len()).filter(|&j| i != j).any(|j| {
175 self.candidate_should_be_dropped_in_favor_of(
182 debug!(candidate = ?candidates[i], "Dropping candidate #{}/{}", i, candidates.len());
183 candidates.swap_remove(i);
185 debug!(candidate = ?candidates[i], "Retaining candidate #{}/{}", i, candidates.len());
188 // If there are *STILL* multiple candidates, give up
189 // and report ambiguity.
191 debug!("multiple matches, ambig");
198 // If there are *NO* candidates, then there are no impls --
199 // that we know of, anyway. Note that in the case where there
200 // are unbound type variables within the obligation, it might
201 // be the case that you could still satisfy the obligation
202 // from another crate by instantiating the type variables with
203 // a type from another crate that does have an impl. This case
204 // is checked for in `evaluate_stack` (and hence users
205 // who might care about this case, like coherence, should use
207 if candidates.is_empty() {
208 // If there's an error type, 'downgrade' our result from
209 // `Err(Unimplemented)` to `Ok(None)`. This helps us avoid
210 // emitting additional spurious errors, since we're guaranteed
211 // to have emitted at least one.
212 if stack.obligation.references_error() {
213 debug!("no results for error type, treating as ambiguous");
216 return Err(Unimplemented);
219 // Just one candidate left.
220 self.filter_negative_and_reservation_impls(candidates.pop().unwrap().candidate)
223 pub(super) fn assemble_candidates<'o>(
225 stack: &TraitObligationStack<'o, 'tcx>,
226 ) -> Result<SelectionCandidateSet<'tcx>, SelectionError<'tcx>> {
227 let TraitObligationStack { obligation, .. } = *stack;
228 let obligation = &Obligation {
229 param_env: obligation.param_env,
230 cause: obligation.cause.clone(),
231 recursion_depth: obligation.recursion_depth,
232 predicate: self.infcx().resolve_vars_if_possible(&obligation.predicate),
235 if obligation.predicate.skip_binder().self_ty().is_ty_var() {
236 // Self is a type variable (e.g., `_: AsRef<str>`).
238 // This is somewhat problematic, as the current scheme can't really
239 // handle it turning to be a projection. This does end up as truly
240 // ambiguous in most cases anyway.
242 // Take the fast path out - this also improves
243 // performance by preventing assemble_candidates_from_impls from
244 // matching every impl for this trait.
245 return Ok(SelectionCandidateSet { vec: vec![], ambiguous: true });
248 let mut candidates = SelectionCandidateSet { vec: Vec::new(), ambiguous: false };
250 self.assemble_candidates_for_trait_alias(obligation, &mut candidates)?;
252 // Other bounds. Consider both in-scope bounds from fn decl
253 // and applicable impls. There is a certain set of precedence rules here.
254 let def_id = obligation.predicate.def_id();
255 let lang_items = self.tcx().lang_items();
257 if lang_items.copy_trait() == Some(def_id) {
258 debug!(obligation_self_ty = ?obligation.predicate.skip_binder().self_ty());
260 // User-defined copy impls are permitted, but only for
261 // structs and enums.
262 self.assemble_candidates_from_impls(obligation, &mut candidates)?;
264 // For other types, we'll use the builtin rules.
265 let copy_conditions = self.copy_clone_conditions(obligation);
266 self.assemble_builtin_bound_candidates(copy_conditions, &mut candidates)?;
267 } else if lang_items.discriminant_kind_trait() == Some(def_id) {
268 // `DiscriminantKind` is automatically implemented for every type.
269 candidates.vec.push(DiscriminantKindCandidate);
270 } else if lang_items.sized_trait() == Some(def_id) {
271 // Sized is never implementable by end-users, it is
272 // always automatically computed.
273 let sized_conditions = self.sized_conditions(obligation);
274 self.assemble_builtin_bound_candidates(sized_conditions, &mut candidates)?;
275 } else if lang_items.unsize_trait() == Some(def_id) {
276 self.assemble_candidates_for_unsizing(obligation, &mut candidates);
278 if lang_items.clone_trait() == Some(def_id) {
279 // Same builtin conditions as `Copy`, i.e., every type which has builtin support
280 // for `Copy` also has builtin support for `Clone`, and tuples/arrays of `Clone`
281 // types have builtin support for `Clone`.
