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::{normalize, normalize_projection_type, normalize_to};
55 pub use self::select::{EvaluationCache, SelectionCache, SelectionContext};
56 pub use self::select::{EvaluationResult, IntercrateAmbiguityCause, OverflowError};
57 pub use self::specialize::specialization_graph::FutureCompatOverlapError;
58 pub use self::specialize::specialization_graph::FutureCompatOverlapErrorKind;
59 pub use self::specialize::{specialization_graph, translate_substs, OverlapError};
60 pub use self::structural_match::search_for_structural_match_violation;
61 pub use self::structural_match::NonStructuralMatchTy;
62 pub use self::util::{elaborate_predicates, elaborate_trait_ref, elaborate_trait_refs};
63 pub use self::util::{expand_trait_aliases, TraitAliasExpander};
65 get_vtable_index_of_object_method, impl_item_is_final, predicate_for_trait_def, upcast_choices,
68 supertrait_def_ids, supertraits, transitive_bounds, SupertraitDefIds, Supertraits,
71 pub use self::chalk_fulfill::FulfillmentContext as ChalkFulfillmentContext;
73 pub use rustc_infer::traits::*;
75 /// Whether to skip the leak check, as part of a future compatibility warning step.
76 #[derive(Copy, Clone, PartialEq, Eq, Debug)]
77 pub enum SkipLeakCheck {
83 fn is_yes(self) -> bool {
84 self == SkipLeakCheck::Yes
88 /// The "default" for skip-leak-check corresponds to the current
89 /// behavior (do not skip the leak check) -- not the behavior we are
90 /// transitioning into.
91 impl Default for SkipLeakCheck {
92 fn default() -> Self {
97 /// The mode that trait queries run in.
98 #[derive(Copy, Clone, PartialEq, Eq, Debug)]
99 pub enum TraitQueryMode {
100 // Standard/un-canonicalized queries get accurate
101 // spans etc. passed in and hence can do reasonable
102 // error reporting on their own.
104 // Canonicalized queries get dummy spans and hence
105 // must generally propagate errors to
106 // pre-canonicalization callsites.
110 /// Creates predicate obligations from the generic bounds.
111 pub fn predicates_for_generics<'tcx>(
112 cause: ObligationCause<'tcx>,
113 param_env: ty::ParamEnv<'tcx>,
114 generic_bounds: ty::InstantiatedPredicates<'tcx>,
115 ) -> impl Iterator<Item = PredicateObligation<'tcx>> {
116 util::predicates_for_generics(cause, 0, param_env, generic_bounds)
119 /// Determines whether the type `ty` is known to meet `bound` and
120 /// returns true if so. Returns false if `ty` either does not meet
121 /// `bound` or is not known to meet bound (note that this is
122 /// conservative towards *no impl*, which is the opposite of the
123 /// `evaluate` methods).
124 pub fn type_known_to_meet_bound_modulo_regions<'a, 'tcx>(
125 infcx: &InferCtxt<'a, 'tcx>,
126 param_env: ty::ParamEnv<'tcx>,
132 "type_known_to_meet_bound_modulo_regions(ty={:?}, bound={:?})",
134 infcx.tcx.def_path_str(def_id)
137 let trait_ref = ty::TraitRef { def_id, substs: infcx.tcx.mk_substs_trait(ty, &[]) };
138 let obligation = Obligation {
140 cause: ObligationCause::misc(span, hir::CRATE_HIR_ID),
142 predicate: trait_ref.without_const().to_predicate(infcx.tcx),
145 let result = infcx.predicate_must_hold_modulo_regions(&obligation);
147 "type_known_to_meet_ty={:?} bound={} => {:?}",
149 infcx.tcx.def_path_str(def_id),
153 if result && ty.has_infer_types_or_consts() {
154 // Because of inference "guessing", selection can sometimes claim
155 // to succeed while the success requires a guess. To ensure
156 // this function's result remains infallible, we must confirm
157 // that guess. While imperfect, I believe this is sound.
159 // The handling of regions in this area of the code is terrible,
160 // see issue #29149. We should be able to improve on this with
162 let mut fulfill_cx = FulfillmentContext::new_ignoring_regions();
164 // We can use a dummy node-id here because we won't pay any mind
165 // to region obligations that arise (there shouldn't really be any
167 let cause = ObligationCause::misc(span, hir::CRATE_HIR_ID);
169 fulfill_cx.register_bound(infcx, param_env, ty, def_id, cause);
171 // Note: we only assume something is `Copy` if we can
172 // *definitively* show that it implements `Copy`. Otherwise,
173 // assume it is move; linear is always ok.
