3 // The job of the coherence phase of typechecking is to ensure that
4 // each trait has at most one implementation for each type. This is
5 // done by the orphan and overlap modules. Then we build up various
6 // mappings. That mapping code resides here.
8 use rustc_errors::{error_code, struct_span_err};
9 use rustc_hir::def_id::{DefId, LocalDefId};
10 use rustc_middle::ty::query::Providers;
11 use rustc_middle::ty::{self, TyCtxt, TypeVisitable};
13 use rustc_trait_selection::traits;
17 mod inherent_impls_overlap;
21 fn check_impl(tcx: TyCtxt<'_>, impl_def_id: LocalDefId, trait_ref: ty::TraitRef<'_>) {
23 "(checking implementation) adding impl for trait '{:?}', item '{}'",
25 tcx.def_path_str(impl_def_id.to_def_id())
28 // Skip impls where one of the self type is an error type.
29 // This occurs with e.g., resolve failures (#30589).
30 if trait_ref.references_error() {
34 enforce_trait_manually_implementable(tcx, impl_def_id, trait_ref.def_id);
35 enforce_empty_impls_for_marker_traits(tcx, impl_def_id, trait_ref.def_id);
38 fn enforce_trait_manually_implementable(
40 impl_def_id: LocalDefId,
43 let impl_header_span = tcx.def_span(impl_def_id);
45 // Disallow *all* explicit impls of traits marked `#[rustc_deny_explicit_impl]`
46 if tcx.has_attr(trait_def_id, sym::rustc_deny_explicit_impl) {
47 let trait_name = tcx.item_name(trait_def_id);
48 let mut err = struct_span_err!(
52 "explicit impls for the `{trait_name}` trait are not permitted"
54 err.span_label(impl_header_span, format!("impl of `{trait_name}` not allowed"));
56 // Maintain explicit error code for `Unsize`, since it has a useful
57 // explanation about using `CoerceUnsized` instead.
58 if Some(trait_def_id) == tcx.lang_items().unsize_trait() {
59 err.code(error_code!(E0328));
66 if let ty::trait_def::TraitSpecializationKind::AlwaysApplicable =
67 tcx.trait_def(trait_def_id).specialization_kind
69 if !tcx.features().specialization && !tcx.features().min_specialization {
73 "implementing `rustc_specialization_trait` traits is unstable",
75 .help("add `#![feature(min_specialization)]` to the crate attributes to enable")
82 /// We allow impls of marker traits to overlap, so they can't override impls
83 /// as that could make it ambiguous which associated item to use.
84 fn enforce_empty_impls_for_marker_traits(
86 impl_def_id: LocalDefId,
89 if !tcx.trait_def(trait_def_id).is_marker {
93 if tcx.associated_item_def_ids(trait_def_id).is_empty() {
99 tcx.def_span(impl_def_id),
101 "impls for marker traits cannot contain items"
106 pub fn provide(providers: &mut Providers) {
107 use self::builtin::coerce_unsized_info;
108 use self::inherent_impls::{crate_incoherent_impls, crate_inherent_impls, inherent_impls};
109 use self::inherent_impls_overlap::crate_inherent_impls_overlap_check;
110 use self::orphan::orphan_check_impl;
112 *providers = Providers {
114 crate_inherent_impls,
115 crate_incoherent_impls,
117 crate_inherent_impls_overlap_check,
124 fn coherent_trait(tcx: TyCtxt<'_>, def_id: DefId) {
125 // Trigger building the specialization graph for the trait. This will detect and report any
127 tcx.ensure().specialization_graph_of(def_id);
129 let impls = tcx.hir().trait_impls(def_id);
130 for &impl_def_id in impls {
131 let trait_ref = tcx.impl_trait_ref(impl_def_id).unwrap();
133 check_impl(tcx, impl_def_id, trait_ref);
134 check_object_overlap(tcx, impl_def_id, trait_ref);
136 tcx.sess.time("unsafety_checking", || unsafety::check_item(tcx, impl_def_id));
137 tcx.sess.time("orphan_checking", || tcx.ensure().orphan_check_impl(impl_def_id));
140 builtin::check_trait(tcx, def_id);
143 /// Checks whether an impl overlaps with the automatic `impl Trait for dyn Trait`.
144 fn check_object_overlap<'tcx>(
146 impl_def_id: LocalDefId,
147 trait_ref: ty::TraitRef<'tcx>,
149 let trait_def_id = trait_ref.def_id;
151 if trait_ref.references_error() {
152 debug!("coherence: skipping impl {:?} with error {:?}", impl_def_id, trait_ref);
156 // check for overlap with the automatic `impl Trait for dyn Trait`
157 if let ty::Dynamic(data, ..) = trait_ref.self_ty().kind() {
158 // This is something like impl Trait1 for Trait2. Illegal
159 // if Trait1 is a supertrait of Trait2 or Trait2 is not object safe.
161 let component_def_ids = data.iter().flat_map(|predicate| {
162 match predicate.skip_binder() {
163 ty::ExistentialPredicate::Trait(tr) => Some(tr.def_id),
164 ty::ExistentialPredicate::AutoTrait(def_id) => Some(def_id),
165 // An associated type projection necessarily comes with
166 // an additional `Trait` requirement.
167 ty::ExistentialPredicate::Projection(..) => None,
171 for component_def_id in component_def_ids {
172 if !tcx.is_object_safe(component_def_id) {
173 // Without the 'object_safe_for_dispatch' feature this is an error
174 // which will be reported by wfcheck. Ignore it here.
175 // This is tested by `coherence-impl-trait-for-trait-object-safe.rs`.
176 // With the feature enabled, the trait is not implemented automatically,
179 let mut supertrait_def_ids = traits::supertrait_def_ids(tcx, component_def_id);
180 if supertrait_def_ids.any(|d| d == trait_def_id) {
181 let span = tcx.def_span(impl_def_id);
186 "the object type `{}` automatically implements the trait `{}`",
188 tcx.def_path_str(trait_def_id)
193 "`{}` automatically implements trait `{}`",
195 tcx.def_path_str(trait_def_id)