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
9 pub mod const_evaluatable;
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, ToPredicate, Ty, TyCtxt, VtblEntry, WithConstness,
35 COMMON_VTABLE_ENTRIES,
41 pub use self::FulfillmentErrorCode::*;
42 pub use self::ImplSource::*;
43 pub use self::ObligationCauseCode::*;
44 pub use self::SelectionError::*;
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::{normalize, normalize_projection_type, normalize_to};
56 pub use self::select::{EvaluationCache, SelectionCache, SelectionContext};
57 pub use self::select::{EvaluationResult, IntercrateAmbiguityCause, OverflowError};
58 pub use self::specialize::specialization_graph::FutureCompatOverlapError;
59 pub use self::specialize::specialization_graph::FutureCompatOverlapErrorKind;
60 pub use self::specialize::{specialization_graph, translate_substs, OverlapError};
61 pub use self::structural_match::search_for_structural_match_violation;
62 pub use self::structural_match::NonStructuralMatchTy;
63 pub use self::util::{elaborate_predicates, elaborate_trait_ref, elaborate_trait_refs};
64 pub use self::util::{expand_trait_aliases, TraitAliasExpander};
66 get_vtable_index_of_object_method, impl_item_is_final, predicate_for_trait_def, upcast_choices,
69 supertrait_def_ids, supertraits, transitive_bounds, transitive_bounds_that_define_assoc_type,
70 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);
307 ty::ParamEnv::new(tcx.intern_predicates(&predicates), unnormalized_env.reveal());
309 // HACK: we are trying to normalize the param-env inside *itself*. The problem is that
310 // normalization expects its param-env to be already normalized, which means we have
313 // The way we handle this is by normalizing the param-env inside an unnormalized version
314 // of the param-env, which means that if the param-env contains unnormalized projections,
315 // we'll have some normalization failures. This is unfortunate.
317 // Lazy normalization would basically handle this by treating just the
318 // normalizing-a-trait-ref-requires-itself cycles as evaluation failures.
320 // Inferred outlives bounds can create a lot of `TypeOutlives` predicates for associated
321 // types, so to make the situation less bad, we normalize all the predicates *but*
322 // the `TypeOutlives` predicates first inside the unnormalized parameter environment, and
323 // then we normalize the `TypeOutlives` bounds inside the normalized parameter environment.
325 // This works fairly well because trait matching does not actually care about param-env
326 // TypeOutlives predicates - these are normally used by regionck.
327 let outlives_predicates: Vec<_> = predicates
328 .drain_filter(|predicate| {
329 matches!(predicate.kind().skip_binder(), ty::PredicateKind::TypeOutlives(..))
334 "normalize_param_env_or_error: predicates=(non-outlives={:?}, outlives={:?})",
335 predicates, outlives_predicates
337 let non_outlives_predicates = match do_normalize_predicates(
344 Ok(predicates) => predicates,
345 // An unnormalized env is better than nothing.
346 Err(ErrorReported) => {
347 debug!("normalize_param_env_or_error: errored resolving non-outlives predicates");
348 return elaborated_env;
352 debug!("normalize_param_env_or_error: non-outlives predicates={:?}", non_outlives_predicates);
354 // Not sure whether it is better to include the unnormalized TypeOutlives predicates
355 // here. I believe they should not matter, because we are ignoring TypeOutlives param-env
356 // predicates here anyway. Keeping them here anyway because it seems safer.
357 let outlives_env: Vec<_> =
358 non_outlives_predicates.iter().chain(&outlives_predicates).cloned().collect();
360 ty::ParamEnv::new(tcx.intern_predicates(&outlives_env), unnormalized_env.reveal());
361 let outlives_predicates = match do_normalize_predicates(
368 Ok(predicates) => predicates,
369 // An unnormalized env is better than nothing.
