1 //! Conversion from AST representation of types to the `ty.rs` representation.
2 //! The main routine here is `ast_ty_to_ty()`; each use is parameterized by an
3 //! instance of `AstConv`.
8 use crate::bounds::Bounds;
9 use crate::collect::PlaceholderHirTyCollector;
11 AmbiguousLifetimeBound, MultipleRelaxedDefaultBounds, TraitObjectDeclaredWithNoTraits,
12 TypeofReservedKeywordUsed, ValueOfAssociatedStructAlreadySpecified,
14 use crate::middle::resolve_lifetime as rl;
15 use crate::require_c_abi_if_c_variadic;
16 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
17 use rustc_errors::{struct_span_err, Applicability, ErrorReported, FatalError};
19 use rustc_hir::def::{CtorOf, DefKind, Namespace, Res};
20 use rustc_hir::def_id::{DefId, LocalDefId};
21 use rustc_hir::intravisit::{walk_generics, Visitor as _};
22 use rustc_hir::lang_items::LangItem;
23 use rustc_hir::{Constness, GenericArg, GenericArgs};
24 use rustc_middle::ty::subst::{self, InternalSubsts, Subst, SubstsRef};
25 use rustc_middle::ty::GenericParamDefKind;
26 use rustc_middle::ty::{self, Const, DefIdTree, Ty, TyCtxt, TypeFoldable};
27 use rustc_session::lint::builtin::AMBIGUOUS_ASSOCIATED_ITEMS;
28 use rustc_span::lev_distance::find_best_match_for_name;
29 use rustc_span::symbol::{Ident, Symbol};
30 use rustc_span::{Span, DUMMY_SP};
31 use rustc_target::spec::abi;
32 use rustc_trait_selection::traits;
33 use rustc_trait_selection::traits::astconv_object_safety_violations;
34 use rustc_trait_selection::traits::error_reporting::report_object_safety_error;
35 use rustc_trait_selection::traits::wf::object_region_bounds;
37 use smallvec::SmallVec;
39 use std::collections::BTreeSet;
43 pub struct PathSeg(pub DefId, pub usize);
45 pub trait AstConv<'tcx> {
46 fn tcx<'a>(&'a self) -> TyCtxt<'tcx>;
48 fn item_def_id(&self) -> Option<DefId>;
50 fn default_constness_for_trait_bounds(&self) -> Constness;
52 /// Returns predicates in scope of the form `X: Foo`, where `X` is
53 /// a type parameter `X` with the given id `def_id`. This is a
54 /// subset of the full set of predicates.
56 /// This is used for one specific purpose: resolving "short-hand"
57 /// associated type references like `T::Item`. In principle, we
58 /// would do that by first getting the full set of predicates in
59 /// scope and then filtering down to find those that apply to `T`,
60 /// but this can lead to cycle errors. The problem is that we have
61 /// to do this resolution *in order to create the predicates in
62 /// the first place*. Hence, we have this "special pass".
63 fn get_type_parameter_bounds(&self, span: Span, def_id: DefId) -> ty::GenericPredicates<'tcx>;
65 /// Returns the lifetime to use when a lifetime is omitted (and not elided).
66 fn re_infer(&self, param: Option<&ty::GenericParamDef>, span: Span)
67 -> Option<ty::Region<'tcx>>;
69 /// Returns the type to use when a type is omitted.
70 fn ty_infer(&self, param: Option<&ty::GenericParamDef>, span: Span) -> Ty<'tcx>;
72 /// Returns `true` if `_` is allowed in type signatures in the current context.
73 fn allow_ty_infer(&self) -> bool;
75 /// Returns the const to use when a const is omitted.
79 param: Option<&ty::GenericParamDef>,
81 ) -> &'tcx Const<'tcx>;
83 /// Projecting an associated type from a (potentially)
84 /// higher-ranked trait reference is more complicated, because of
85 /// the possibility of late-bound regions appearing in the
86 /// associated type binding. This is not legal in function
87 /// signatures for that reason. In a function body, we can always
88 /// handle it because we can use inference variables to remove the
89 /// late-bound regions.
90 fn projected_ty_from_poly_trait_ref(
94 item_segment: &hir::PathSegment<'_>,
95 poly_trait_ref: ty::PolyTraitRef<'tcx>,
98 /// Normalize an associated type coming from the user.
99 fn normalize_ty(&self, span: Span, ty: Ty<'tcx>) -> Ty<'tcx>;
101 /// Invoked when we encounter an error from some prior pass
102 /// (e.g., resolve) that is translated into a ty-error. This is
103 /// used to help suppress derived errors typeck might otherwise
105 fn set_tainted_by_errors(&self);
107 fn record_ty(&self, hir_id: hir::HirId, ty: Ty<'tcx>, span: Span);
110 pub enum SizedByDefault {
115 struct ConvertedBinding<'a, 'tcx> {
117 kind: ConvertedBindingKind<'a, 'tcx>,
121 enum ConvertedBindingKind<'a, 'tcx> {
123 Constraint(&'a [hir::GenericBound<'a>]),
126 /// New-typed boolean indicating whether explicit late-bound lifetimes
127 /// are present in a set of generic arguments.
129 /// For example if we have some method `fn f<'a>(&'a self)` implemented
130 /// for some type `T`, although `f` is generic in the lifetime `'a`, `'a`
131 /// is late-bound so should not be provided explicitly. Thus, if `f` is
132 /// instantiated with some generic arguments providing `'a` explicitly,
133 /// we taint those arguments with `ExplicitLateBound::Yes` so that we
134 /// can provide an appropriate diagnostic later.
135 #[derive(Copy, Clone, PartialEq)]
136 pub enum ExplicitLateBound {
141 /// Denotes the "position" of a generic argument, indicating if it is a generic type,
142 /// generic function or generic method call.
143 #[derive(Copy, Clone, PartialEq)]
144 pub(crate) enum GenericArgPosition {
146 Value, // e.g., functions
150 /// A marker denoting that the generic arguments that were
151 /// provided did not match the respective generic parameters.
152 #[derive(Clone, Default)]
153 pub struct GenericArgCountMismatch {
154 /// Indicates whether a fatal error was reported (`Some`), or just a lint (`None`).
155 pub reported: Option<ErrorReported>,
156 /// A list of spans of arguments provided that were not valid.
157 pub invalid_args: Vec<Span>,
160 /// Decorates the result of a generic argument count mismatch
161 /// check with whether explicit late bounds were provided.
163 pub struct GenericArgCountResult {
164 pub explicit_late_bound: ExplicitLateBound,
165 pub correct: Result<(), GenericArgCountMismatch>,
168 pub trait CreateSubstsForGenericArgsCtxt<'a, 'tcx> {
169 fn args_for_def_id(&mut self, def_id: DefId) -> (Option<&'a GenericArgs<'a>>, bool);
173 param: &ty::GenericParamDef,
174 arg: &GenericArg<'_>,
175 ) -> subst::GenericArg<'tcx>;
179 substs: Option<&[subst::GenericArg<'tcx>]>,
180 param: &ty::GenericParamDef,
182 ) -> subst::GenericArg<'tcx>;
185 impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
186 pub fn ast_region_to_region(
188 lifetime: &hir::Lifetime,
189 def: Option<&ty::GenericParamDef>,
190 ) -> ty::Region<'tcx> {
191 let tcx = self.tcx();
192 let lifetime_name = |def_id| tcx.hir().name(tcx.hir().local_def_id_to_hir_id(def_id));
194 let r = match tcx.named_region(lifetime.hir_id) {
195 Some(rl::Region::Static) => tcx.lifetimes.re_static,
197 Some(rl::Region::LateBound(debruijn, id, _)) => {
198 let name = lifetime_name(id.expect_local());
199 tcx.mk_region(ty::ReLateBound(debruijn, ty::BrNamed(id, name)))
202 Some(rl::Region::LateBoundAnon(debruijn, index)) => {
203 tcx.mk_region(ty::ReLateBound(debruijn, ty::BrAnon(index)))
206 Some(rl::Region::EarlyBound(index, id, _)) => {
207 let name = lifetime_name(id.expect_local());
208 tcx.mk_region(ty::ReEarlyBound(ty::EarlyBoundRegion { def_id: id, index, name }))
211 Some(rl::Region::Free(scope, id)) => {
212 let name = lifetime_name(id.expect_local());
213 tcx.mk_region(ty::ReFree(ty::FreeRegion {
215 bound_region: ty::BrNamed(id, name),
218 // (*) -- not late-bound, won't change
222 self.re_infer(def, lifetime.span).unwrap_or_else(|| {
223 // This indicates an illegal lifetime
224 // elision. `resolve_lifetime` should have
225 // reported an error in this case -- but if
226 // not, let's error out.
227 tcx.sess.delay_span_bug(lifetime.span, "unelided lifetime in signature");
229 // Supply some dummy value. We don't have an
230 // `re_error`, annoyingly, so use `'static`.
231 tcx.lifetimes.re_static
236 debug!("ast_region_to_region(lifetime={:?}) yields {:?}", lifetime, r);
241 /// Given a path `path` that refers to an item `I` with the declared generics `decl_generics`,
242 /// returns an appropriate set of substitutions for this particular reference to `I`.
243 pub fn ast_path_substs_for_ty(
247 item_segment: &hir::PathSegment<'_>,
248 ) -> SubstsRef<'tcx> {
249 let (substs, assoc_bindings, _) = self.create_substs_for_ast_path(
253 item_segment.generic_args(),
254 item_segment.infer_args,
258 if let Some(b) = assoc_bindings.first() {
259 Self::prohibit_assoc_ty_binding(self.tcx(), b.span);
265 /// Given the type/lifetime/const arguments provided to some path (along with
266 /// an implicit `Self`, if this is a trait reference), returns the complete
267 /// set of substitutions. This may involve applying defaulted type parameters.
