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::ty::fold::TypeFoldable;
32 use rustc_middle::ty::subst::{InternalSubsts, SubstsRef};
33 use rustc_middle::ty::{
34 self, GenericParamDefKind, ParamEnv, ToPredicate, Ty, TyCtxt, WithConstness,
40 pub use self::FulfillmentErrorCode::*;
41 pub use self::ImplSource::*;
42 pub use self::ObligationCauseCode::*;
43 pub use self::SelectionError::*;
45 pub use self::coherence::{add_placeholder_note, orphan_check, overlapping_impls};
46 pub use self::coherence::{OrphanCheckErr, OverlapResult};
47 pub use self::engine::TraitEngineExt;
48 pub use self::fulfill::{FulfillmentContext, PendingPredicateObligation};
49 pub use self::object_safety::astconv_object_safety_violations;
50 pub use self::object_safety::is_vtable_safe_method;
51 pub use self::object_safety::MethodViolationCode;
52 pub use self::object_safety::ObjectSafetyViolation;
53 pub use self::on_unimplemented::{OnUnimplementedDirective, OnUnimplementedNote};
54 pub use self::project::{
55 normalize, normalize_projection_type, normalize_to, poly_project_and_unify_type,
57 pub use self::select::{EvaluationCache, SelectionCache, SelectionContext};
58 pub use self::select::{EvaluationResult, IntercrateAmbiguityCause, OverflowError};
59 pub use self::specialize::specialization_graph::FutureCompatOverlapError;
60 pub use self::specialize::specialization_graph::FutureCompatOverlapErrorKind;
61 pub use self::specialize::{specialization_graph, translate_substs, OverlapError};
62 pub use self::structural_match::search_for_structural_match_violation;
63 pub use self::structural_match::NonStructuralMatchTy;
64 pub use self::util::{elaborate_predicates, elaborate_trait_ref, elaborate_trait_refs};
65 pub use self::util::{expand_trait_aliases, TraitAliasExpander};
67 get_vtable_index_of_object_method, impl_item_is_final, predicate_for_trait_def, upcast_choices,
70 supertrait_def_ids, supertraits, transitive_bounds, SupertraitDefIds, Supertraits,
73 pub use self::chalk_fulfill::FulfillmentContext as ChalkFulfillmentContext;
75 pub use rustc_infer::traits::*;
77 /// Whether to skip the leak check, as part of a future compatibility warning step.
78 #[derive(Copy, Clone, PartialEq, Eq, Debug)]
79 pub enum SkipLeakCheck {
85 fn is_yes(self) -> bool {
86 self == SkipLeakCheck::Yes
90 /// The "default" for skip-leak-check corresponds to the current
91 /// behavior (do not skip the leak check) -- not the behavior we are
92 /// transitioning into.
93 impl Default for SkipLeakCheck {
94 fn default() -> Self {
99 /// The mode that trait queries run in.
100 #[derive(Copy, Clone, PartialEq, Eq, Debug)]
101 pub enum TraitQueryMode {
102 // Standard/un-canonicalized queries get accurate
103 // spans etc. passed in and hence can do reasonable
104 // error reporting on their own.
106 // Canonicalized queries get dummy spans and hence
107 // must generally propagate errors to
108 // pre-canonicalization callsites.
112 /// Creates predicate obligations from the generic bounds.
113 pub fn predicates_for_generics<'tcx>(
114 cause: ObligationCause<'tcx>,
115 param_env: ty::ParamEnv<'tcx>,
116 generic_bounds: ty::InstantiatedPredicates<'tcx>,
117 ) -> impl Iterator<Item = PredicateObligation<'tcx>> {
118 util::predicates_for_generics(cause, 0, param_env, generic_bounds)
121 /// Determines whether the type `ty` is known to meet `bound` and
122 /// returns true if so. Returns false if `ty` either does not meet
123 /// `bound` or is not known to meet bound (note that this is
124 /// conservative towards *no impl*, which is the opposite of the
125 /// `evaluate` methods).