174 match fulfill_cx.select_all_or_error(infcx) {
177 "type_known_to_meet_bound_modulo_regions: ty={:?} bound={} success",
179 infcx.tcx.def_path_str(def_id)
185 "type_known_to_meet_bound_modulo_regions: ty={:?} bound={} errors={:?}",
187 infcx.tcx.def_path_str(def_id),
198 fn do_normalize_predicates<'tcx>(
200 region_context: DefId,
201 cause: ObligationCause<'tcx>,
202 elaborated_env: ty::ParamEnv<'tcx>,
203 predicates: Vec<ty::Predicate<'tcx>>,
204 ) -> Result<Vec<ty::Predicate<'tcx>>, ErrorReported> {
206 "do_normalize_predicates(predicates={:?}, region_context={:?}, cause={:?})",
207 predicates, region_context, cause,
209 let span = cause.span;
210 tcx.infer_ctxt().enter(|infcx| {
211 // FIXME. We should really... do something with these region
212 // obligations. But this call just continues the older
213 // behavior (i.e., doesn't cause any new bugs), and it would
214 // take some further refactoring to actually solve them. In
215 // particular, we would have to handle implied bounds
216 // properly, and that code is currently largely confined to
217 // regionck (though I made some efforts to extract it
220 // @arielby: In any case, these obligations are checked
221 // by wfcheck anyway, so I'm not sure we have to check
222 // them here too, and we will remove this function when
223 // we move over to lazy normalization *anyway*.
224 let fulfill_cx = FulfillmentContext::new_ignoring_regions();
226 match fully_normalize(&infcx, fulfill_cx, cause, elaborated_env, &predicates) {
227 Ok(predicates) => predicates,
229 infcx.report_fulfillment_errors(&errors, None, false);
230 return Err(ErrorReported);
234 debug!("do_normalize_predictes: normalized predicates = {:?}", predicates);
236 // We can use the `elaborated_env` here; the region code only
237 // cares about declarations like `'a: 'b`.
238 let outlives_env = OutlivesEnvironment::new(elaborated_env);
240 infcx.resolve_regions_and_report_errors(
243 RegionckMode::default(),
246 let predicates = match infcx.fully_resolve(&predicates) {
247 Ok(predicates) => predicates,
249 // If we encounter a fixup error, it means that some type
250 // variable wound up unconstrained. I actually don't know
251 // if this can happen, and I certainly don't expect it to
252 // happen often, but if it did happen it probably
253 // represents a legitimate failure due to some kind of
254 // unconstrained variable, and it seems better not to ICE,
255 // all things considered.
256 tcx.sess.span_err(span, &fixup_err.to_string());
257 return Err(ErrorReported);
260 if predicates.needs_infer() {
261 tcx.sess.delay_span_bug(span, "encountered inference variables after `fully_resolve`");
269 // FIXME: this is gonna need to be removed ...
270 /// Normalizes the parameter environment, reporting errors if they occur.
271 pub fn normalize_param_env_or_error<'tcx>(
273 region_context: DefId,
274 unnormalized_env: ty::ParamEnv<'tcx>,
275 cause: ObligationCause<'tcx>,
276 ) -> ty::ParamEnv<'tcx> {
277 // I'm not wild about reporting errors here; I'd prefer to
278 // have the errors get reported at a defined place (e.g.,
279 // during typeck). Instead I have all parameter
280 // environments, in effect, going through this function
281 // and hence potentially reporting errors. This ensures of
282 // course that we never forget to normalize (the
283 // alternative seemed like it would involve a lot of
284 // manual invocations of this fn -- and then we'd have to
285 // deal with the errors at each of those sites).
287 // In any case, in practice, typeck constructs all the
288 // parameter environments once for every fn as it goes,
289 // and errors will get reported then; so after typeck we
290 // can be sure that no errors should occur.
293 "normalize_param_env_or_error(region_context={:?}, unnormalized_env={:?}, cause={:?})",
294 region_context, unnormalized_env, cause
297 let mut predicates: Vec<_> =
298 util::elaborate_predicates(tcx, unnormalized_env.caller_bounds().into_iter())
299 .map(|obligation| obligation.predicate)
302 debug!("normalize_param_env_or_error: elaborated-predicates={:?}", predicates);
304 let elaborated_env = ty::ParamEnv::new(
305 tcx.intern_predicates(&predicates),
306 unnormalized_env.reveal(),
307 unnormalized_env.def_id,
310 // HACK: we are trying to normalize the param-env inside *itself*. The problem is that
311 // normalization expects its param-env to be already normalized, which means we have
314 // The way we handle this is by normalizing the param-env inside an unnormalized version
315 // of the param-env, which means that if the param-env contains unnormalized projections,
316 // we'll have some normalization failures. This is unfortunate.