268 /// Also returns back constraints on associated types.
273 /// T: std::ops::Index<usize, Output = u32>
274 /// ^1 ^^^^^^^^^^^^^^2 ^^^^3 ^^^^^^^^^^^4
277 /// 1. The `self_ty` here would refer to the type `T`.
278 /// 2. The path in question is the path to the trait `std::ops::Index`,
279 /// which will have been resolved to a `def_id`
280 /// 3. The `generic_args` contains info on the `<...>` contents. The `usize` type
281 /// parameters are returned in the `SubstsRef`, the associated type bindings like
282 /// `Output = u32` are returned in the `Vec<ConvertedBinding...>` result.
284 /// Note that the type listing given here is *exactly* what the user provided.
286 /// For (generic) associated types
289 /// <Vec<u8> as Iterable<u8>>::Iter::<'a>
292 /// We have the parent substs are the substs for the parent trait:
293 /// `[Vec<u8>, u8]` and `generic_args` are the arguments for the associated
294 /// type itself: `['a]`. The returned `SubstsRef` concatenates these two
295 /// lists: `[Vec<u8>, u8, 'a]`.
296 fn create_substs_for_ast_path<'a>(
300 parent_substs: &[subst::GenericArg<'tcx>],
301 generic_args: &'a hir::GenericArgs<'_>,
303 self_ty: Option<Ty<'tcx>>,
304 ) -> (SubstsRef<'tcx>, Vec<ConvertedBinding<'a, 'tcx>>, GenericArgCountResult) {
305 // If the type is parameterized by this region, then replace this
306 // region with the current anon region binding (in other words,
307 // whatever & would get replaced with).
309 "create_substs_for_ast_path(def_id={:?}, self_ty={:?}, \
311 def_id, self_ty, generic_args
314 let tcx = self.tcx();
315 let generic_params = tcx.generics_of(def_id);
317 if generic_params.has_self {
318 if generic_params.parent.is_some() {
319 // The parent is a trait so it should have at least one subst
320 // for the `Self` type.
321 assert!(!parent_substs.is_empty())
323 // This item (presumably a trait) needs a self-type.
324 assert!(self_ty.is_some());
327 assert!(self_ty.is_none() && parent_substs.is_empty());
330 let arg_count = Self::check_generic_arg_count(
335 GenericArgPosition::Type,
340 // Skip processing if type has no generic parameters.
341 // Traits always have `Self` as a generic parameter, which means they will not return early
342 // here and so associated type bindings will be handled regardless of whether there are any
343 // non-`Self` generic parameters.
344 if generic_params.params.len() == 0 {
345 return (tcx.intern_substs(&[]), vec![], arg_count);
348 let is_object = self_ty.map_or(false, |ty| ty == self.tcx().types.trait_object_dummy_self);
350 struct SubstsForAstPathCtxt<'a, 'tcx> {
351 astconv: &'a (dyn AstConv<'tcx> + 'a),
353 generic_args: &'a GenericArgs<'a>,
355 missing_type_params: Vec<String>,
356 inferred_params: Vec<Span>,
361 impl<'tcx, 'a> SubstsForAstPathCtxt<'tcx, 'a> {
362 fn default_needs_object_self(&mut self, param: &ty::GenericParamDef) -> bool {
363 let tcx = self.astconv.tcx();
364 if let GenericParamDefKind::Type { has_default, .. } = param.kind {
365 if self.is_object && has_default {
366 let default_ty = tcx.at(self.span).type_of(param.def_id);
367 let self_param = tcx.types.self_param;
368 if default_ty.walk().any(|arg| arg == self_param.into()) {
369 // There is no suitable inference default for a type parameter
370 // that references self, in an object type.
380 impl<'a, 'tcx> CreateSubstsForGenericArgsCtxt<'a, 'tcx> for SubstsForAstPathCtxt<'a, 'tcx> {
381 fn args_for_def_id(&mut self, did: DefId) -> (Option<&'a GenericArgs<'a>>, bool) {
382 if did == self.def_id {
383 (Some(self.generic_args), self.infer_args)
385 // The last component of this tuple is unimportant.
392 param: &ty::GenericParamDef,
393 arg: &GenericArg<'_>,
394 ) -> subst::GenericArg<'tcx> {
395 let tcx = self.astconv.tcx();
396 match (¶m.kind, arg) {
397 (GenericParamDefKind::Lifetime, GenericArg::Lifetime(lt)) => {
398 self.astconv.ast_region_to_region(<, Some(param)).into()
400 (&GenericParamDefKind::Type { has_default, .. }, GenericArg::Type(ty)) => {
402 tcx.check_optional_stability(
407 // Default generic parameters may not be marked
408 // with stability attributes, i.e. when the
409 // default parameter was defined at the same time
410 // as the rest of the type. As such, we ignore missing
411 // stability attributes.
415 if let (hir::TyKind::Infer, false) =
416 (&ty.kind, self.astconv.allow_ty_infer())
418 self.inferred_params.push(ty.span);
419 tcx.ty_error().into()
421 self.astconv.ast_ty_to_ty(&ty).into()
424 (GenericParamDefKind::Const, GenericArg::Const(ct)) => {
425 ty::Const::from_opt_const_arg_anon_const(
427 ty::WithOptConstParam {
428 did: tcx.hir().local_def_id(ct.value.hir_id),
429 const_param_did: Some(param.def_id),
440 substs: Option<&[subst::GenericArg<'tcx>]>,
441 param: &ty::GenericParamDef,
443 ) -> subst::GenericArg<'tcx> {
444 let tcx = self.astconv.tcx();
446 GenericParamDefKind::Lifetime => tcx.lifetimes.re_static.into(),
447 GenericParamDefKind::Type { has_default, .. } => {
448 if !infer_args && has_default {
449 // No type parameter provided, but a default exists.
451 // If we are converting an object type, then the
452 // `Self` parameter is unknown. However, some of the
453 // other type parameters may reference `Self` in their
454 // defaults. This will lead to an ICE if we are not
456 if self.default_needs_object_self(param) {
457 self.missing_type_params.push(param.name.to_string());
458 tcx.ty_error().into()
460 // This is a default type parameter.
464 tcx.at(self.span).type_of(param.def_id).subst_spanned(
472 } else if infer_args {
473 // No type parameters were provided, we can infer all.
474 let param = if !self.default_needs_object_self(param) {
479 self.astconv.ty_infer(param, self.span).into()
481 // We've already errored above about the mismatch.
482 tcx.ty_error().into()
485 GenericParamDefKind::Const => {
486 let ty = tcx.at(self.span).type_of(param.def_id);
487 // FIXME(const_generics:defaults)
489 // No const parameters were provided, we can infer all.
490 self.astconv.ct_infer(ty, Some(param), self.span).into()
492 // We've already errored above about the mismatch.
493 tcx.const_error(ty).into()
500 let mut substs_ctx = SubstsForAstPathCtxt {
505 missing_type_params: vec![],
506 inferred_params: vec![],
510 let substs = Self::create_substs_for_generic_args(
520 self.complain_about_missing_type_params(
521 substs_ctx.missing_type_params,
524 generic_args.args.is_empty(),
527 // Convert associated-type bindings or constraints into a separate vector.
528 // Example: Given this:
530 // T: Iterator<Item = u32>
532 // The `T` is passed in as a self-type; the `Item = u32` is
533 // not a "type parameter" of the `Iterator` trait, but rather
534 // a restriction on `<T as Iterator>::Item`, so it is passed
536 let assoc_bindings = generic_args
540 let kind = match binding.kind {
541 hir::TypeBindingKind::Equality { ref ty } => {
542 ConvertedBindingKind::Equality(self.ast_ty_to_ty(ty))
544 hir::TypeBindingKind::Constraint { ref bounds } => {
545 ConvertedBindingKind::Constraint(bounds)
548 ConvertedBinding { item_name: binding.ident, kind, span: binding.span }
553 "create_substs_for_ast_path(generic_params={:?}, self_ty={:?}) -> {:?}",
554 generic_params, self_ty, substs
557 (substs, assoc_bindings, arg_count)
560 crate fn create_substs_for_associated_item(
565 item_segment: &hir::PathSegment<'_>,
566 parent_substs: SubstsRef<'tcx>,
567 ) -> SubstsRef<'tcx> {
568 if tcx.generics_of(item_def_id).params.is_empty() {
569 self.prohibit_generics(slice::from_ref(item_segment));
573 self.create_substs_for_ast_path(
577 item_segment.generic_args(),
578 item_segment.infer_args,
585 /// Instantiates the path for the given trait reference, assuming that it's
586 /// bound to a valid trait type. Returns the `DefId` of the defining trait.
587 /// The type _cannot_ be a type other than a trait type.
589 /// If the `projections` argument is `None`, then assoc type bindings like `Foo<T = X>`
590 /// are disallowed. Otherwise, they are pushed onto the vector given.
591 pub fn instantiate_mono_trait_ref(
593 trait_ref: &hir::TraitRef<'_>,
595 ) -> ty::TraitRef<'tcx> {
596 self.prohibit_generics(trait_ref.path.segments.split_last().unwrap().1);
598 self.ast_path_to_mono_trait_ref(
600 trait_ref.trait_def_id().unwrap_or_else(|| FatalError.raise()),
602 trait_ref.path.segments.last().unwrap(),
606 /// The given trait-ref must actually be a trait.