126 pub fn type_known_to_meet_bound_modulo_regions<'a, 'tcx>(
127 infcx: &InferCtxt<'a, 'tcx>,
128 param_env: ty::ParamEnv<'tcx>,
134 "type_known_to_meet_bound_modulo_regions(ty={:?}, bound={:?})",
136 infcx.tcx.def_path_str(def_id)
139 let trait_ref = ty::TraitRef { def_id, substs: infcx.tcx.mk_substs_trait(ty, &[]) };
140 let obligation = Obligation {
142 cause: ObligationCause::misc(span, hir::CRATE_HIR_ID),
144 predicate: trait_ref.without_const().to_predicate(infcx.tcx),
147 let result = infcx.predicate_must_hold_modulo_regions(&obligation);
149 "type_known_to_meet_ty={:?} bound={} => {:?}",
151 infcx.tcx.def_path_str(def_id),
155 if result && ty.has_infer_types_or_consts() {
156 // Because of inference "guessing", selection can sometimes claim
157 // to succeed while the success requires a guess. To ensure
158 // this function's result remains infallible, we must confirm
159 // that guess. While imperfect, I believe this is sound.
161 // The handling of regions in this area of the code is terrible,
162 // see issue #29149. We should be able to improve on this with
164 let mut fulfill_cx = FulfillmentContext::new_ignoring_regions();
166 // We can use a dummy node-id here because we won't pay any mind
167 // to region obligations that arise (there shouldn't really be any
169 let cause = ObligationCause::misc(span, hir::CRATE_HIR_ID);
171 fulfill_cx.register_bound(infcx, param_env, ty, def_id, cause);
173 // Note: we only assume something is `Copy` if we can
174 // *definitively* show that it implements `Copy`. Otherwise,
175 // assume it is move; linear is always ok.
176 match fulfill_cx.select_all_or_error(infcx) {
179 "type_known_to_meet_bound_modulo_regions: ty={:?} bound={} success",
181 infcx.tcx.def_path_str(def_id)
187 "type_known_to_meet_bound_modulo_regions: ty={:?} bound={} errors={:?}",
189 infcx.tcx.def_path_str(def_id),
200 fn do_normalize_predicates<'tcx>(
202 region_context: DefId,
203 cause: ObligationCause<'tcx>,
204 elaborated_env: ty::ParamEnv<'tcx>,
205 predicates: Vec<ty::Predicate<'tcx>>,
206 ) -> Result<Vec<ty::Predicate<'tcx>>, ErrorReported> {
208 "do_normalize_predicates(predicates={:?}, region_context={:?}, cause={:?})",
209 predicates, region_context, cause,
211 let span = cause.span;
212 tcx.infer_ctxt().enter(|infcx| {
213 // FIXME. We should really... do something with these region
214 // obligations. But this call just continues the older
215 // behavior (i.e., doesn't cause any new bugs), and it would
216 // take some further refactoring to actually solve them. In
217 // particular, we would have to handle implied bounds
218 // properly, and that code is currently largely confined to
219 // regionck (though I made some efforts to extract it
222 // @arielby: In any case, these obligations are checked
223 // by wfcheck anyway, so I'm not sure we have to check
224 // them here too, and we will remove this function when
225 // we move over to lazy normalization *anyway*.
226 let fulfill_cx = FulfillmentContext::new_ignoring_regions();
228 match fully_normalize(&infcx, fulfill_cx, cause, elaborated_env, &predicates) {
229 Ok(predicates) => predicates,
231 infcx.report_fulfillment_errors(&errors, None, false);
232 return Err(ErrorReported);
236 debug!("do_normalize_predictes: normalized predicates = {:?}", predicates);
238 // We can use the `elaborated_env` here; the region code only
239 // cares about declarations like `'a: 'b`.