370 Err(ErrorReported) => {
371 debug!("normalize_param_env_or_error: errored resolving outlives predicates");
372 return elaborated_env;
375 debug!("normalize_param_env_or_error: outlives predicates={:?}", outlives_predicates);
377 let mut predicates = non_outlives_predicates;
378 predicates.extend(outlives_predicates);
379 debug!("normalize_param_env_or_error: final predicates={:?}", predicates);
380 ty::ParamEnv::new(tcx.intern_predicates(&predicates), unnormalized_env.reveal())
383 pub fn fully_normalize<'a, 'tcx, T>(
384 infcx: &InferCtxt<'a, 'tcx>,
385 mut fulfill_cx: FulfillmentContext<'tcx>,
386 cause: ObligationCause<'tcx>,
387 param_env: ty::ParamEnv<'tcx>,
389 ) -> Result<T, Vec<FulfillmentError<'tcx>>>
391 T: TypeFoldable<'tcx>,
393 debug!("fully_normalize_with_fulfillcx(value={:?})", value);
394 let selcx = &mut SelectionContext::new(infcx);
395 let Normalized { value: normalized_value, obligations } =
396 project::normalize(selcx, param_env, cause, value);
398 "fully_normalize: normalized_value={:?} obligations={:?}",
399 normalized_value, obligations
401 for obligation in obligations {
402 fulfill_cx.register_predicate_obligation(selcx.infcx(), obligation);
405 debug!("fully_normalize: select_all_or_error start");
406 fulfill_cx.select_all_or_error(infcx)?;
407 debug!("fully_normalize: select_all_or_error complete");
408 let resolved_value = infcx.resolve_vars_if_possible(normalized_value);
409 debug!("fully_normalize: resolved_value={:?}", resolved_value);
413 /// Normalizes the predicates and checks whether they hold in an empty environment. If this
414 /// returns true, then either normalize encountered an error or one of the predicates did not
415 /// hold. Used when creating vtables to check for unsatisfiable methods.
416 pub fn impossible_predicates<'tcx>(
418 predicates: Vec<ty::Predicate<'tcx>>,
420 debug!("impossible_predicates(predicates={:?})", predicates);
422 let result = tcx.infer_ctxt().enter(|infcx| {
423 let param_env = ty::ParamEnv::reveal_all();
424 let mut selcx = SelectionContext::new(&infcx);
425 let mut fulfill_cx = FulfillmentContext::new();
426 let cause = ObligationCause::dummy();
427 let Normalized { value: predicates, obligations } =
428 normalize(&mut selcx, param_env, cause.clone(), predicates);
429 for obligation in obligations {
430 fulfill_cx.register_predicate_obligation(&infcx, obligation);
432 for predicate in predicates {
433 let obligation = Obligation::new(cause.clone(), param_env, predicate);
434 fulfill_cx.register_predicate_obligation(&infcx, obligation);
437 fulfill_cx.select_all_or_error(&infcx).is_err()
439 debug!("impossible_predicates = {:?}", result);
443 fn subst_and_check_impossible_predicates<'tcx>(
445 key: (DefId, SubstsRef<'tcx>),
447 debug!("subst_and_check_impossible_predicates(key={:?})", key);
449 let mut predicates = tcx.predicates_of(key.0).instantiate(tcx, key.1).predicates;
450 predicates.retain(|predicate| !predicate.needs_subst());
451 let result = impossible_predicates(tcx, predicates);
453 debug!("subst_and_check_impossible_predicates(key={:?}) = {:?}", key, result);
457 /// Given a trait `trait_ref`, iterates the vtable entries
458 /// that come from `trait_ref`, including its supertraits.
459 fn vtable_entries<'tcx>(
461 trait_ref: ty::PolyTraitRef<'tcx>,
462 ) -> &'tcx [VtblEntry<'tcx>] {
463 debug!("vtable_entries({:?})", trait_ref);
465 let entries = COMMON_VTABLE_ENTRIES.iter().cloned().chain(
466 supertraits(tcx, trait_ref).flat_map(move |trait_ref| {
467 let trait_methods = tcx
468 .associated_items(trait_ref.def_id())
469 .in_definition_order()
470 .filter(|item| item.kind == ty::AssocKind::Fn);
472 // Now list each method's DefId and InternalSubsts (for within its trait).