607 pub(super) fn instantiate_poly_trait_ref_inner(
609 trait_ref: &hir::TraitRef<'_>,
611 constness: Constness,
613 bounds: &mut Bounds<'tcx>,
615 ) -> GenericArgCountResult {
616 let trait_def_id = trait_ref.trait_def_id().unwrap_or_else(|| FatalError.raise());
618 debug!("instantiate_poly_trait_ref({:?}, def_id={:?})", trait_ref, trait_def_id);
620 self.prohibit_generics(trait_ref.path.segments.split_last().unwrap().1);
622 let (substs, assoc_bindings, arg_count) = self.create_substs_for_ast_trait_ref(
626 trait_ref.path.segments.last().unwrap(),
628 let poly_trait_ref = ty::Binder::bind(ty::TraitRef::new(trait_def_id, substs));
630 bounds.trait_bounds.push((poly_trait_ref, span, constness));
632 let mut dup_bindings = FxHashMap::default();
633 for binding in &assoc_bindings {
634 // Specify type to assert that error was already reported in `Err` case.
635 let _: Result<_, ErrorReported> = self.add_predicates_for_ast_type_binding(
636 trait_ref.hir_ref_id,
644 // Okay to ignore `Err` because of `ErrorReported` (see above).
648 "instantiate_poly_trait_ref({:?}, bounds={:?}) -> {:?}",
649 trait_ref, bounds, poly_trait_ref
655 /// Given a trait bound like `Debug`, applies that trait bound the given self-type to construct
656 /// a full trait reference. The resulting trait reference is returned. This may also generate
657 /// auxiliary bounds, which are added to `bounds`.
662 /// poly_trait_ref = Iterator<Item = u32>
666 /// this would return `Foo: Iterator` and add `<Foo as Iterator>::Item = u32` into `bounds`.
668 /// **A note on binders:** against our usual convention, there is an implied bounder around
669 /// the `self_ty` and `poly_trait_ref` parameters here. So they may reference bound regions.
670 /// If for example you had `for<'a> Foo<'a>: Bar<'a>`, then the `self_ty` would be `Foo<'a>`
671 /// where `'a` is a bound region at depth 0. Similarly, the `poly_trait_ref` would be
672 /// `Bar<'a>`. The returned poly-trait-ref will have this binder instantiated explicitly,
674 pub fn instantiate_poly_trait_ref(
676 poly_trait_ref: &hir::PolyTraitRef<'_>,
677 constness: Constness,
679 bounds: &mut Bounds<'tcx>,
680 ) -> GenericArgCountResult {
681 self.instantiate_poly_trait_ref_inner(
682 &poly_trait_ref.trait_ref,
691 pub fn instantiate_lang_item_trait_ref(
693 lang_item: hir::LangItem,
696 args: &GenericArgs<'_>,
698 bounds: &mut Bounds<'tcx>,
700 let trait_def_id = self.tcx().require_lang_item(lang_item, Some(span));
702 let (substs, assoc_bindings, _) =
703 self.create_substs_for_ast_path(span, trait_def_id, &[], args, false, Some(self_ty));
704 let poly_trait_ref = ty::Binder::bind(ty::TraitRef::new(trait_def_id, substs));
705 bounds.trait_bounds.push((poly_trait_ref, span, Constness::NotConst));
707 let mut dup_bindings = FxHashMap::default();
708 for binding in assoc_bindings {
709 let _: Result<_, ErrorReported> = self.add_predicates_for_ast_type_binding(
721 fn ast_path_to_mono_trait_ref(
726 trait_segment: &hir::PathSegment<'_>,
727 ) -> ty::TraitRef<'tcx> {
728 let (substs, assoc_bindings, _) =
729 self.create_substs_for_ast_trait_ref(span, trait_def_id, self_ty, trait_segment);
730 if let Some(b) = assoc_bindings.first() {
731 Self::prohibit_assoc_ty_binding(self.tcx(), b.span);
733 ty::TraitRef::new(trait_def_id, substs)
736 fn create_substs_for_ast_trait_ref<'a>(
741 trait_segment: &'a hir::PathSegment<'a>,
742 ) -> (SubstsRef<'tcx>, Vec<ConvertedBinding<'a, 'tcx>>, GenericArgCountResult) {
743 debug!("create_substs_for_ast_trait_ref(trait_segment={:?})", trait_segment);
745 self.complain_about_internal_fn_trait(span, trait_def_id, trait_segment);
747 self.create_substs_for_ast_path(
751 trait_segment.generic_args(),
752 trait_segment.infer_args,
757 fn trait_defines_associated_type_named(&self, trait_def_id: DefId, assoc_name: Ident) -> bool {
759 .associated_items(trait_def_id)
760 .find_by_name_and_kind(self.tcx(), assoc_name, ty::AssocKind::Type, trait_def_id)
764 // Returns `true` if a bounds list includes `?Sized`.
765 pub fn is_unsized(&self, ast_bounds: &[hir::GenericBound<'_>], span: Span) -> bool {
766 let tcx = self.tcx();
768 // Try to find an unbound in bounds.
769 let mut unbound = None;
770 for ab in ast_bounds {
771 if let &hir::GenericBound::Trait(ref ptr, hir::TraitBoundModifier::Maybe) = ab {
772 if unbound.is_none() {
773 unbound = Some(&ptr.trait_ref);
775 tcx.sess.emit_err(MultipleRelaxedDefaultBounds { span });
780 let kind_id = tcx.lang_items().require(LangItem::Sized);
783 // FIXME(#8559) currently requires the unbound to be built-in.
784 if let Ok(kind_id) = kind_id {
785 if tpb.path.res != Res::Def(DefKind::Trait, kind_id) {
788 "default bound relaxed for a type parameter, but \
789 this does nothing because the given bound is not \
790 a default; only `?Sized` is supported",
795 _ if kind_id.is_ok() => {
798 // No lang item for `Sized`, so we can't add it as a bound.
805 /// This helper takes a *converted* parameter type (`param_ty`)
806 /// and an *unconverted* list of bounds:
810 /// ^ ^^^^^ `ast_bounds` parameter, in HIR form
812 /// `param_ty`, in ty form
815 /// It adds these `ast_bounds` into the `bounds` structure.
817 /// **A note on binders:** there is an implied binder around
818 /// `param_ty` and `ast_bounds`. See `instantiate_poly_trait_ref`
819 /// for more details.
823 ast_bounds: &[hir::GenericBound<'_>],
824 bounds: &mut Bounds<'tcx>,
826 let constness = self.default_constness_for_trait_bounds();
827 for ast_bound in ast_bounds {
829 hir::GenericBound::Trait(ref b, hir::TraitBoundModifier::None) => {
830 self.instantiate_poly_trait_ref(b, constness, param_ty, bounds);
832 hir::GenericBound::Trait(ref b, hir::TraitBoundModifier::MaybeConst) => {
833 self.instantiate_poly_trait_ref(b, Constness::NotConst, param_ty, bounds);
835 hir::GenericBound::Trait(_, hir::TraitBoundModifier::Maybe) => {}
836 hir::GenericBound::LangItemTrait(lang_item, span, hir_id, args) => self
837 .instantiate_lang_item_trait_ref(
838 lang_item, span, hir_id, args, param_ty, bounds,
840 hir::GenericBound::Outlives(ref l) => {
841 bounds.region_bounds.push((self.ast_region_to_region(l, None), l.span))
847 /// Translates a list of bounds from the HIR into the `Bounds` data structure.
848 /// The self-type for the bounds is given by `param_ty`.
853 /// fn foo<T: Bar + Baz>() { }
854 /// ^ ^^^^^^^^^ ast_bounds
858 /// The `sized_by_default` parameter indicates if, in this context, the `param_ty` should be
859 /// considered `Sized` unless there is an explicit `?Sized` bound. This would be true in the
860 /// example above, but is not true in supertrait listings like `trait Foo: Bar + Baz`.
862 /// `span` should be the declaration size of the parameter.
863 pub fn compute_bounds(
866 ast_bounds: &[hir::GenericBound<'_>],
867 sized_by_default: SizedByDefault,
870 let mut bounds = Bounds::default();
872 self.add_bounds(param_ty, ast_bounds, &mut bounds);
873 bounds.trait_bounds.sort_by_key(|(t, _, _)| t.def_id());
875 bounds.implicitly_sized = if let SizedByDefault::Yes = sized_by_default {
876 if !self.is_unsized(ast_bounds, span) { Some(span) } else { None }
884 /// Given an HIR binding like `Item = Foo` or `Item: Foo`, pushes the corresponding predicates
887 /// **A note on binders:** given something like `T: for<'a> Iterator<Item = &'a u32>`, the
888 /// `trait_ref` here will be `for<'a> T: Iterator`. The `binding` data however is from *inside*
889 /// the binder (e.g., `&'a u32`) and hence may reference bound regions.
890 fn add_predicates_for_ast_type_binding(
892 hir_ref_id: hir::HirId,
893 trait_ref: ty::PolyTraitRef<'tcx>,
894 binding: &ConvertedBinding<'_, 'tcx>,
895 bounds: &mut Bounds<'tcx>,
897 dup_bindings: &mut FxHashMap<DefId, Span>,
899 ) -> Result<(), ErrorReported> {
900 let tcx = self.tcx();
903 // Given something like `U: SomeTrait<T = X>`, we want to produce a
904 // predicate like `<U as SomeTrait>::T = X`. This is somewhat
905 // subtle in the event that `T` is defined in a supertrait of
906 // `SomeTrait`, because in that case we need to upcast.
908 // That is, consider this case:
911 // trait SubTrait: SuperTrait<i32> { }
912 // trait SuperTrait<A> { type T; }
914 // ... B: SubTrait<T = foo> ...
