1 //! Trait Resolution. See the [rustc dev guide] for more information on how this works.
3 //! [rustc dev guide]: https://rustc-dev-guide.rust-lang.org/traits/resolution.html
11 pub mod error_reporting;
24 use crate::infer::outlives::env::OutlivesEnvironment;
25 use crate::infer::{InferCtxt, RegionckMode, TyCtxtInferExt};
26 use crate::traits::error_reporting::InferCtxtExt as _;
27 use crate::traits::query::evaluate_obligation::InferCtxtExt as _;
28 use rustc_errors::ErrorReported;
30 use rustc_hir::def_id::DefId;
31 use rustc_middle::middle::region;
32 use rustc_middle::ty::fold::TypeFoldable;
33 use rustc_middle::ty::subst::{InternalSubsts, SubstsRef};
34 use rustc_middle::ty::{
35 self, GenericParamDefKind, ParamEnv, ToPredicate, Ty, TyCtxt, WithConstness,
41 pub use self::FulfillmentErrorCode::*;
42 pub use self::ObligationCauseCode::*;
43 pub use self::SelectionError::*;
44 pub use self::Vtable::*;
46 pub use self::coherence::{add_placeholder_note, orphan_check, overlapping_impls};
47 pub use self::coherence::{OrphanCheckErr, OverlapResult};
48 pub use self::engine::TraitEngineExt;
49 pub use self::fulfill::{FulfillmentContext, PendingPredicateObligation};
50 pub use self::object_safety::astconv_object_safety_violations;
51 pub use self::object_safety::is_vtable_safe_method;
52 pub use self::object_safety::MethodViolationCode;
53 pub use self::object_safety::ObjectSafetyViolation;
54 pub use self::on_unimplemented::{OnUnimplementedDirective, OnUnimplementedNote};
55 pub use self::project::{
56 normalize, normalize_projection_type, normalize_to, poly_project_and_unify_type,
58 pub use self::select::{EvaluationCache, SelectionCache, SelectionContext};
59 pub use self::select::{EvaluationResult, IntercrateAmbiguityCause, OverflowError};
60 pub use self::specialize::specialization_graph::FutureCompatOverlapError;
61 pub use self::specialize::specialization_graph::FutureCompatOverlapErrorKind;
62 pub use self::specialize::{specialization_graph, translate_substs, OverlapError};
63 pub use self::structural_match::search_for_structural_match_violation;
64 pub use self::structural_match::type_marked_structural;
65 pub use self::structural_match::NonStructuralMatchTy;
66 pub use self::util::{elaborate_predicates, elaborate_trait_ref, elaborate_trait_refs};
67 pub use self::util::{expand_trait_aliases, TraitAliasExpander};
69 get_vtable_index_of_object_method, impl_item_is_final, predicate_for_trait_def, upcast_choices,
72 supertrait_def_ids, supertraits, transitive_bounds, SupertraitDefIds, Supertraits,
75 pub use self::chalk_fulfill::FulfillmentContext as ChalkFulfillmentContext;
77 pub use rustc_infer::traits::*;
79 /// Whether to skip the leak check, as part of a future compatibility warning step.
80 #[derive(Copy, Clone, PartialEq, Eq, Debug)]
81 pub enum SkipLeakCheck {
87 fn is_yes(self) -> bool {
88 self == SkipLeakCheck::Yes
92 /// The "default" for skip-leak-check corresponds to the current
93 /// behavior (do not skip the leak check) -- not the behavior we are
94 /// transitioning into.
95 impl Default for SkipLeakCheck {
96 fn default() -> Self {
101 /// The mode that trait queries run in.
102 #[derive(Copy, Clone, PartialEq, Eq, Debug)]
103 pub enum TraitQueryMode {
104 // Standard/un-canonicalized queries get accurate
105 // spans etc. passed in and hence can do reasonable
106 // error reporting on their own.
108 // Canonicalized queries get dummy spans and hence
109 // must generally propagate errors to
110 // pre-canonicalization callsites.
114 /// Creates predicate obligations from the generic bounds.