318 // Lazy normalization would basically handle this by treating just the
319 // normalizing-a-trait-ref-requires-itself cycles as evaluation failures.
321 // Inferred outlives bounds can create a lot of `TypeOutlives` predicates for associated
322 // types, so to make the situation less bad, we normalize all the predicates *but*
323 // the `TypeOutlives` predicates first inside the unnormalized parameter environment, and
324 // then we normalize the `TypeOutlives` bounds inside the normalized parameter environment.
326 // This works fairly well because trait matching does not actually care about param-env
327 // TypeOutlives predicates - these are normally used by regionck.
328 let outlives_predicates: Vec<_> = predicates
329 .drain_filter(|predicate| match predicate.skip_binders() {
330 ty::PredicateAtom::TypeOutlives(..) => true,
336 "normalize_param_env_or_error: predicates=(non-outlives={:?}, outlives={:?})",
337 predicates, outlives_predicates
339 let non_outlives_predicates = match do_normalize_predicates(
346 Ok(predicates) => predicates,
347 // An unnormalized env is better than nothing.
348 Err(ErrorReported) => {
349 debug!("normalize_param_env_or_error: errored resolving non-outlives predicates");
350 return elaborated_env;
354 debug!("normalize_param_env_or_error: non-outlives predicates={:?}", non_outlives_predicates);
356 // Not sure whether it is better to include the unnormalized TypeOutlives predicates
357 // here. I believe they should not matter, because we are ignoring TypeOutlives param-env
358 // predicates here anyway. Keeping them here anyway because it seems safer.
359 let outlives_env: Vec<_> =
360 non_outlives_predicates.iter().chain(&outlives_predicates).cloned().collect();
362 ty::ParamEnv::new(tcx.intern_predicates(&outlives_env), unnormalized_env.reveal(), None);
363 let outlives_predicates = match do_normalize_predicates(
370 Ok(predicates) => predicates,
371 // An unnormalized env is better than nothing.
372 Err(ErrorReported) => {
373 debug!("normalize_param_env_or_error: errored resolving outlives predicates");
374 return elaborated_env;
377 debug!("normalize_param_env_or_error: outlives predicates={:?}", outlives_predicates);
379 let mut predicates = non_outlives_predicates;
380 predicates.extend(outlives_predicates);
381 debug!("normalize_param_env_or_error: final predicates={:?}", predicates);
383 tcx.intern_predicates(&predicates),
384 unnormalized_env.reveal(),
385 unnormalized_env.def_id,
389 pub fn fully_normalize<'a, 'tcx, T>(
390 infcx: &InferCtxt<'a, 'tcx>,
391 mut fulfill_cx: FulfillmentContext<'tcx>,
392 cause: ObligationCause<'tcx>,
393 param_env: ty::ParamEnv<'tcx>,
395 ) -> Result<T, Vec<FulfillmentError<'tcx>>>
397 T: TypeFoldable<'tcx>,
399 debug!("fully_normalize_with_fulfillcx(value={:?})", value);
400 let selcx = &mut SelectionContext::new(infcx);
401 let Normalized { value: normalized_value, obligations } =
402 project::normalize(selcx, param_env, cause, value);
404 "fully_normalize: normalized_value={:?} obligations={:?}",
405 normalized_value, obligations
407 for obligation in obligations {
408 fulfill_cx.register_predicate_obligation(selcx.infcx(), obligation);
411 debug!("fully_normalize: select_all_or_error start");
412 fulfill_cx.select_all_or_error(infcx)?;
413 debug!("fully_normalize: select_all_or_error complete");
414 let resolved_value = infcx.resolve_vars_if_possible(&normalized_value);
415 debug!("fully_normalize: resolved_value={:?}", resolved_value);
419 /// Normalizes the predicates and checks whether they hold in an empty environment. If this
420 /// returns true, then either normalize encountered an error or one of the predicates did not
421 /// hold. Used when creating vtables to check for unsatisfiable methods.