917 // We want to produce `<B as SuperTrait<i32>>::T == foo`.
919 // Find any late-bound regions declared in `ty` that are not
920 // declared in the trait-ref. These are not well-formed.
924 // for<'a> <T as Iterator>::Item = &'a str // <-- 'a is bad
925 // for<'a> <T as FnMut<(&'a u32,)>>::Output = &'a str // <-- 'a is ok
926 if let ConvertedBindingKind::Equality(ty) = binding.kind {
927 let late_bound_in_trait_ref =
928 tcx.collect_constrained_late_bound_regions(&trait_ref);
929 let late_bound_in_ty =
930 tcx.collect_referenced_late_bound_regions(&ty::Binder::bind(ty));
931 debug!("late_bound_in_trait_ref = {:?}", late_bound_in_trait_ref);
932 debug!("late_bound_in_ty = {:?}", late_bound_in_ty);
934 // FIXME: point at the type params that don't have appropriate lifetimes:
935 // struct S1<F: for<'a> Fn(&i32, &i32) -> &'a i32>(F);
937 self.validate_late_bound_regions(
938 late_bound_in_trait_ref,
945 "binding for associated type `{}` references {}, \
946 which does not appear in the trait input types",
956 if self.trait_defines_associated_type_named(trait_ref.def_id(), binding.item_name) {
957 // Simple case: X is defined in the current trait.
960 // Otherwise, we have to walk through the supertraits to find
962 self.one_bound_for_assoc_type(
963 || traits::supertraits(tcx, trait_ref),
964 || trait_ref.print_only_trait_path().to_string(),
967 || match binding.kind {
968 ConvertedBindingKind::Equality(ty) => Some(ty.to_string()),
974 let (assoc_ident, def_scope) =
975 tcx.adjust_ident_and_get_scope(binding.item_name, candidate.def_id(), hir_ref_id);
977 // We have already adjusted the item name above, so compare with `ident.normalize_to_macros_2_0()` instead
978 // of calling `filter_by_name_and_kind`.
980 .associated_items(candidate.def_id())
981 .filter_by_name_unhygienic(assoc_ident.name)
983 i.kind == ty::AssocKind::Type && i.ident.normalize_to_macros_2_0() == assoc_ident
985 .expect("missing associated type");
987 if !assoc_ty.vis.is_accessible_from(def_scope, tcx) {
991 &format!("associated type `{}` is private", binding.item_name),
993 .span_label(binding.span, "private associated type")
996 tcx.check_stability(assoc_ty.def_id, Some(hir_ref_id), binding.span);
1000 .entry(assoc_ty.def_id)
1001 .and_modify(|prev_span| {
1002 self.tcx().sess.emit_err(ValueOfAssociatedStructAlreadySpecified {
1004 prev_span: *prev_span,
1005 item_name: binding.item_name,
1006 def_path: tcx.def_path_str(assoc_ty.container.id()),
1009 .or_insert(binding.span);
1012 match binding.kind {
1013 ConvertedBindingKind::Equality(ref ty) => {
1014 // "Desugar" a constraint like `T: Iterator<Item = u32>` this to
1015 // the "projection predicate" for:
1017 // `<T as Iterator>::Item = u32`
1018 bounds.projection_bounds.push((
1019 candidate.map_bound(|trait_ref| ty::ProjectionPredicate {
1020 projection_ty: ty::ProjectionTy::from_ref_and_name(
1030 ConvertedBindingKind::Constraint(ast_bounds) => {
1031 // "Desugar" a constraint like `T: Iterator<Item: Debug>` to
1033 // `<T as Iterator>::Item: Debug`
1035 // Calling `skip_binder` is okay, because `add_bounds` expects the `param_ty`
1036 // parameter to have a skipped binder.
1037 let param_ty = tcx.mk_projection(assoc_ty.def_id, candidate.skip_binder().substs);
1038 self.add_bounds(param_ty, ast_bounds, bounds);
1048 item_segment: &hir::PathSegment<'_>,
1050 let substs = self.ast_path_substs_for_ty(span, did, item_segment);
1051 self.normalize_ty(span, self.tcx().at(span).type_of(did).subst(self.tcx(), substs))
1054 fn conv_object_ty_poly_trait_ref(
1057 trait_bounds: &[hir::PolyTraitRef<'_>],
1058 lifetime: &hir::Lifetime,
1061 let tcx = self.tcx();
1063 let mut bounds = Bounds::default();
1064 let mut potential_assoc_types = Vec::new();
1065 let dummy_self = self.tcx().types.trait_object_dummy_self;
1066 for trait_bound in trait_bounds.iter().rev() {
1067 if let GenericArgCountResult {
1069 Err(GenericArgCountMismatch { invalid_args: cur_potential_assoc_types, .. }),
1071 } = self.instantiate_poly_trait_ref(
1073 Constness::NotConst,
1077 potential_assoc_types.extend(cur_potential_assoc_types.into_iter());
1081 // Expand trait aliases recursively and check that only one regular (non-auto) trait
1082 // is used and no 'maybe' bounds are used.
1083 let expanded_traits =
1084 traits::expand_trait_aliases(tcx, bounds.trait_bounds.iter().map(|&(a, b, _)| (a, b)));
1085 let (mut auto_traits, regular_traits): (Vec<_>, Vec<_>) =
1086 expanded_traits.partition(|i| tcx.trait_is_auto(i.trait_ref().def_id()));
1087 if regular_traits.len() > 1 {
1088 let first_trait = ®ular_traits[0];
1089 let additional_trait = ®ular_traits[1];
1090 let mut err = struct_span_err!(
1092 additional_trait.bottom().1,
1094 "only auto traits can be used as additional traits in a trait object"
1096 additional_trait.label_with_exp_info(
1098 "additional non-auto trait",
1101 first_trait.label_with_exp_info(&mut err, "first non-auto trait", "first use");
1103 "consider creating a new trait with all of these as super-traits and using that \
1104 trait here instead: `trait NewTrait: {} {{}}`",
1107 .map(|t| t.trait_ref().print_only_trait_path().to_string())
1108 .collect::<Vec<_>>()
1112 "auto-traits like `Send` and `Sync` are traits that have special properties; \
1113 for more information on them, visit \
1114 <https://doc.rust-lang.org/reference/special-types-and-traits.html#auto-traits>",
1119 if regular_traits.is_empty() && auto_traits.is_empty() {
1120 tcx.sess.emit_err(TraitObjectDeclaredWithNoTraits { span });
1121 return tcx.ty_error();
1124 // Check that there are no gross object safety violations;
1125 // most importantly, that the supertraits don't contain `Self`,
1127 for item in ®ular_traits {
1128 let object_safety_violations =
1129 astconv_object_safety_violations(tcx, item.trait_ref().def_id());
1130 if !object_safety_violations.is_empty() {
1131 report_object_safety_error(
1134 item.trait_ref().def_id(),
1135 &object_safety_violations[..],
1138 return tcx.ty_error();
1142 // Use a `BTreeSet` to keep output in a more consistent order.
1143 let mut associated_types: FxHashMap<Span, BTreeSet<DefId>> = FxHashMap::default();
1145 let regular_traits_refs_spans = bounds
1148 .filter(|(trait_ref, _, _)| !tcx.trait_is_auto(trait_ref.def_id()));
1150 for (base_trait_ref, span, constness) in regular_traits_refs_spans {
1151 assert_eq!(constness, Constness::NotConst);
1153 for obligation in traits::elaborate_trait_ref(tcx, base_trait_ref) {
1155 "conv_object_ty_poly_trait_ref: observing object predicate `{:?}`",
1156 obligation.predicate
1159 let bound_predicate = obligation.predicate.bound_atom();
1160 match bound_predicate.skip_binder() {
1161 ty::PredicateAtom::Trait(pred, _) => {
1162 let pred = bound_predicate.rebind(pred);
1163 associated_types.entry(span).or_default().extend(
1164 tcx.associated_items(pred.def_id())
1165 .in_definition_order()
1166 .filter(|item| item.kind == ty::AssocKind::Type)
1167 .map(|item| item.def_id),
1170 ty::PredicateAtom::Projection(pred) => {
1171 let pred = bound_predicate.rebind(pred);
1172 // A `Self` within the original bound will be substituted with a
1173 // `trait_object_dummy_self`, so check for that.
1174 let references_self =
1175 pred.skip_binder().ty.walk().any(|arg| arg == dummy_self.into());
1177 // If the projection output contains `Self`, force the user to
1178 // elaborate it explicitly to avoid a lot of complexity.
1180 // The "classicaly useful" case is the following:
1182 // trait MyTrait: FnMut() -> <Self as MyTrait>::MyOutput {
1187 // Here, the user could theoretically write `dyn MyTrait<Output = X>`,
1188 // but actually supporting that would "expand" to an infinitely-long type
1189 // `fix $ τ → dyn MyTrait<MyOutput = X, Output = <τ as MyTrait>::MyOutput`.
1191 // Instead, we force the user to write
1192 // `dyn MyTrait<MyOutput = X, Output = X>`, which is uglier but works. See
1193 // the discussion in #56288 for alternatives.
1194 if !references_self {
1195 // Include projections defined on supertraits.
1196 bounds.projection_bounds.push((pred, span));
1204 for (projection_bound, _) in &bounds.projection_bounds {
1205 for def_ids in associated_types.values_mut() {
1206 def_ids.remove(&projection_bound.projection_def_id());
1210 self.complain_about_missing_associated_types(
1212 potential_assoc_types,
1216 // De-duplicate auto traits so that, e.g., `dyn Trait + Send + Send` is the same as
1217 // `dyn Trait + Send`.