473 // If the method can never be called from this object, produce `Vacant`.
474 trait_methods.map(move |trait_method| {
475 debug!("vtable_entries: trait_method={:?}", trait_method);
476 let def_id = trait_method.def_id;
478 // Some methods cannot be called on an object; skip those.
479 if !is_vtable_safe_method(tcx, trait_ref.def_id(), &trait_method) {
480 debug!("vtable_entries: not vtable safe");
481 return VtblEntry::Vacant;
484 // The method may have some early-bound lifetimes; add regions for those.
485 let substs = trait_ref.map_bound(|trait_ref| {
486 InternalSubsts::for_item(tcx, def_id, |param, _| match param.kind {
487 GenericParamDefKind::Lifetime => tcx.lifetimes.re_erased.into(),
488 GenericParamDefKind::Type { .. } | GenericParamDefKind::Const { .. } => {
489 trait_ref.substs[param.index as usize]
494 // The trait type may have higher-ranked lifetimes in it;
495 // erase them if they appear, so that we get the type
496 // at some particular call site.
498 tcx.normalize_erasing_late_bound_regions(ty::ParamEnv::reveal_all(), substs);
500 // It's possible that the method relies on where-clauses that
501 // do not hold for this particular set of type parameters.
502 // Note that this method could then never be called, so we
503 // do not want to try and codegen it, in that case (see #23435).
504 let predicates = tcx.predicates_of(def_id).instantiate_own(tcx, substs);
505 if impossible_predicates(tcx, predicates.predicates) {
506 debug!("vtable_entries: predicates do not hold");
507 return VtblEntry::Vacant;
510 VtblEntry::Method(def_id, substs)
515 tcx.arena.alloc_from_iter(entries)
518 /// Find slot base for trait methods within vtable entries of another trait
519 fn vtable_trait_first_method_offset<'tcx>(
522 ty::PolyTraitRef<'tcx>, // trait_to_be_found
523 ty::PolyTraitRef<'tcx>, // trait_owning_vtable
526 let (trait_to_be_found, trait_owning_vtable) = key;
528 let mut supertraits = util::supertraits(tcx, trait_owning_vtable);
530 // For each of the non-matching predicates that
531 // we pass over, we sum up the set of number of vtable
532 // entries, so that we can compute the offset for the selected
534 let vtable_base = ty::COMMON_VTABLE_ENTRIES.len()
537 .take_while(|t| *t != trait_to_be_found)
538 .map(|t| util::count_own_vtable_entries(tcx, t))
544 pub fn provide(providers: &mut ty::query::Providers) {
545 object_safety::provide(providers);
546 structural_match::provide(providers);
547 *providers = ty::query::Providers {
548 specialization_graph_of: specialize::specialization_graph_provider,
549 specializes: specialize::specializes,
550 codegen_fulfill_obligation: codegen::codegen_fulfill_obligation,
552 subst_and_check_impossible_predicates,
553 mir_abstract_const: |tcx, def_id| {
554 let def_id = def_id.expect_local();
555 if let Some(def) = ty::WithOptConstParam::try_lookup(def_id, tcx) {
556 tcx.mir_abstract_const_of_const_arg(def)
558 const_evaluatable::mir_abstract_const(tcx, ty::WithOptConstParam::unknown(def_id))
561 mir_abstract_const_of_const_arg: |tcx, (did, param_did)| {
562 const_evaluatable::mir_abstract_const(
564 ty::WithOptConstParam { did, const_param_did: Some(param_did) },
567 try_unify_abstract_consts: const_evaluatable::try_unify_abstract_consts,