115 pub fn predicates_for_generics<'tcx>(
116 cause: ObligationCause<'tcx>,
117 param_env: ty::ParamEnv<'tcx>,
118 generic_bounds: ty::InstantiatedPredicates<'tcx>,
119 ) -> impl Iterator<Item = PredicateObligation<'tcx>> {
120 util::predicates_for_generics(cause, 0, param_env, generic_bounds)
123 /// Determines whether the type `ty` is known to meet `bound` and
124 /// returns true if so. Returns false if `ty` either does not meet
125 /// `bound` or is not known to meet bound (note that this is
126 /// conservative towards *no impl*, which is the opposite of the
127 /// `evaluate` methods).
128 pub fn type_known_to_meet_bound_modulo_regions<'a, 'tcx>(
129 infcx: &InferCtxt<'a, 'tcx>,
130 param_env: ty::ParamEnv<'tcx>,
136 "type_known_to_meet_bound_modulo_regions(ty={:?}, bound={:?})",
138 infcx.tcx.def_path_str(def_id)
141 let trait_ref = ty::TraitRef { def_id, substs: infcx.tcx.mk_substs_trait(ty, &[]) };
142 let obligation = Obligation {
144 cause: ObligationCause::misc(span, hir::CRATE_HIR_ID),
146 predicate: trait_ref.without_const().to_predicate(infcx.tcx),
149 let result = infcx.predicate_must_hold_modulo_regions(&obligation);
151 "type_known_to_meet_ty={:?} bound={} => {:?}",
153 infcx.tcx.def_path_str(def_id),
157 if result && ty.has_infer_types_or_consts() {
158 // Because of inference "guessing", selection can sometimes claim
159 // to succeed while the success requires a guess. To ensure
160 // this function's result remains infallible, we must confirm
161 // that guess. While imperfect, I believe this is sound.
163 // The handling of regions in this area of the code is terrible,
164 // see issue #29149. We should be able to improve on this with
166 let mut fulfill_cx = FulfillmentContext::new_ignoring_regions();
168 // We can use a dummy node-id here because we won't pay any mind
169 // to region obligations that arise (there shouldn't really be any
171 let cause = ObligationCause::misc(span, hir::CRATE_HIR_ID);
173 fulfill_cx.register_bound(infcx, param_env, ty, def_id, cause);
175 // Note: we only assume something is `Copy` if we can
176 // *definitively* show that it implements `Copy`. Otherwise,
177 // assume it is move; linear is always ok.
178 match fulfill_cx.select_all_or_error(infcx) {
181 "type_known_to_meet_bound_modulo_regions: ty={:?} bound={} success",
183 infcx.tcx.def_path_str(def_id)
189 "type_known_to_meet_bound_modulo_regions: ty={:?} bound={} errors={:?}",
191 infcx.tcx.def_path_str(def_id),
202 fn do_normalize_predicates<'tcx>(
204 region_context: DefId,
205 cause: ObligationCause<'tcx>,
206 elaborated_env: ty::ParamEnv<'tcx>,
207 predicates: Vec<ty::Predicate<'tcx>>,
208 ) -> Result<Vec<ty::Predicate<'tcx>>, ErrorReported> {
210 "do_normalize_predicates(predicates={:?}, region_context={:?}, cause={:?})",
211 predicates, region_context, cause,
213 let span = cause.span;
214 tcx.infer_ctxt().enter(|infcx| {
215 // FIXME. We should really... do something with these region
216 // obligations. But this call just continues the older
217 // behavior (i.e., doesn't cause any new bugs), and it would
218 // take some further refactoring to actually solve them. In
219 // particular, we would have to handle implied bounds
220 // properly, and that code is currently largely confined to
221 // regionck (though I made some efforts to extract it
224 // @arielby: In any case, these obligations are checked
225 // by wfcheck anyway, so I'm not sure we have to check
226 // them here too, and we will remove this function when
227 // we move over to lazy normalization *anyway*.