1218 auto_traits.sort_by_key(|i| i.trait_ref().def_id());
1219 auto_traits.dedup_by_key(|i| i.trait_ref().def_id());
1220 debug!("regular_traits: {:?}", regular_traits);
1221 debug!("auto_traits: {:?}", auto_traits);
1223 // Transform a `PolyTraitRef` into a `PolyExistentialTraitRef` by
1224 // removing the dummy `Self` type (`trait_object_dummy_self`).
1225 let trait_ref_to_existential = |trait_ref: ty::TraitRef<'tcx>| {
1226 if trait_ref.self_ty() != dummy_self {
1227 // FIXME: There appears to be a missing filter on top of `expand_trait_aliases`,
1228 // which picks up non-supertraits where clauses - but also, the object safety
1229 // completely ignores trait aliases, which could be object safety hazards. We
1230 // `delay_span_bug` here to avoid an ICE in stable even when the feature is
1231 // disabled. (#66420)
1232 tcx.sess.delay_span_bug(
1235 "trait_ref_to_existential called on {:?} with non-dummy Self",
1240 ty::ExistentialTraitRef::erase_self_ty(tcx, trait_ref)
1243 // Erase the `dummy_self` (`trait_object_dummy_self`) used above.
1244 let existential_trait_refs =
1245 regular_traits.iter().map(|i| i.trait_ref().map_bound(trait_ref_to_existential));
1246 let existential_projections = bounds.projection_bounds.iter().map(|(bound, _)| {
1247 bound.map_bound(|b| {
1248 let trait_ref = trait_ref_to_existential(b.projection_ty.trait_ref(tcx));
1249 ty::ExistentialProjection {
1251 item_def_id: b.projection_ty.item_def_id,
1252 substs: trait_ref.substs,
1257 // Calling `skip_binder` is okay because the predicates are re-bound.
1258 let regular_trait_predicates = existential_trait_refs
1259 .map(|trait_ref| ty::ExistentialPredicate::Trait(trait_ref.skip_binder()));
1260 let auto_trait_predicates = auto_traits
1262 .map(|trait_ref| ty::ExistentialPredicate::AutoTrait(trait_ref.trait_ref().def_id()));
1263 let mut v = regular_trait_predicates
1264 .chain(auto_trait_predicates)
1266 existential_projections
1267 .map(|x| ty::ExistentialPredicate::Projection(x.skip_binder())),
1269 .collect::<SmallVec<[_; 8]>>();
1270 v.sort_by(|a, b| a.stable_cmp(tcx, b));
1272 let existential_predicates = ty::Binder::bind(tcx.mk_existential_predicates(v.into_iter()));
1274 // Use explicitly-specified region bound.
1275 let region_bound = if !lifetime.is_elided() {
1276 self.ast_region_to_region(lifetime, None)
1278 self.compute_object_lifetime_bound(span, existential_predicates).unwrap_or_else(|| {
1279 if tcx.named_region(lifetime.hir_id).is_some() {
1280 self.ast_region_to_region(lifetime, None)
1282 self.re_infer(None, span).unwrap_or_else(|| {
1283 let mut err = struct_span_err!(
1287 "the lifetime bound for this object type cannot be deduced \
1288 from context; please supply an explicit bound"
1291 // We will have already emitted an error E0106 complaining about a
1292 // missing named lifetime in `&dyn Trait`, so we elide this one.
1297 tcx.lifetimes.re_static
1302 debug!("region_bound: {:?}", region_bound);
1304 let ty = tcx.mk_dynamic(existential_predicates, region_bound);
1305 debug!("trait_object_type: {:?}", ty);
1309 fn report_ambiguous_associated_type(
1316 let mut err = struct_span_err!(self.tcx().sess, span, E0223, "ambiguous associated type");
1317 if let (Some(_), Ok(snippet)) = (
1318 self.tcx().sess.confused_type_with_std_module.borrow().get(&span),
1319 self.tcx().sess.source_map().span_to_snippet(span),
1321 err.span_suggestion(
1323 "you are looking for the module in `std`, not the primitive type",
1324 format!("std::{}", snippet),
1325 Applicability::MachineApplicable,
1328 err.span_suggestion(
1330 "use fully-qualified syntax",
1331 format!("<{} as {}>::{}", type_str, trait_str, name),
1332 Applicability::HasPlaceholders,
1338 // Search for a bound on a type parameter which includes the associated item
1339 // given by `assoc_name`. `ty_param_def_id` is the `DefId` of the type parameter
1340 // This function will fail if there are no suitable bounds or there is
1342 fn find_bound_for_assoc_item(
1344 ty_param_def_id: LocalDefId,
1347 ) -> Result<ty::PolyTraitRef<'tcx>, ErrorReported> {
1348 let tcx = self.tcx();
1351 "find_bound_for_assoc_item(ty_param_def_id={:?}, assoc_name={:?}, span={:?})",
1352 ty_param_def_id, assoc_name, span,
1356 &self.get_type_parameter_bounds(span, ty_param_def_id.to_def_id()).predicates;
1358 debug!("find_bound_for_assoc_item: predicates={:#?}", predicates);
1360 let param_hir_id = tcx.hir().local_def_id_to_hir_id(ty_param_def_id);
1361 let param_name = tcx.hir().ty_param_name(param_hir_id);
1362 self.one_bound_for_assoc_type(
1364 traits::transitive_bounds(
1366 predicates.iter().filter_map(|(p, _)| {
1367 p.to_opt_poly_trait_ref().map(|trait_ref| trait_ref.value)
1371 || param_name.to_string(),
1378 // Checks that `bounds` contains exactly one element and reports appropriate
1379 // errors otherwise.
1380 fn one_bound_for_assoc_type<I>(
1382 all_candidates: impl Fn() -> I,
1383 ty_param_name: impl Fn() -> String,
1386 is_equality: impl Fn() -> Option<String>,
1387 ) -> Result<ty::PolyTraitRef<'tcx>, ErrorReported>
1389 I: Iterator<Item = ty::PolyTraitRef<'tcx>>,
1391 let mut matching_candidates = all_candidates()
1392 .filter(|r| self.trait_defines_associated_type_named(r.def_id(), assoc_name));
1394 let bound = match matching_candidates.next() {
1395 Some(bound) => bound,
1397 self.complain_about_assoc_type_not_found(
1403 return Err(ErrorReported);
1407 debug!("one_bound_for_assoc_type: bound = {:?}", bound);
1409 if let Some(bound2) = matching_candidates.next() {
1410 debug!("one_bound_for_assoc_type: bound2 = {:?}", bound2);
1412 let is_equality = is_equality();
1413 let bounds = array::IntoIter::new([bound, bound2]).chain(matching_candidates);
1414 let mut err = if is_equality.is_some() {
1415 // More specific Error Index entry.
1420 "ambiguous associated type `{}` in bounds of `{}`",
1429 "ambiguous associated type `{}` in bounds of `{}`",
1434 err.span_label(span, format!("ambiguous associated type `{}`", assoc_name));
1436 let mut where_bounds = vec![];
1437 for bound in bounds {
1438 let bound_id = bound.def_id();
1439 let bound_span = self
1441 .associated_items(bound_id)
1442 .find_by_name_and_kind(self.tcx(), assoc_name, ty::AssocKind::Type, bound_id)
1443 .and_then(|item| self.tcx().hir().span_if_local(item.def_id));
1445 if let Some(bound_span) = bound_span {
1449 "ambiguous `{}` from `{}`",
1451 bound.print_only_trait_path(),
1454 if let Some(constraint) = &is_equality {
1455 where_bounds.push(format!(
1456 " T: {trait}::{assoc} = {constraint}",
1457 trait=bound.print_only_trait_path(),
1459 constraint=constraint,
1462 err.span_suggestion(
1464 "use fully qualified syntax to disambiguate",
1468 bound.print_only_trait_path(),
1471 Applicability::MaybeIncorrect,
1476 "associated type `{}` could derive from `{}`",
1478 bound.print_only_trait_path(),
1482 if !where_bounds.is_empty() {
1484 "consider introducing a new type parameter `T` and adding `where` constraints:\
1485 \n where\n T: {},\n{}",
1487 where_bounds.join(",\n"),
1491 if !where_bounds.is_empty() {
1492 return Err(ErrorReported);
1498 // Create a type from a path to an associated type.
1499 // For a path `A::B::C::D`, `qself_ty` and `qself_def` are the type and def for `A::B::C`
1500 // and item_segment is the path segment for `D`. We return a type and a def for
1502 // Will fail except for `T::A` and `Self::A`; i.e., if `qself_ty`/`qself_def` are not a type
1503 // parameter or `Self`.
1504 pub fn associated_path_to_ty(
1506 hir_ref_id: hir::HirId,
1510 assoc_segment: &hir::PathSegment<'_>,
1511 permit_variants: bool,
1512 ) -> Result<(Ty<'tcx>, DefKind, DefId), ErrorReported> {
1513 let tcx = self.tcx();
1514 let assoc_ident = assoc_segment.ident;
1516 debug!("associated_path_to_ty: {:?}::{}", qself_ty, assoc_ident);
1518 // Check if we have an enum variant.