282 let clone_conditions = self.copy_clone_conditions(obligation);
283 self.assemble_builtin_bound_candidates(clone_conditions, &mut candidates)?;
286 self.assemble_generator_candidates(obligation, &mut candidates)?;
287 self.assemble_closure_candidates(obligation, &mut candidates)?;
288 self.assemble_fn_pointer_candidates(obligation, &mut candidates)?;
289 self.assemble_candidates_from_impls(obligation, &mut candidates)?;
290 self.assemble_candidates_from_object_ty(obligation, &mut candidates);
293 self.assemble_candidates_from_projected_tys(obligation, &mut candidates);
294 self.assemble_candidates_from_caller_bounds(stack, &mut candidates)?;
295 // Auto implementations have lower priority, so we only
296 // consider triggering a default if there is no other impl that can apply.
297 if candidates.vec.is_empty() {
298 self.assemble_candidates_from_auto_impls(obligation, &mut candidates)?;
300 debug!("candidate list size: {}", candidates.vec.len());
304 fn assemble_candidates_from_projected_tys(
306 obligation: &TraitObligation<'tcx>,
307 candidates: &mut SelectionCandidateSet<'tcx>,
309 debug!(?obligation, "assemble_candidates_from_projected_tys");
311 // Before we go into the whole placeholder thing, just
312 // quickly check if the self-type is a projection at all.
313 match obligation.predicate.skip_binder().trait_ref.self_ty().kind() {
314 ty::Projection(_) | ty::Opaque(..) => {}
315 ty::Infer(ty::TyVar(_)) => {
317 obligation.cause.span,
318 "Self=_ should have been handled by assemble_candidates"
326 .probe(|_| self.match_projection_obligation_against_definition_bounds(obligation));
328 for predicate_index in result {
329 candidates.vec.push(ProjectionCandidate(predicate_index));
333 /// Given an obligation like `<SomeTrait for T>`, searches the obligations that the caller
334 /// supplied to find out whether it is listed among them.
336 /// Never affects the inference environment.
337 fn assemble_candidates_from_caller_bounds<'o>(
339 stack: &TraitObligationStack<'o, 'tcx>,
340 candidates: &mut SelectionCandidateSet<'tcx>,
341 ) -> Result<(), SelectionError<'tcx>> {
342 debug!(?stack.obligation, "assemble_candidates_from_caller_bounds");
344 let all_bounds = stack
349 .filter_map(|o| o.to_opt_poly_trait_ref());
351 // Micro-optimization: filter out predicates relating to different traits.
352 let matching_bounds =
353 all_bounds.filter(|p| p.def_id() == stack.obligation.predicate.def_id());
355 // Keep only those bounds which may apply, and propagate overflow if it occurs.
356 let mut param_candidates = vec![];
357 for bound in matching_bounds {
358 let wc = self.evaluate_where_clause(stack, bound)?;
360 param_candidates.push(ParamCandidate(bound));
364 candidates.vec.extend(param_candidates);
369 fn assemble_generator_candidates(
371 obligation: &TraitObligation<'tcx>,
372 candidates: &mut SelectionCandidateSet<'tcx>,
373 ) -> Result<(), SelectionError<'tcx>> {
374 if self.tcx().lang_items().gen_trait() != Some(obligation.predicate.def_id()) {
378 // Okay to skip binder because the substs on generator types never
379 // touch bound regions, they just capture the in-scope
380 // type/region parameters.
381 let self_ty = obligation.self_ty().skip_binder();
382 match self_ty.kind() {
383 ty::Generator(..) => {
384 debug!(?self_ty, ?obligation, "assemble_generator_candidates",);
386 candidates.vec.push(GeneratorCandidate);
388 ty::Infer(ty::TyVar(_)) => {
389 debug!("assemble_generator_candidates: ambiguous self-type");
390 candidates.ambiguous = true;
398 /// Checks for the artificial impl that the compiler will create for an obligation like `X :
399 /// FnMut<..>` where `X` is a closure type.
401 /// Note: the type parameters on a closure candidate are modeled as *output* type
402 /// parameters and hence do not affect whether this trait is a match or not. They will be
403 /// unified during the confirmation step.