228 let fulfill_cx = FulfillmentContext::new_ignoring_regions();
230 match fully_normalize(&infcx, fulfill_cx, cause, elaborated_env, &predicates) {
231 Ok(predicates) => predicates,
233 infcx.report_fulfillment_errors(&errors, None, false);
234 return Err(ErrorReported);
238 debug!("do_normalize_predictes: normalized predicates = {:?}", predicates);
240 let region_scope_tree = region::ScopeTree::default();
242 // We can use the `elaborated_env` here; the region code only
243 // cares about declarations like `'a: 'b`.
244 let outlives_env = OutlivesEnvironment::new(elaborated_env);
246 infcx.resolve_regions_and_report_errors(
250 RegionckMode::default(),
253 let predicates = match infcx.fully_resolve(&predicates) {
254 Ok(predicates) => predicates,
256 // If we encounter a fixup error, it means that some type
257 // variable wound up unconstrained. I actually don't know
258 // if this can happen, and I certainly don't expect it to
259 // happen often, but if it did happen it probably
260 // represents a legitimate failure due to some kind of
261 // unconstrained variable, and it seems better not to ICE,
262 // all things considered.
263 tcx.sess.span_err(span, &fixup_err.to_string());
264 return Err(ErrorReported);
267 if predicates.needs_infer() {
268 tcx.sess.delay_span_bug(span, "encountered inference variables after `fully_resolve`");
276 // FIXME: this is gonna need to be removed ...
277 /// Normalizes the parameter environment, reporting errors if they occur.
278 pub fn normalize_param_env_or_error<'tcx>(
280 region_context: DefId,
281 unnormalized_env: ty::ParamEnv<'tcx>,
282 cause: ObligationCause<'tcx>,
283 ) -> ty::ParamEnv<'tcx> {
284 // I'm not wild about reporting errors here; I'd prefer to
285 // have the errors get reported at a defined place (e.g.,
286 // during typeck). Instead I have all parameter
287 // environments, in effect, going through this function
288 // and hence potentially reporting errors. This ensures of
289 // course that we never forget to normalize (the
290 // alternative seemed like it would involve a lot of
291 // manual invocations of this fn -- and then we'd have to
292 // deal with the errors at each of those sites).
294 // In any case, in practice, typeck constructs all the
295 // parameter environments once for every fn as it goes,
296 // and errors will get reported then; so after typeck we
297 // can be sure that no errors should occur.
300 "normalize_param_env_or_error(region_context={:?}, unnormalized_env={:?}, cause={:?})",
301 region_context, unnormalized_env, cause
304 let mut predicates: Vec<_> =
305 util::elaborate_predicates(tcx, unnormalized_env.caller_bounds.into_iter().cloned())
306 .map(|obligation| obligation.predicate)
309 debug!("normalize_param_env_or_error: elaborated-predicates={:?}", predicates);
311 let elaborated_env = ty::ParamEnv::new(
312 tcx.intern_predicates(&predicates),
313 unnormalized_env.reveal,
314 unnormalized_env.def_id,
317 // HACK: we are trying to normalize the param-env inside *itself*. The problem is that
318 // normalization expects its param-env to be already normalized, which means we have
321 // The way we handle this is by normalizing the param-env inside an unnormalized version
322 // of the param-env, which means that if the param-env contains unnormalized projections,
323 // we'll have some normalization failures. This is unfortunate.
325 // Lazy normalization would basically handle this by treating just the
326 // normalizing-a-trait-ref-requires-itself cycles as evaluation failures.
328 // Inferred outlives bounds can create a lot of `TypeOutlives` predicates for associated
329 // types, so to make the situation less bad, we normalize all the predicates *but*
330 // the `TypeOutlives` predicates first inside the unnormalized parameter environment, and
331 // then we normalize the `TypeOutlives` bounds inside the normalized parameter environment.
333 // This works fairly well because trait matching does not actually care about param-env
334 // TypeOutlives predicates - these are normally used by regionck.
335 let outlives_predicates: Vec<_> = predicates
336 .drain_filter(|predicate| match predicate {
337 ty::PredicateKind::TypeOutlives(..) => true,
343 "normalize_param_env_or_error: predicates=(non-outlives={:?}, outlives={:?})",
344 predicates, outlives_predicates
346 let non_outlives_predicates = match do_normalize_predicates(
353 Ok(predicates) => predicates,
354 // An unnormalized env is better than nothing.