1519 let mut variant_resolution = None;
1520 if let ty::Adt(adt_def, _) = qself_ty.kind() {
1521 if adt_def.is_enum() {
1522 let variant_def = adt_def
1525 .find(|vd| tcx.hygienic_eq(assoc_ident, vd.ident, adt_def.did));
1526 if let Some(variant_def) = variant_def {
1527 if permit_variants {
1528 tcx.check_stability(variant_def.def_id, Some(hir_ref_id), span);
1529 self.prohibit_generics(slice::from_ref(assoc_segment));
1530 return Ok((qself_ty, DefKind::Variant, variant_def.def_id));
1532 variant_resolution = Some(variant_def.def_id);
1538 // Find the type of the associated item, and the trait where the associated
1539 // item is declared.
1540 let bound = match (&qself_ty.kind(), qself_res) {
1541 (_, Res::SelfTy(Some(_), Some((impl_def_id, _)))) => {
1542 // `Self` in an impl of a trait -- we have a concrete self type and a
1544 let trait_ref = match tcx.impl_trait_ref(impl_def_id) {
1545 Some(trait_ref) => trait_ref,
1547 // A cycle error occurred, most likely.
1548 return Err(ErrorReported);
1552 self.one_bound_for_assoc_type(
1553 || traits::supertraits(tcx, ty::Binder::bind(trait_ref)),
1554 || "Self".to_string(),
1562 Res::SelfTy(Some(param_did), None) | Res::Def(DefKind::TyParam, param_did),
1563 ) => self.find_bound_for_assoc_item(param_did.expect_local(), assoc_ident, span)?,
1565 if variant_resolution.is_some() {
1566 // Variant in type position
1567 let msg = format!("expected type, found variant `{}`", assoc_ident);
1568 tcx.sess.span_err(span, &msg);
1569 } else if qself_ty.is_enum() {
1570 let mut err = struct_span_err!(
1574 "no variant named `{}` found for enum `{}`",
1579 let adt_def = qself_ty.ty_adt_def().expect("enum is not an ADT");
1580 if let Some(suggested_name) = find_best_match_for_name(
1584 .map(|variant| variant.ident.name)
1585 .collect::<Vec<Symbol>>(),
1589 err.span_suggestion(
1591 "there is a variant with a similar name",
1592 suggested_name.to_string(),
1593 Applicability::MaybeIncorrect,
1598 format!("variant not found in `{}`", qself_ty),
1602 if let Some(sp) = tcx.hir().span_if_local(adt_def.did) {
1603 let sp = tcx.sess.source_map().guess_head_span(sp);
1604 err.span_label(sp, format!("variant `{}` not found here", assoc_ident));
1608 } else if !qself_ty.references_error() {
1609 // Don't print `TyErr` to the user.
1610 self.report_ambiguous_associated_type(
1612 &qself_ty.to_string(),
1617 return Err(ErrorReported);
1621 let trait_did = bound.def_id();
1622 let (assoc_ident, def_scope) =
1623 tcx.adjust_ident_and_get_scope(assoc_ident, trait_did, hir_ref_id);
1625 // We have already adjusted the item name above, so compare with `ident.normalize_to_macros_2_0()` instead
1626 // of calling `filter_by_name_and_kind`.
1628 .associated_items(trait_did)
1629 .in_definition_order()
1631 i.kind.namespace() == Namespace::TypeNS
1632 && i.ident.normalize_to_macros_2_0() == assoc_ident
1634 .expect("missing associated type");
1636 let ty = self.projected_ty_from_poly_trait_ref(span, item.def_id, assoc_segment, bound);
1637 let ty = self.normalize_ty(span, ty);
1639 let kind = DefKind::AssocTy;
1640 if !item.vis.is_accessible_from(def_scope, tcx) {
1641 let kind = kind.descr(item.def_id);
1642 let msg = format!("{} `{}` is private", kind, assoc_ident);
1644 .struct_span_err(span, &msg)
1645 .span_label(span, &format!("private {}", kind))
1648 tcx.check_stability(item.def_id, Some(hir_ref_id), span);
1650 if let Some(variant_def_id) = variant_resolution {
1651 tcx.struct_span_lint_hir(AMBIGUOUS_ASSOCIATED_ITEMS, hir_ref_id, span, |lint| {
1652 let mut err = lint.build("ambiguous associated item");
1653 let mut could_refer_to = |kind: DefKind, def_id, also| {
1654 let note_msg = format!(
1655 "`{}` could{} refer to the {} defined here",
1660 err.span_note(tcx.def_span(def_id), ¬e_msg);
1663 could_refer_to(DefKind::Variant, variant_def_id, "");
1664 could_refer_to(kind, item.def_id, " also");
1666 err.span_suggestion(
1668 "use fully-qualified syntax",
1669 format!("<{} as {}>::{}", qself_ty, tcx.item_name(trait_did), assoc_ident),
1670 Applicability::MachineApplicable,
1676 Ok((ty, kind, item.def_id))
1682 opt_self_ty: Option<Ty<'tcx>>,
1684 trait_segment: &hir::PathSegment<'_>,
1685 item_segment: &hir::PathSegment<'_>,
1687 let tcx = self.tcx();
1689 let trait_def_id = tcx.parent(item_def_id).unwrap();
1691 debug!("qpath_to_ty: trait_def_id={:?}", trait_def_id);
1693 let self_ty = if let Some(ty) = opt_self_ty {
1696 let path_str = tcx.def_path_str(trait_def_id);
1698 let def_id = self.item_def_id();
1700 debug!("qpath_to_ty: self.item_def_id()={:?}", def_id);
1702 let parent_def_id = def_id
1703 .and_then(|def_id| {
1704 def_id.as_local().map(|def_id| tcx.hir().local_def_id_to_hir_id(def_id))
1706 .map(|hir_id| tcx.hir().get_parent_did(hir_id).to_def_id());
1708 debug!("qpath_to_ty: parent_def_id={:?}", parent_def_id);
1710 // If the trait in segment is the same as the trait defining the item,
1711 // use the `<Self as ..>` syntax in the error.
1712 let is_part_of_self_trait_constraints = def_id == Some(trait_def_id);
1713 let is_part_of_fn_in_self_trait = parent_def_id == Some(trait_def_id);
1715 let type_name = if is_part_of_self_trait_constraints || is_part_of_fn_in_self_trait {
1721 self.report_ambiguous_associated_type(
1725 item_segment.ident.name,
1727 return tcx.ty_error();
1730 debug!("qpath_to_ty: self_type={:?}", self_ty);
1732 let trait_ref = self.ast_path_to_mono_trait_ref(span, trait_def_id, self_ty, trait_segment);
1734 let item_substs = self.create_substs_for_associated_item(
1742 debug!("qpath_to_ty: trait_ref={:?}", trait_ref);
1744 self.normalize_ty(span, tcx.mk_projection(item_def_id, item_substs))
1747 pub fn prohibit_generics<'a, T: IntoIterator<Item = &'a hir::PathSegment<'a>>>(
1751 let mut has_err = false;
1752 for segment in segments {
1753 let (mut err_for_lt, mut err_for_ty, mut err_for_ct) = (false, false, false);
1754 for arg in segment.generic_args().args {
1755 let (span, kind) = match arg {
1756 hir::GenericArg::Lifetime(lt) => {
1762 (lt.span, "lifetime")
1764 hir::GenericArg::Type(ty) => {
1772 hir::GenericArg::Const(ct) => {
1781 let mut err = struct_span_err!(
1785 "{} arguments are not allowed for this type",
1788 err.span_label(span, format!("{} argument not allowed", kind));
1790 if err_for_lt && err_for_ty && err_for_ct {
1795 // Only emit the first error to avoid overloading the user with error messages.
1796 if let [binding, ..] = segment.generic_args().bindings {
1798 Self::prohibit_assoc_ty_binding(self.tcx(), binding.span);
1804 // FIXME(eddyb, varkor) handle type paths here too, not just value ones.
1805 pub fn def_ids_for_value_path_segments(
1807 segments: &[hir::PathSegment<'_>],
1808 self_ty: Option<Ty<'tcx>>,
1812 // We need to extract the type parameters supplied by the user in
1813 // the path `path`. Due to the current setup, this is a bit of a
1814 // tricky-process; the problem is that resolve only tells us the
1815 // end-point of the path resolution, and not the intermediate steps.
1816 // Luckily, we can (at least for now) deduce the intermediate steps
1817 // just from the end-point.
1819 // There are basically five cases to consider:
1821 // 1. Reference to a constructor of a struct:
1823 // struct Foo<T>(...)
1825 // In this case, the parameters are declared in the type space.
1827 // 2. Reference to a constructor of an enum variant:
1829 // enum E<T> { Foo(...) }
1831 // In this case, the parameters are defined in the type space,
1832 // but may be specified either on the type or the variant.
1834 // 3. Reference to a fn item or a free constant:
1838 // In this case, the path will again always have the form
1839 // `a::b::foo::<T>` where only the final segment should have
1840 // type parameters. However, in this case, those parameters are
1841 // declared on a value, and hence are in the `FnSpace`.
1843 // 4. Reference to a method or an associated constant:
1845 // impl<A> SomeStruct<A> {
1849 // Here we can have a path like
1850 // `a::b::SomeStruct::<A>::foo::<B>`, in which case parameters
1851 // may appear in two places. The penultimate segment,
1852 // `SomeStruct::<A>`, contains parameters in TypeSpace, and the
1853 // final segment, `foo::<B>` contains parameters in fn space.
1855 // The first step then is to categorize the segments appropriately.
1857 let tcx = self.tcx();
1859 assert!(!segments.is_empty());
1860 let last = segments.len() - 1;
1862 let mut path_segs = vec![];
1865 // Case 1. Reference to a struct constructor.
1866 DefKind::Ctor(CtorOf::Struct, ..) => {
1867 // Everything but the final segment should have no
1868 // parameters at all.
1869 let generics = tcx.generics_of(def_id);
1870 // Variant and struct constructors use the
1871 // generics of their parent type definition.