404 fn assemble_closure_candidates(
406 obligation: &TraitObligation<'tcx>,
407 candidates: &mut SelectionCandidateSet<'tcx>,
408 ) -> Result<(), SelectionError<'tcx>> {
409 let kind = match self.tcx().fn_trait_kind_from_lang_item(obligation.predicate.def_id()) {
416 // Okay to skip binder because the substs on closure types never
417 // touch bound regions, they just capture the in-scope
418 // type/region parameters
419 match *obligation.self_ty().skip_binder().kind() {
420 ty::Closure(_, closure_substs) => {
421 debug!(?kind, ?obligation, "assemble_unboxed_candidates");
422 match self.infcx.closure_kind(closure_substs) {
423 Some(closure_kind) => {
424 debug!(?closure_kind, "assemble_unboxed_candidates");
425 if closure_kind.extends(kind) {
426 candidates.vec.push(ClosureCandidate);
430 debug!("assemble_unboxed_candidates: closure_kind not yet known");
431 candidates.vec.push(ClosureCandidate);
435 ty::Infer(ty::TyVar(_)) => {
436 debug!("assemble_unboxed_closure_candidates: ambiguous self-type");
437 candidates.ambiguous = true;
445 /// Implements one of the `Fn()` family for a fn pointer.
446 fn assemble_fn_pointer_candidates(
448 obligation: &TraitObligation<'tcx>,
449 candidates: &mut SelectionCandidateSet<'tcx>,
450 ) -> Result<(), SelectionError<'tcx>> {
451 // We provide impl of all fn traits for fn pointers.
452 if self.tcx().fn_trait_kind_from_lang_item(obligation.predicate.def_id()).is_none() {
456 // Okay to skip binder because what we are inspecting doesn't involve bound regions.
457 let self_ty = obligation.self_ty().skip_binder();
458 match *self_ty.kind() {
459 ty::Infer(ty::TyVar(_)) => {
460 debug!("assemble_fn_pointer_candidates: ambiguous self-type");
461 candidates.ambiguous = true; // Could wind up being a fn() type.
463 // Provide an impl, but only for suitable `fn` pointers.
466 unsafety: hir::Unsafety::Normal,
470 } = self_ty.fn_sig(self.tcx()).skip_binder()
472 candidates.vec.push(FnPointerCandidate);
475 // Provide an impl for suitable functions, rejecting `#[target_feature]` functions (RFC 2396).
476 ty::FnDef(def_id, _) => {
478 unsafety: hir::Unsafety::Normal,
482 } = self_ty.fn_sig(self.tcx()).skip_binder()
484 if self.tcx().codegen_fn_attrs(def_id).target_features.is_empty() {
485 candidates.vec.push(FnPointerCandidate);
495 /// Searches for impls that might apply to `obligation`.
496 fn assemble_candidates_from_impls(
498 obligation: &TraitObligation<'tcx>,
499 candidates: &mut SelectionCandidateSet<'tcx>,
500 ) -> Result<(), SelectionError<'tcx>> {
501 debug!(?obligation, "assemble_candidates_from_impls");
503 // Essentially any user-written impl will match with an error type,
504 // so creating `ImplCandidates` isn't useful. However, we might
505 // end up finding a candidate elsewhere (e.g. a `BuiltinCandidate` for `Sized)
506 // This helps us avoid overflow: see issue #72839
507 // Since compilation is already guaranteed to fail, this is just
508 // to try to show the 'nicest' possible errors to the user.
509 if obligation.references_error() {
513 self.tcx().for_each_relevant_impl(
514 obligation.predicate.def_id(),
515 obligation.predicate.skip_binder().trait_ref.self_ty(),
517 self.infcx.probe(|_| {
518 if let Ok(_substs) = self.match_impl(impl_def_id, obligation) {
519 candidates.vec.push(ImplCandidate(impl_def_id));
528 fn assemble_candidates_from_auto_impls(
530 obligation: &TraitObligation<'tcx>,
531 candidates: &mut SelectionCandidateSet<'tcx>,
532 ) -> Result<(), SelectionError<'tcx>> {
533 // Okay to skip binder here because the tests we do below do not involve bound regions.
534 let self_ty = obligation.self_ty().skip_binder();
535 debug!(?self_ty, "assemble_candidates_from_auto_impls");
537 let def_id = obligation.predicate.def_id();
539 if self.tcx().trait_is_auto(def_id) {
540 match self_ty.kind() {
542 // For object types, we don't know what the closed
543 // over types are. This means we conservatively
544 // say nothing; a candidate may be added by
545 // `assemble_candidates_from_object_ty`.