240 let outlives_env = OutlivesEnvironment::new(elaborated_env);
242 infcx.resolve_regions_and_report_errors(
245 RegionckMode::default(),
248 let predicates = match infcx.fully_resolve(&predicates) {
249 Ok(predicates) => predicates,
251 // If we encounter a fixup error, it means that some type
252 // variable wound up unconstrained. I actually don't know
253 // if this can happen, and I certainly don't expect it to
254 // happen often, but if it did happen it probably
255 // represents a legitimate failure due to some kind of
256 // unconstrained variable, and it seems better not to ICE,
257 // all things considered.
258 tcx.sess.span_err(span, &fixup_err.to_string());
259 return Err(ErrorReported);
262 if predicates.needs_infer() {
263 tcx.sess.delay_span_bug(span, "encountered inference variables after `fully_resolve`");
271 // FIXME: this is gonna need to be removed ...
272 /// Normalizes the parameter environment, reporting errors if they occur.
273 pub fn normalize_param_env_or_error<'tcx>(
275 region_context: DefId,
276 unnormalized_env: ty::ParamEnv<'tcx>,
277 cause: ObligationCause<'tcx>,
278 ) -> ty::ParamEnv<'tcx> {
279 // I'm not wild about reporting errors here; I'd prefer to
280 // have the errors get reported at a defined place (e.g.,
281 // during typeck). Instead I have all parameter
282 // environments, in effect, going through this function
283 // and hence potentially reporting errors. This ensures of
284 // course that we never forget to normalize (the
285 // alternative seemed like it would involve a lot of
286 // manual invocations of this fn -- and then we'd have to
287 // deal with the errors at each of those sites).
289 // In any case, in practice, typeck constructs all the
290 // parameter environments once for every fn as it goes,
291 // and errors will get reported then; so after typeck we
292 // can be sure that no errors should occur.
295 "normalize_param_env_or_error(region_context={:?}, unnormalized_env={:?}, cause={:?})",
296 region_context, unnormalized_env, cause
299 let mut predicates: Vec<_> =
300 util::elaborate_predicates(tcx, unnormalized_env.caller_bounds().into_iter())
301 .map(|obligation| obligation.predicate)
304 debug!("normalize_param_env_or_error: elaborated-predicates={:?}", predicates);
306 let elaborated_env = ty::ParamEnv::new(
307 tcx.intern_predicates(&predicates),
308 unnormalized_env.reveal(),
309 unnormalized_env.def_id,
312 // HACK: we are trying to normalize the param-env inside *itself*. The problem is that
313 // normalization expects its param-env to be already normalized, which means we have
316 // The way we handle this is by normalizing the param-env inside an unnormalized version
317 // of the param-env, which means that if the param-env contains unnormalized projections,
318 // we'll have some normalization failures. This is unfortunate.
320 // Lazy normalization would basically handle this by treating just the
321 // normalizing-a-trait-ref-requires-itself cycles as evaluation failures.
323 // Inferred outlives bounds can create a lot of `TypeOutlives` predicates for associated
324 // types, so to make the situation less bad, we normalize all the predicates *but*
325 // the `TypeOutlives` predicates first inside the unnormalized parameter environment, and
326 // then we normalize the `TypeOutlives` bounds inside the normalized parameter environment.
328 // This works fairly well because trait matching does not actually care about param-env
329 // TypeOutlives predicates - these are normally used by regionck.
330 let outlives_predicates: Vec<_> = predicates
331 .drain_filter(|predicate| match predicate.kind() {
332 ty::PredicateKind::TypeOutlives(..) => true,
338 "normalize_param_env_or_error: predicates=(non-outlives={:?}, outlives={:?})",
339 predicates, outlives_predicates
341 let non_outlives_predicates = match do_normalize_predicates(
348 Ok(predicates) => predicates,
349 // An unnormalized env is better than nothing.