1872 let generics_def_id = generics.parent.unwrap_or(def_id);
1873 path_segs.push(PathSeg(generics_def_id, last));
1876 // Case 2. Reference to a variant constructor.
1877 DefKind::Ctor(CtorOf::Variant, ..) | DefKind::Variant => {
1878 let adt_def = self_ty.map(|t| t.ty_adt_def().unwrap());
1879 let (generics_def_id, index) = if let Some(adt_def) = adt_def {
1880 debug_assert!(adt_def.is_enum());
1882 } else if last >= 1 && segments[last - 1].args.is_some() {
1883 // Everything but the penultimate segment should have no
1884 // parameters at all.
1885 let mut def_id = def_id;
1887 // `DefKind::Ctor` -> `DefKind::Variant`
1888 if let DefKind::Ctor(..) = kind {
1889 def_id = tcx.parent(def_id).unwrap()
1892 // `DefKind::Variant` -> `DefKind::Enum`
1893 let enum_def_id = tcx.parent(def_id).unwrap();
1894 (enum_def_id, last - 1)
1896 // FIXME: lint here recommending `Enum::<...>::Variant` form
1897 // instead of `Enum::Variant::<...>` form.
1899 // Everything but the final segment should have no
1900 // parameters at all.
1901 let generics = tcx.generics_of(def_id);
1902 // Variant and struct constructors use the
1903 // generics of their parent type definition.
1904 (generics.parent.unwrap_or(def_id), last)
1906 path_segs.push(PathSeg(generics_def_id, index));
1909 // Case 3. Reference to a top-level value.
1910 DefKind::Fn | DefKind::Const | DefKind::ConstParam | DefKind::Static => {
1911 path_segs.push(PathSeg(def_id, last));
1914 // Case 4. Reference to a method or associated const.
1915 DefKind::AssocFn | DefKind::AssocConst => {
1916 if segments.len() >= 2 {
1917 let generics = tcx.generics_of(def_id);
1918 path_segs.push(PathSeg(generics.parent.unwrap(), last - 1));
1920 path_segs.push(PathSeg(def_id, last));
1923 kind => bug!("unexpected definition kind {:?} for {:?}", kind, def_id),
1926 debug!("path_segs = {:?}", path_segs);
1931 // Check a type `Path` and convert it to a `Ty`.
1934 opt_self_ty: Option<Ty<'tcx>>,
1935 path: &hir::Path<'_>,
1936 permit_variants: bool,
1938 let tcx = self.tcx();
1941 "res_to_ty(res={:?}, opt_self_ty={:?}, path_segments={:?})",
1942 path.res, opt_self_ty, path.segments
1945 let span = path.span;
1947 Res::Def(DefKind::OpaqueTy, did) => {
1948 // Check for desugared `impl Trait`.
1949 assert!(ty::is_impl_trait_defn(tcx, did).is_none());
1950 let item_segment = path.segments.split_last().unwrap();
1951 self.prohibit_generics(item_segment.1);
1952 let substs = self.ast_path_substs_for_ty(span, did, item_segment.0);
1953 self.normalize_ty(span, tcx.mk_opaque(did, substs))
1960 | DefKind::ForeignTy,
1963 assert_eq!(opt_self_ty, None);
1964 self.prohibit_generics(path.segments.split_last().unwrap().1);
1965 self.ast_path_to_ty(span, did, path.segments.last().unwrap())
1967 Res::Def(kind @ DefKind::Variant, def_id) if permit_variants => {
1968 // Convert "variant type" as if it were a real type.
1969 // The resulting `Ty` is type of the variant's enum for now.
1970 assert_eq!(opt_self_ty, None);
1973 self.def_ids_for_value_path_segments(&path.segments, None, kind, def_id);
1974 let generic_segs: FxHashSet<_> =
1975 path_segs.iter().map(|PathSeg(_, index)| index).collect();
1976 self.prohibit_generics(path.segments.iter().enumerate().filter_map(
1978 if !generic_segs.contains(&index) { Some(seg) } else { None }
1982 let PathSeg(def_id, index) = path_segs.last().unwrap();
1983 self.ast_path_to_ty(span, *def_id, &path.segments[*index])
1985 Res::Def(DefKind::TyParam, def_id) => {
1986 assert_eq!(opt_self_ty, None);
1987 self.prohibit_generics(path.segments);
1989 let hir_id = tcx.hir().local_def_id_to_hir_id(def_id.expect_local());
1990 let item_id = tcx.hir().get_parent_node(hir_id);
1991 let item_def_id = tcx.hir().local_def_id(item_id);
1992 let generics = tcx.generics_of(item_def_id);
1993 let index = generics.param_def_id_to_index[&def_id];
1994 tcx.mk_ty_param(index, tcx.hir().name(hir_id))
1996 Res::SelfTy(Some(_), None) => {
1997 // `Self` in trait or type alias.
1998 assert_eq!(opt_self_ty, None);
1999 self.prohibit_generics(path.segments);
2000 tcx.types.self_param
2002 Res::SelfTy(_, Some((def_id, forbid_generic))) => {
2003 // `Self` in impl (we know the concrete type).
2004 assert_eq!(opt_self_ty, None);
2005 self.prohibit_generics(path.segments);
2006 // Try to evaluate any array length constants.
2007 let normalized_ty = self.normalize_ty(span, tcx.at(span).type_of(def_id));
2008 if forbid_generic && normalized_ty.needs_subst() {
2009 let mut err = tcx.sess.struct_span_err(
2011 "generic `Self` types are currently not permitted in anonymous constants",
2013 if let Some(hir::Node::Item(&hir::Item {
2014 kind: hir::ItemKind::Impl { self_ty, .. },
2016 })) = tcx.hir().get_if_local(def_id)
2018 err.span_note(self_ty.span, "not a concrete type");
2026 Res::Def(DefKind::AssocTy, def_id) => {
2027 debug_assert!(path.segments.len() >= 2);
2028 self.prohibit_generics(&path.segments[..path.segments.len() - 2]);
2033 &path.segments[path.segments.len() - 2],
2034 path.segments.last().unwrap(),
2037 Res::PrimTy(prim_ty) => {
2038 assert_eq!(opt_self_ty, None);
2039 self.prohibit_generics(path.segments);
2041 hir::PrimTy::Bool => tcx.types.bool,
2042 hir::PrimTy::Char => tcx.types.char,
2043 hir::PrimTy::Int(it) => tcx.mk_mach_int(it),
2044 hir::PrimTy::Uint(uit) => tcx.mk_mach_uint(uit),
2045 hir::PrimTy::Float(ft) => tcx.mk_mach_float(ft),
2046 hir::PrimTy::Str => tcx.types.str_,
2050 self.set_tainted_by_errors();
2051 self.tcx().ty_error()
2053 _ => span_bug!(span, "unexpected resolution: {:?}", path.res),
2057 /// Parses the programmer's textual representation of a type into our
2058 /// internal notion of a type.
2059 pub fn ast_ty_to_ty(&self, ast_ty: &hir::Ty<'_>) -> Ty<'tcx> {
2060 self.ast_ty_to_ty_inner(ast_ty, false)
2063 /// Turns a `hir::Ty` into a `Ty`. For diagnostics' purposes we keep track of whether trait
2064 /// objects are borrowed like `&dyn Trait` to avoid emitting redundant errors.