548 // Since the contents of foreign types is unknown,
549 // we don't add any `..` impl. Default traits could
550 // still be provided by a manual implementation for
551 // this trait and type.
553 ty::Param(..) | ty::Projection(..) => {
554 // In these cases, we don't know what the actual
555 // type is. Therefore, we cannot break it down
556 // into its constituent types. So we don't
557 // consider the `..` impl but instead just add no
558 // candidates: this means that typeck will only
559 // succeed if there is another reason to believe
560 // that this obligation holds. That could be a
561 // where-clause or, in the case of an object type,
562 // it could be that the object type lists the
563 // trait (e.g., `Foo+Send : Send`). See
564 // `compile-fail/typeck-default-trait-impl-send-param.rs`
565 // for an example of a test case that exercises
568 ty::Infer(ty::TyVar(_)) => {
569 // The auto impl might apply; we don't know.
570 candidates.ambiguous = true;
572 ty::Generator(_, _, movability)
573 if self.tcx().lang_items().unpin_trait() == Some(def_id) =>
576 hir::Movability::Static => {
577 // Immovable generators are never `Unpin`, so
578 // suppress the normal auto-impl candidate for it.
580 hir::Movability::Movable => {
581 // Movable generators are always `Unpin`, so add an
582 // unconditional builtin candidate.
583 candidates.vec.push(BuiltinCandidate { has_nested: false });
588 _ => candidates.vec.push(AutoImplCandidate(def_id)),
595 /// Searches for impls that might apply to `obligation`.
596 fn assemble_candidates_from_object_ty(
598 obligation: &TraitObligation<'tcx>,
599 candidates: &mut SelectionCandidateSet<'tcx>,
602 self_ty = ?obligation.self_ty().skip_binder(),
603 "assemble_candidates_from_object_ty",
606 self.infcx.probe(|_snapshot| {
607 // The code below doesn't care about regions, and the
608 // self-ty here doesn't escape this probe, so just erase
610 let self_ty = self.tcx().erase_late_bound_regions(&obligation.self_ty());
611 let poly_trait_ref = match self_ty.kind() {
612 ty::Dynamic(ref data, ..) => {
613 if data.auto_traits().any(|did| did == obligation.predicate.def_id()) {
615 "assemble_candidates_from_object_ty: matched builtin bound, \
618 candidates.vec.push(BuiltinObjectCandidate);
622 if let Some(principal) = data.principal() {
623 if !self.infcx.tcx.features().object_safe_for_dispatch {
624 principal.with_self_ty(self.tcx(), self_ty)
625 } else if self.tcx().is_object_safe(principal.def_id()) {
626 principal.with_self_ty(self.tcx(), self_ty)
631 // Only auto trait bounds exist.
635 ty::Infer(ty::TyVar(_)) => {
636 debug!("assemble_candidates_from_object_ty: ambiguous");
637 candidates.ambiguous = true; // could wind up being an object type
643 debug!(?poly_trait_ref, "assemble_candidates_from_object_ty");
645 let poly_trait_predicate = self.infcx().resolve_vars_if_possible(&obligation.predicate);
646 let placeholder_trait_predicate =
647 self.infcx().replace_bound_vars_with_placeholders(&poly_trait_predicate);
649 // Count only those upcast versions that match the trait-ref
650 // we are looking for. Specifically, do not only check for the
651 // correct trait, but also the correct type parameters.
652 // For example, we may be trying to upcast `Foo` to `Bar<i32>`,
653 // but `Foo` is declared as `trait Foo: Bar<u32>`.
654 let candidate_supertraits = util::supertraits(self.tcx(), poly_trait_ref)
656 .filter(|&(_, upcast_trait_ref)| {
657 self.infcx.probe(|_| {
658 self.match_normalize_trait_ref(
661 placeholder_trait_predicate.trait_ref,
666 .map(|(idx, _)| ObjectCandidate(idx));
668 candidates.vec.extend(candidate_supertraits);
672 /// Searches for unsizing that might apply to `obligation`.
673 fn assemble_candidates_for_unsizing(
675 obligation: &TraitObligation<'tcx>,
676 candidates: &mut SelectionCandidateSet<'tcx>,
678 // We currently never consider higher-ranked obligations e.g.