350 Err(ErrorReported) => {
351 debug!("normalize_param_env_or_error: errored resolving non-outlives predicates");
352 return elaborated_env;
356 debug!("normalize_param_env_or_error: non-outlives predicates={:?}", non_outlives_predicates);
358 // Not sure whether it is better to include the unnormalized TypeOutlives predicates
359 // here. I believe they should not matter, because we are ignoring TypeOutlives param-env
360 // predicates here anyway. Keeping them here anyway because it seems safer.
361 let outlives_env: Vec<_> =
362 non_outlives_predicates.iter().chain(&outlives_predicates).cloned().collect();
364 ty::ParamEnv::new(tcx.intern_predicates(&outlives_env), unnormalized_env.reveal(), None);
365 let outlives_predicates = match do_normalize_predicates(
372 Ok(predicates) => predicates,
373 // An unnormalized env is better than nothing.
374 Err(ErrorReported) => {
375 debug!("normalize_param_env_or_error: errored resolving outlives predicates");
376 return elaborated_env;
379 debug!("normalize_param_env_or_error: outlives predicates={:?}", outlives_predicates);
381 let mut predicates = non_outlives_predicates;
382 predicates.extend(outlives_predicates);
383 debug!("normalize_param_env_or_error: final predicates={:?}", predicates);
385 tcx.intern_predicates(&predicates),
386 unnormalized_env.reveal(),
387 unnormalized_env.def_id,
391 pub fn fully_normalize<'a, 'tcx, T>(
392 infcx: &InferCtxt<'a, 'tcx>,
393 mut fulfill_cx: FulfillmentContext<'tcx>,
394 cause: ObligationCause<'tcx>,
395 param_env: ty::ParamEnv<'tcx>,
397 ) -> Result<T, Vec<FulfillmentError<'tcx>>>
399 T: TypeFoldable<'tcx>,
401 debug!("fully_normalize_with_fulfillcx(value={:?})", value);
402 let selcx = &mut SelectionContext::new(infcx);
403 let Normalized { value: normalized_value, obligations } =
404 project::normalize(selcx, param_env, cause, value);
406 "fully_normalize: normalized_value={:?} obligations={:?}",
407 normalized_value, obligations
409 for obligation in obligations {
410 fulfill_cx.register_predicate_obligation(selcx.infcx(), obligation);
413 debug!("fully_normalize: select_all_or_error start");
414 fulfill_cx.select_all_or_error(infcx)?;
415 debug!("fully_normalize: select_all_or_error complete");
416 let resolved_value = infcx.resolve_vars_if_possible(&normalized_value);
417 debug!("fully_normalize: resolved_value={:?}", resolved_value);
421 /// Normalizes the predicates and checks whether they hold in an empty environment. If this
422 /// returns true, then either normalize encountered an error or one of the predicates did not
423 /// hold. Used when creating vtables to check for unsatisfiable methods.
424 pub fn impossible_predicates<'tcx>(
426 predicates: Vec<ty::Predicate<'tcx>>,
428 debug!("impossible_predicates(predicates={:?})", predicates);
430 let result = tcx.infer_ctxt().enter(|infcx| {
431 let param_env = ty::ParamEnv::reveal_all();
432 let mut selcx = SelectionContext::new(&infcx);
433 let mut fulfill_cx = FulfillmentContext::new();
434 let cause = ObligationCause::dummy();
435 let Normalized { value: predicates, obligations } =
436 normalize(&mut selcx, param_env, cause.clone(), &predicates);
437 for obligation in obligations {
438 fulfill_cx.register_predicate_obligation(&infcx, obligation);
440 for predicate in predicates {
441 let obligation = Obligation::new(cause.clone(), param_env, predicate);
442 fulfill_cx.register_predicate_obligation(&infcx, obligation);
445 fulfill_cx.select_all_or_error(&infcx).is_err()
447 debug!("impossible_predicates(predicates={:?}) = {:?}", predicates, result);
451 fn subst_and_check_impossible_predicates<'tcx>(
453 key: (DefId, SubstsRef<'tcx>),
455 debug!("subst_and_check_impossible_predicates(key={:?})", key);
457 let mut predicates = tcx.predicates_of(key.0).instantiate(tcx, key.1).predicates;
458 predicates.retain(|predicate| !predicate.needs_subst());
459 let result = impossible_predicates(tcx, predicates);
461 debug!("subst_and_check_impossible_predicates(key={:?}) = {:?}", key, result);
465 /// Given a trait `trait_ref`, iterates the vtable entries
466 /// that come from `trait_ref`, including its supertraits.