355 Err(ErrorReported) => {
356 debug!("normalize_param_env_or_error: errored resolving non-outlives predicates");
357 return elaborated_env;
361 debug!("normalize_param_env_or_error: non-outlives predicates={:?}", non_outlives_predicates);
363 // Not sure whether it is better to include the unnormalized TypeOutlives predicates
364 // here. I believe they should not matter, because we are ignoring TypeOutlives param-env
365 // predicates here anyway. Keeping them here anyway because it seems safer.
366 let outlives_env: Vec<_> =
367 non_outlives_predicates.iter().chain(&outlives_predicates).cloned().collect();
369 ty::ParamEnv::new(tcx.intern_predicates(&outlives_env), unnormalized_env.reveal, None);
370 let outlives_predicates = match do_normalize_predicates(
377 Ok(predicates) => predicates,
378 // An unnormalized env is better than nothing.
379 Err(ErrorReported) => {
380 debug!("normalize_param_env_or_error: errored resolving outlives predicates");
381 return elaborated_env;
384 debug!("normalize_param_env_or_error: outlives predicates={:?}", outlives_predicates);
386 let mut predicates = non_outlives_predicates;
387 predicates.extend(outlives_predicates);
388 debug!("normalize_param_env_or_error: final predicates={:?}", predicates);
390 tcx.intern_predicates(&predicates),
391 unnormalized_env.reveal,
392 unnormalized_env.def_id,
396 pub fn fully_normalize<'a, 'tcx, T>(
397 infcx: &InferCtxt<'a, 'tcx>,
398 mut fulfill_cx: FulfillmentContext<'tcx>,
399 cause: ObligationCause<'tcx>,
400 param_env: ty::ParamEnv<'tcx>,
402 ) -> Result<T, Vec<FulfillmentError<'tcx>>>
404 T: TypeFoldable<'tcx>,
406 debug!("fully_normalize_with_fulfillcx(value={:?})", value);
407 let selcx = &mut SelectionContext::new(infcx);
408 let Normalized { value: normalized_value, obligations } =
409 project::normalize(selcx, param_env, cause, value);
411 "fully_normalize: normalized_value={:?} obligations={:?}",
412 normalized_value, obligations
414 for obligation in obligations {
415 fulfill_cx.register_predicate_obligation(selcx.infcx(), obligation);
418 debug!("fully_normalize: select_all_or_error start");
419 fulfill_cx.select_all_or_error(infcx)?;
420 debug!("fully_normalize: select_all_or_error complete");
421 let resolved_value = infcx.resolve_vars_if_possible(&normalized_value);
422 debug!("fully_normalize: resolved_value={:?}", resolved_value);
426 /// Normalizes the predicates and checks whether they hold in an empty
427 /// environment. If this returns false, then either normalize
428 /// encountered an error or one of the predicates did not hold. Used
429 /// when creating vtables to check for unsatisfiable methods.
430 pub fn normalize_and_test_predicates<'tcx>(
432 predicates: Vec<ty::Predicate<'tcx>>,
434 debug!("normalize_and_test_predicates(predicates={:?})", predicates);
436 let result = tcx.infer_ctxt().enter(|infcx| {
437 let param_env = ty::ParamEnv::reveal_all();
438 let mut selcx = SelectionContext::new(&infcx);
439 let mut fulfill_cx = FulfillmentContext::new();
440 let cause = ObligationCause::dummy();
441 let Normalized { value: predicates, obligations } =
442 normalize(&mut selcx, param_env, cause.clone(), &predicates);
443 for obligation in obligations {
444 fulfill_cx.register_predicate_obligation(&infcx, obligation);
446 for predicate in predicates {
447 let obligation = Obligation::new(cause.clone(), param_env, predicate);
448 fulfill_cx.register_predicate_obligation(&infcx, obligation);
451 fulfill_cx.select_all_or_error(&infcx).is_ok()
453 debug!("normalize_and_test_predicates(predicates={:?}) = {:?}", predicates, result);
457 fn substitute_normalize_and_test_predicates<'tcx>(
459 key: (DefId, SubstsRef<'tcx>),
461 debug!("substitute_normalize_and_test_predicates(key={:?})", key);
463 let predicates = tcx.predicates_of(key.0).instantiate(tcx, key.1).predicates;
464 let result = normalize_and_test_predicates(tcx, predicates);
466 debug!("substitute_normalize_and_test_predicates(key={:?}) = {:?}", key, result);
470 /// Given a trait `trait_ref`, iterates the vtable entries
471 /// that come from `trait_ref`, including its supertraits.