679 // `for<'a> &'a T: Unsize<Trait+'a>` to be implemented. This is not
680 // because they are a priori invalid, and we could potentially add support
681 // for them later, it's just that there isn't really a strong need for it.
682 // A `T: Unsize<U>` obligation is always used as part of a `T: CoerceUnsize<U>`
683 // impl, and those are generally applied to concrete types.
685 // That said, one might try to write a fn with a where clause like
686 // for<'a> Foo<'a, T>: Unsize<Foo<'a, Trait>>
687 // where the `'a` is kind of orthogonal to the relevant part of the `Unsize`.
688 // Still, you'd be more likely to write that where clause as
690 // so it seems ok if we (conservatively) fail to accept that `Unsize`
691 // obligation above. Should be possible to extend this in the future.
692 let source = match obligation.self_ty().no_bound_vars() {
695 // Don't add any candidates if there are bound regions.
699 let target = obligation.predicate.skip_binder().trait_ref.substs.type_at(1);
701 debug!(?source, ?target, "assemble_candidates_for_unsizing");
703 let may_apply = match (source.kind(), target.kind()) {
704 // Trait+Kx+'a -> Trait+Ky+'b (upcasts).
705 (&ty::Dynamic(ref data_a, ..), &ty::Dynamic(ref data_b, ..)) => {
706 // Upcasts permit two things:
708 // 1. Dropping auto traits, e.g., `Foo + Send` to `Foo`
709 // 2. Tightening the region bound, e.g., `Foo + 'a` to `Foo + 'b` if `'a: 'b`
711 // Note that neither of these changes requires any
712 // change at runtime. Eventually this will be
715 // We always upcast when we can because of reason
716 // #2 (region bounds).
717 data_a.principal_def_id() == data_b.principal_def_id()
720 // All of a's auto traits need to be in b's auto traits.
721 .all(|b| data_a.auto_traits().any(|a| a == b))
725 (_, &ty::Dynamic(..)) => true,
727 // Ambiguous handling is below `T` -> `Trait`, because inference
728 // variables can still implement `Unsize<Trait>` and nested
729 // obligations will have the final say (likely deferred).
730 (&ty::Infer(ty::TyVar(_)), _) | (_, &ty::Infer(ty::TyVar(_))) => {
731 debug!("assemble_candidates_for_unsizing: ambiguous");
732 candidates.ambiguous = true;
737 (&ty::Array(..), &ty::Slice(_)) => true,
739 // `Struct<T>` -> `Struct<U>`
740 (&ty::Adt(def_id_a, _), &ty::Adt(def_id_b, _)) if def_id_a.is_struct() => {
744 // `(.., T)` -> `(.., U)`
745 (&ty::Tuple(tys_a), &ty::Tuple(tys_b)) => tys_a.len() == tys_b.len(),
751 candidates.vec.push(BuiltinUnsizeCandidate);
755 fn assemble_candidates_for_trait_alias(
757 obligation: &TraitObligation<'tcx>,
758 candidates: &mut SelectionCandidateSet<'tcx>,
759 ) -> Result<(), SelectionError<'tcx>> {
760 // Okay to skip binder here because the tests we do below do not involve bound regions.
761 let self_ty = obligation.self_ty().skip_binder();
762 debug!(?self_ty, "assemble_candidates_for_trait_alias");
764 let def_id = obligation.predicate.def_id();
766 if self.tcx().is_trait_alias(def_id) {
767 candidates.vec.push(TraitAliasCandidate(def_id));
773 /// Assembles the trait which are built-in to the language itself:
774 /// `Copy`, `Clone` and `Sized`.
775 fn assemble_builtin_bound_candidates(
777 conditions: BuiltinImplConditions<'tcx>,
778 candidates: &mut SelectionCandidateSet<'tcx>,
779 ) -> Result<(), SelectionError<'tcx>> {
781 BuiltinImplConditions::Where(nested) => {
782 debug!(?nested, "builtin_bound");
785 .push(BuiltinCandidate { has_nested: !nested.skip_binder().is_empty() });
787 BuiltinImplConditions::None => {}
788 BuiltinImplConditions::Ambiguous => {
789 debug!("assemble_builtin_bound_candidates: ambiguous builtin");
790 candidates.ambiguous = true;