2065 fn ast_ty_to_ty_inner(&self, ast_ty: &hir::Ty<'_>, borrowed: bool) -> Ty<'tcx> {
2066 debug!("ast_ty_to_ty(id={:?}, ast_ty={:?} ty_ty={:?})", ast_ty.hir_id, ast_ty, ast_ty.kind);
2068 let tcx = self.tcx();
2070 let result_ty = match ast_ty.kind {
2071 hir::TyKind::Slice(ref ty) => tcx.mk_slice(self.ast_ty_to_ty(&ty)),
2072 hir::TyKind::Ptr(ref mt) => {
2073 tcx.mk_ptr(ty::TypeAndMut { ty: self.ast_ty_to_ty(&mt.ty), mutbl: mt.mutbl })
2075 hir::TyKind::Rptr(ref region, ref mt) => {
2076 let r = self.ast_region_to_region(region, None);
2077 debug!("ast_ty_to_ty: r={:?}", r);
2078 let t = self.ast_ty_to_ty_inner(&mt.ty, true);
2079 tcx.mk_ref(r, ty::TypeAndMut { ty: t, mutbl: mt.mutbl })
2081 hir::TyKind::Never => tcx.types.never,
2082 hir::TyKind::Tup(ref fields) => {
2083 tcx.mk_tup(fields.iter().map(|t| self.ast_ty_to_ty(&t)))
2085 hir::TyKind::BareFn(ref bf) => {
2086 require_c_abi_if_c_variadic(tcx, &bf.decl, bf.abi, ast_ty.span);
2087 tcx.mk_fn_ptr(self.ty_of_fn(
2091 &hir::Generics::empty(),
2095 hir::TyKind::TraitObject(ref bounds, ref lifetime) => {
2096 self.conv_object_ty_poly_trait_ref(ast_ty.span, bounds, lifetime, borrowed)
2098 hir::TyKind::Path(hir::QPath::Resolved(ref maybe_qself, ref path)) => {
2099 debug!("ast_ty_to_ty: maybe_qself={:?} path={:?}", maybe_qself, path);
2100 let opt_self_ty = maybe_qself.as_ref().map(|qself| self.ast_ty_to_ty(qself));
2101 self.res_to_ty(opt_self_ty, path, false)
2103 hir::TyKind::OpaqueDef(item_id, ref lifetimes) => {
2104 let opaque_ty = tcx.hir().expect_item(item_id.id);
2105 let def_id = tcx.hir().local_def_id(item_id.id).to_def_id();
2107 match opaque_ty.kind {
2108 hir::ItemKind::OpaqueTy(hir::OpaqueTy { impl_trait_fn, .. }) => {
2109 self.impl_trait_ty_to_ty(def_id, lifetimes, impl_trait_fn.is_some())
2111 ref i => bug!("`impl Trait` pointed to non-opaque type?? {:#?}", i),
2114 hir::TyKind::Path(hir::QPath::TypeRelative(ref qself, ref segment)) => {
2115 debug!("ast_ty_to_ty: qself={:?} segment={:?}", qself, segment);
2116 let ty = self.ast_ty_to_ty(qself);
2118 let res = if let hir::TyKind::Path(hir::QPath::Resolved(_, ref path)) = qself.kind {
2123 self.associated_path_to_ty(ast_ty.hir_id, ast_ty.span, ty, res, segment, false)
2124 .map(|(ty, _, _)| ty)
2125 .unwrap_or_else(|_| tcx.ty_error())
2127 hir::TyKind::Path(hir::QPath::LangItem(lang_item, span)) => {
2128 let def_id = tcx.require_lang_item(lang_item, Some(span));
2129 let (substs, _, _) = self.create_substs_for_ast_path(
2133 &GenericArgs::none(),
2137 self.normalize_ty(span, tcx.at(span).type_of(def_id).subst(tcx, substs))
2139 hir::TyKind::Array(ref ty, ref length) => {
2140 let length_def_id = tcx.hir().local_def_id(length.hir_id);
2141 let length = ty::Const::from_anon_const(tcx, length_def_id);
2142 let array_ty = tcx.mk_ty(ty::Array(self.ast_ty_to_ty(&ty), length));
2143 self.normalize_ty(ast_ty.span, array_ty)
2145 hir::TyKind::Typeof(ref _e) => {
2146 tcx.sess.emit_err(TypeofReservedKeywordUsed { span: ast_ty.span });
2149 hir::TyKind::Infer => {
2150 // Infer also appears as the type of arguments or return
2151 // values in a ExprKind::Closure, or as
2152 // the type of local variables. Both of these cases are
2153 // handled specially and will not descend into this routine.
2154 self.ty_infer(None, ast_ty.span)
2156 hir::TyKind::Err => tcx.ty_error(),
2159 debug!("ast_ty_to_ty: result_ty={:?}", result_ty);
2161 self.record_ty(ast_ty.hir_id, result_ty, ast_ty.span);
2165 pub fn impl_trait_ty_to_ty(
2168 lifetimes: &[hir::GenericArg<'_>],
2169 replace_parent_lifetimes: bool,
2171 debug!("impl_trait_ty_to_ty(def_id={:?}, lifetimes={:?})", def_id, lifetimes);
2172 let tcx = self.tcx();
2174 let generics = tcx.generics_of(def_id);
2176 debug!("impl_trait_ty_to_ty: generics={:?}", generics);
2177 let substs = InternalSubsts::for_item(tcx, def_id, |param, _| {
2178 if let Some(i) = (param.index as usize).checked_sub(generics.parent_count) {
2179 // Our own parameters are the resolved lifetimes.
2181 GenericParamDefKind::Lifetime => {
2182 if let hir::GenericArg::Lifetime(lifetime) = &lifetimes[i] {
2183 self.ast_region_to_region(lifetime, None).into()
2192 // For RPIT (return position impl trait), only lifetimes
2193 // mentioned in the impl Trait predicate are captured by
2194 // the opaque type, so the lifetime parameters from the
2195 // parent item need to be replaced with `'static`.
2197 // For `impl Trait` in the types of statics, constants,
2198 // locals and type aliases. These capture all parent
2199 // lifetimes, so they can use their identity subst.
2200 GenericParamDefKind::Lifetime if replace_parent_lifetimes => {
2201 tcx.lifetimes.re_static.into()
2203 _ => tcx.mk_param_from_def(param),
2207 debug!("impl_trait_ty_to_ty: substs={:?}", substs);
2209 let ty = tcx.mk_opaque(def_id, substs);
2210 debug!("impl_trait_ty_to_ty: {}", ty);
2214 pub fn ty_of_arg(&self, ty: &hir::Ty<'_>, expected_ty: Option<Ty<'tcx>>) -> Ty<'tcx> {
2216 hir::TyKind::Infer if expected_ty.is_some() => {
2217 self.record_ty(ty.hir_id, expected_ty.unwrap(), ty.span);
2218 expected_ty.unwrap()
2220 _ => self.ast_ty_to_ty(ty),
2226 unsafety: hir::Unsafety,
2228 decl: &hir::FnDecl<'_>,
2229 generics: &hir::Generics<'_>,
2230 ident_span: Option<Span>,
2231 ) -> ty::PolyFnSig<'tcx> {
2234 let tcx = self.tcx();
2236 // We proactively collect all the inferred type params to emit a single error per fn def.
2237 let mut visitor = PlaceholderHirTyCollector::default();
2238 for ty in decl.inputs {
2239 visitor.visit_ty(ty);
2241 walk_generics(&mut visitor, generics);
2243 let input_tys = decl.inputs.iter().map(|a| self.ty_of_arg(a, None));
2244 let output_ty = match decl.output {
2245 hir::FnRetTy::Return(ref output) => {
2246 visitor.visit_ty(output);
2247 self.ast_ty_to_ty(output)
2249 hir::FnRetTy::DefaultReturn(..) => tcx.mk_unit(),
2252 debug!("ty_of_fn: output_ty={:?}", output_ty);
2255 ty::Binder::bind(tcx.mk_fn_sig(input_tys, output_ty, decl.c_variadic, unsafety, abi));
2257 if !self.allow_ty_infer() {
2258 // We always collect the spans for placeholder types when evaluating `fn`s, but we
2259 // only want to emit an error complaining about them if infer types (`_`) are not
2260 // allowed. `allow_ty_infer` gates this behavior. We check for the presence of
2261 // `ident_span` to not emit an error twice when we have `fn foo(_: fn() -> _)`.
2262 crate::collect::placeholder_type_error(
2264 ident_span.map(|sp| sp.shrink_to_hi()),
2265 &generics.params[..],
2271 // Find any late-bound regions declared in return type that do
2272 // not appear in the arguments. These are not well-formed.
2275 // for<'a> fn() -> &'a str <-- 'a is bad
2276 // for<'a> fn(&'a String) -> &'a str <-- 'a is ok
2277 let inputs = bare_fn_ty.inputs();
2278 let late_bound_in_args =
2279 tcx.collect_constrained_late_bound_regions(&inputs.map_bound(|i| i.to_owned()));
2280 let output = bare_fn_ty.output();
2281 let late_bound_in_ret = tcx.collect_referenced_late_bound_regions(&output);
2283 self.validate_late_bound_regions(late_bound_in_args, late_bound_in_ret, |br_name| {
2288 "return type references {}, which is not constrained by the fn input types",
2296 fn validate_late_bound_regions(
2298 constrained_regions: FxHashSet<ty::BoundRegion>,
2299 referenced_regions: FxHashSet<ty::BoundRegion>,
2300 generate_err: impl Fn(&str) -> rustc_errors::DiagnosticBuilder<'tcx>,
2302 for br in referenced_regions.difference(&constrained_regions) {
2303 let br_name = match *br {
2304 ty::BrNamed(_, name) => format!("lifetime `{}`", name),
2305 ty::BrAnon(_) | ty::BrEnv => "an anonymous lifetime".to_string(),
2308 let mut err = generate_err(&br_name);
2310 if let ty::BrAnon(_) = *br {
2311 // The only way for an anonymous lifetime to wind up
2312 // in the return type but **also** be unconstrained is
2313 // if it only appears in "associated types" in the
2314 // input. See #47511 and #62200 for examples. In this case,
2315 // though we can easily give a hint that ought to be
2318 "lifetimes appearing in an associated type are not considered constrained",
2326 /// Given the bounds on an object, determines what single region bound (if any) we can
2327 /// use to summarize this type. The basic idea is that we will use the bound the user
2328 /// provided, if they provided one, and otherwise search the supertypes of trait bounds
2329 /// for region bounds. It may be that we can derive no bound at all, in which case
2330 /// we return `None`.
2331 fn compute_object_lifetime_bound(
2334 existential_predicates: ty::Binder<&'tcx ty::List<ty::ExistentialPredicate<'tcx>>>,
2335 ) -> Option<ty::Region<'tcx>> // if None, use the default
2337 let tcx = self.tcx();
2339 debug!("compute_opt_region_bound(existential_predicates={:?})", existential_predicates);
2341 // No explicit region bound specified. Therefore, examine trait
2342 // bounds and see if we can derive region bounds from those.
2343 let derived_region_bounds = object_region_bounds(tcx, existential_predicates);
2345 // If there are no derived region bounds, then report back that we
2346 // can find no region bound. The caller will use the default.
2347 if derived_region_bounds.is_empty() {
2351 // If any of the derived region bounds are 'static, that is always
2353 if derived_region_bounds.iter().any(|&r| ty::ReStatic == *r) {
2354 return Some(tcx.lifetimes.re_static);
2357 // Determine whether there is exactly one unique region in the set
2358 // of derived region bounds. If so, use that. Otherwise, report an
2360 let r = derived_region_bounds[0];
2361 if derived_region_bounds[1..].iter().any(|r1| r != *r1) {
2362 tcx.sess.emit_err(AmbiguousLifetimeBound { span });