467 #[inline] // FIXME(#35870): avoid closures being unexported due to `impl Trait`.
468 fn vtable_methods<'tcx>(
470 trait_ref: ty::PolyTraitRef<'tcx>,
471 ) -> &'tcx [Option<(DefId, SubstsRef<'tcx>)>] {
472 debug!("vtable_methods({:?})", trait_ref);
474 tcx.arena.alloc_from_iter(supertraits(tcx, trait_ref).flat_map(move |trait_ref| {
475 let trait_methods = tcx
476 .associated_items(trait_ref.def_id())
477 .in_definition_order()
478 .filter(|item| item.kind == ty::AssocKind::Fn);
480 // Now list each method's DefId and InternalSubsts (for within its trait).
481 // If the method can never be called from this object, produce None.
482 trait_methods.map(move |trait_method| {
483 debug!("vtable_methods: trait_method={:?}", trait_method);
484 let def_id = trait_method.def_id;
486 // Some methods cannot be called on an object; skip those.
487 if !is_vtable_safe_method(tcx, trait_ref.def_id(), &trait_method) {
488 debug!("vtable_methods: not vtable safe");
492 // The method may have some early-bound lifetimes; add regions for those.
493 let substs = trait_ref.map_bound(|trait_ref| {
494 InternalSubsts::for_item(tcx, def_id, |param, _| match param.kind {
495 GenericParamDefKind::Lifetime => tcx.lifetimes.re_erased.into(),
496 GenericParamDefKind::Type { .. } | GenericParamDefKind::Const => {
497 trait_ref.substs[param.index as usize]
502 // The trait type may have higher-ranked lifetimes in it;
503 // erase them if they appear, so that we get the type
504 // at some particular call site.
506 tcx.normalize_erasing_late_bound_regions(ty::ParamEnv::reveal_all(), &substs);
508 // It's possible that the method relies on where-clauses that
509 // do not hold for this particular set of type parameters.
510 // Note that this method could then never be called, so we
511 // do not want to try and codegen it, in that case (see #23435).
512 let predicates = tcx.predicates_of(def_id).instantiate_own(tcx, substs);
513 if impossible_predicates(tcx, predicates.predicates) {
514 debug!("vtable_methods: predicates do not hold");
518 Some((def_id, substs))
523 /// Check whether a `ty` implements given trait(trait_def_id).
525 /// NOTE: Always return `false` for a type which needs inference.
526 fn type_implements_trait<'tcx>(
529 DefId, // trait_def_id,
535 let (trait_def_id, ty, params, param_env) = key;
538 "type_implements_trait: trait_def_id={:?}, type={:?}, params={:?}, param_env={:?}",
539 trait_def_id, ty, params, param_env
542 let trait_ref = ty::TraitRef { def_id: trait_def_id, substs: tcx.mk_substs_trait(ty, params) };
544 let obligation = Obligation {
545 cause: ObligationCause::dummy(),
548 predicate: trait_ref.without_const().to_predicate(tcx),
550 tcx.infer_ctxt().enter(|infcx| infcx.predicate_must_hold_modulo_regions(&obligation))
553 pub fn provide(providers: &mut ty::query::Providers) {
554 object_safety::provide(providers);
555 structural_match::provide(providers);
556 *providers = ty::query::Providers {
557 specialization_graph_of: specialize::specialization_graph_provider,
558 specializes: specialize::specializes,
559 codegen_fulfill_obligation: codegen::codegen_fulfill_obligation,
561 type_implements_trait,
562 subst_and_check_impossible_predicates,