472 #[inline] // FIXME(#35870): avoid closures being unexported due to `impl Trait`.
473 fn vtable_methods<'tcx>(
475 trait_ref: ty::PolyTraitRef<'tcx>,
476 ) -> &'tcx [Option<(DefId, SubstsRef<'tcx>)>] {
477 debug!("vtable_methods({:?})", trait_ref);
479 tcx.arena.alloc_from_iter(supertraits(tcx, trait_ref).flat_map(move |trait_ref| {
480 let trait_methods = tcx
481 .associated_items(trait_ref.def_id())
482 .in_definition_order()
483 .filter(|item| item.kind == ty::AssocKind::Fn);
485 // Now list each method's DefId and InternalSubsts (for within its trait).
486 // If the method can never be called from this object, produce None.
487 trait_methods.map(move |trait_method| {
488 debug!("vtable_methods: trait_method={:?}", trait_method);
489 let def_id = trait_method.def_id;
491 // Some methods cannot be called on an object; skip those.
492 if !is_vtable_safe_method(tcx, trait_ref.def_id(), &trait_method) {
493 debug!("vtable_methods: not vtable safe");
497 // The method may have some early-bound lifetimes; add regions for those.
498 let substs = trait_ref.map_bound(|trait_ref| {
499 InternalSubsts::for_item(tcx, def_id, |param, _| match param.kind {
500 GenericParamDefKind::Lifetime => tcx.lifetimes.re_erased.into(),
501 GenericParamDefKind::Type { .. } | GenericParamDefKind::Const => {
502 trait_ref.substs[param.index as usize]
507 // The trait type may have higher-ranked lifetimes in it;
508 // erase them if they appear, so that we get the type
509 // at some particular call site.
511 tcx.normalize_erasing_late_bound_regions(ty::ParamEnv::reveal_all(), &substs);
513 // It's possible that the method relies on where-clauses that
514 // do not hold for this particular set of type parameters.
515 // Note that this method could then never be called, so we
516 // do not want to try and codegen it, in that case (see #23435).
517 let predicates = tcx.predicates_of(def_id).instantiate_own(tcx, substs);
518 if !normalize_and_test_predicates(tcx, predicates.predicates) {
519 debug!("vtable_methods: predicates do not hold");
523 Some((def_id, substs))
528 /// Check whether a `ty` implements given trait(trait_def_id).
530 /// NOTE: Always return `false` for a type which needs inference.
531 fn type_implements_trait<'tcx>(
534 DefId, // trait_def_id,
540 let (trait_def_id, ty, params, param_env) = key;
543 "type_implements_trait: trait_def_id={:?}, type={:?}, params={:?}, param_env={:?}",
544 trait_def_id, ty, params, param_env
547 // Do not check on infer_types to avoid panic in evaluate_obligation.
548 if ty.has_infer_types() {
552 let ty = tcx.erase_regions(&ty);
554 let trait_ref = ty::TraitRef { def_id: trait_def_id, substs: tcx.mk_substs_trait(ty, params) };
556 let obligation = Obligation {
557 cause: ObligationCause::dummy(),
560 predicate: trait_ref.without_const().to_predicate(tcx),
562 tcx.infer_ctxt().enter(|infcx| infcx.predicate_must_hold_modulo_regions(&obligation))
565 pub fn provide(providers: &mut ty::query::Providers<'_>) {
566 object_safety::provide(providers);
567 *providers = ty::query::Providers {
568 specialization_graph_of: specialize::specialization_graph_provider,
569 specializes: specialize::specializes,
570 codegen_fulfill_obligation: codegen::codegen_fulfill_obligation,
572 substitute_normalize_and_test_predicates,
573 type_implements_trait,