422 pub fn impossible_predicates<'tcx>(
424 predicates: Vec<ty::Predicate<'tcx>>,
426 debug!("impossible_predicates(predicates={:?})", predicates);
428 let result = tcx.infer_ctxt().enter(|infcx| {
429 let param_env = ty::ParamEnv::reveal_all();
430 let mut selcx = SelectionContext::new(&infcx);
431 let mut fulfill_cx = FulfillmentContext::new();
432 let cause = ObligationCause::dummy();
433 let Normalized { value: predicates, obligations } =
434 normalize(&mut selcx, param_env, cause.clone(), &predicates);
435 for obligation in obligations {
436 fulfill_cx.register_predicate_obligation(&infcx, obligation);
438 for predicate in predicates {
439 let obligation = Obligation::new(cause.clone(), param_env, predicate);
440 fulfill_cx.register_predicate_obligation(&infcx, obligation);
443 fulfill_cx.select_all_or_error(&infcx).is_err()
445 debug!("impossible_predicates(predicates={:?}) = {:?}", predicates, result);
449 fn subst_and_check_impossible_predicates<'tcx>(
451 key: (DefId, SubstsRef<'tcx>),
453 debug!("subst_and_check_impossible_predicates(key={:?})", key);
455 let mut predicates = tcx.predicates_of(key.0).instantiate(tcx, key.1).predicates;
456 predicates.retain(|predicate| !predicate.needs_subst());
457 let result = impossible_predicates(tcx, predicates);
459 debug!("subst_and_check_impossible_predicates(key={:?}) = {:?}", key, result);
463 /// Given a trait `trait_ref`, iterates the vtable entries
464 /// that come from `trait_ref`, including its supertraits.
465 #[inline] // FIXME(#35870): avoid closures being unexported due to `impl Trait`.
466 fn vtable_methods<'tcx>(
468 trait_ref: ty::PolyTraitRef<'tcx>,
469 ) -> &'tcx [Option<(DefId, SubstsRef<'tcx>)>] {
470 debug!("vtable_methods({:?})", trait_ref);
472 tcx.arena.alloc_from_iter(supertraits(tcx, trait_ref).flat_map(move |trait_ref| {
473 let trait_methods = tcx
474 .associated_items(trait_ref.def_id())
475 .in_definition_order()
476 .filter(|item| item.kind == ty::AssocKind::Fn);
478 // Now list each method's DefId and InternalSubsts (for within its trait).
479 // If the method can never be called from this object, produce None.
480 trait_methods.map(move |trait_method| {
481 debug!("vtable_methods: trait_method={:?}", trait_method);
482 let def_id = trait_method.def_id;
484 // Some methods cannot be called on an object; skip those.
485 if !is_vtable_safe_method(tcx, trait_ref.def_id(), &trait_method) {
486 debug!("vtable_methods: not vtable safe");
490 // The method may have some early-bound lifetimes; add regions for those.
491 let substs = trait_ref.map_bound(|trait_ref| {
492 InternalSubsts::for_item(tcx, def_id, |param, _| match param.kind {
493 GenericParamDefKind::Lifetime => tcx.lifetimes.re_erased.into(),
494 GenericParamDefKind::Type { .. } | GenericParamDefKind::Const => {
495 trait_ref.substs[param.index as usize]
500 // The trait type may have higher-ranked lifetimes in it;
501 // erase them if they appear, so that we get the type
502 // at some particular call site.
504 tcx.normalize_erasing_late_bound_regions(ty::ParamEnv::reveal_all(), &substs);
506 // It's possible that the method relies on where-clauses that
507 // do not hold for this particular set of type parameters.
508 // Note that this method could then never be called, so we
509 // do not want to try and codegen it, in that case (see #23435).
510 let predicates = tcx.predicates_of(def_id).instantiate_own(tcx, substs);
511 if impossible_predicates(tcx, predicates.predicates) {
512 debug!("vtable_methods: predicates do not hold");
516 Some((def_id, substs))
521 /// Check whether a `ty` implements given trait(trait_def_id).
523 /// NOTE: Always return `false` for a type which needs inference.
524 fn type_implements_trait<'tcx>(
527 DefId, // trait_def_id,
533 let (trait_def_id, ty, params, param_env) = key;
536 "type_implements_trait: trait_def_id={:?}, type={:?}, params={:?}, param_env={:?}",
537 trait_def_id, ty, params, param_env
540 let trait_ref = ty::TraitRef { def_id: trait_def_id, substs: tcx.mk_substs_trait(ty, params) };
542 let obligation = Obligation {
543 cause: ObligationCause::dummy(),
546 predicate: trait_ref.without_const().to_predicate(tcx),
548 tcx.infer_ctxt().enter(|infcx| infcx.predicate_must_hold_modulo_regions(&obligation))
551 pub fn provide(providers: &mut ty::query::Providers) {
552 object_safety::provide(providers);
553 structural_match::provide(providers);
554 *providers = ty::query::Providers {
555 specialization_graph_of: specialize::specialization_graph_provider,
556 specializes: specialize::specializes,
557 codegen_fulfill_obligation: codegen::codegen_fulfill_obligation,
559 type_implements_trait,
560 subst_and_check_impossible_predicates,