compiler/rustc_hir_analysis/src/coherence/mod.rs

   1 // Coherence phase
   2 //
   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.
   7
   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, TypeVisitableExt};
  12 use rustc_span::sym;
  13 use rustc_trait_selection::traits;
  14
  15 mod builtin;
  16 mod inherent_impls;
  17 mod inherent_impls_overlap;
  18 mod orphan;
  19 mod unsafety;
  20
  21 fn check_impl(tcx: TyCtxt<'_>, impl_def_id: LocalDefId, trait_ref: ty::TraitRef<'_>) {
  22     debug!(
  23         "(checking implementation) adding impl for trait '{:?}', item '{}'",
  24         trait_ref,
  25         tcx.def_path_str(impl_def_id.to_def_id())
  26     );
  27
  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() {
  31         return;
  32     }
  33
  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);
  36 }
  37
  38 fn enforce_trait_manually_implementable(
  39     tcx: TyCtxt<'_>,
  40     impl_def_id: LocalDefId,
  41     trait_def_id: DefId,
  42 ) {
  43     let impl_header_span = tcx.def_span(impl_def_id);
  44
  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!(
  49             tcx.sess,
  50             impl_header_span,
  51             E0322,
  52             "explicit impls for the `{trait_name}` trait are not permitted"
  53         );
  54         err.span_label(impl_header_span, format!("impl of `{trait_name}` not allowed"));
  55
  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));
  60         }
  61
  62         err.emit();
  63         return;
  64     }
  65
  66     if let ty::trait_def::TraitSpecializationKind::AlwaysApplicable =
  67         tcx.trait_def(trait_def_id).specialization_kind
  68     {
  69         if !tcx.features().specialization && !tcx.features().min_specialization {
  70             tcx.sess
  71                 .struct_span_err(
  72                     impl_header_span,
  73                     "implementing `rustc_specialization_trait` traits is unstable",
  74                 )
  75                 .help("add `#![feature(min_specialization)]` to the crate attributes to enable")
  76                 .emit();
  77             return;
  78         }
  79     }
  80 }
  81
  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(
  85     tcx: TyCtxt<'_>,
  86     impl_def_id: LocalDefId,
  87     trait_def_id: DefId,
  88 ) {
  89     if !tcx.trait_def(trait_def_id).is_marker {
  90         return;
  91     }
  92
  93     if tcx.associated_item_def_ids(trait_def_id).is_empty() {
  94         return;
  95     }
  96
  97     struct_span_err!(
  98         tcx.sess,
  99         tcx.def_span(impl_def_id),
 100         E0715,
 101         "impls for marker traits cannot contain items"
 102     )
 103     .emit();
 104 }
 105
 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;
 111
 112     *providers = Providers {
 113         coherent_trait,
 114         crate_inherent_impls,
 115         crate_incoherent_impls,
 116         inherent_impls,
 117         crate_inherent_impls_overlap_check,
 118         coerce_unsized_info,
 119         orphan_check_impl,
 120         ..*providers
 121     };
 122 }
 123
 124 fn coherent_trait(tcx: TyCtxt<'_>, def_id: DefId) {
 125     // Trigger building the specialization graph for the trait. This will detect and report any
 126     // overlap errors.
 127     tcx.ensure().specialization_graph_of(def_id);
 128
 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().subst_identity();
 132
 133         check_impl(tcx, impl_def_id, trait_ref);
 134         check_object_overlap(tcx, impl_def_id, trait_ref);
 135
 136         unsafety::check_item(tcx, impl_def_id);
 137         tcx.ensure().orphan_check_impl(impl_def_id);
 138     }
 139
 140     builtin::check_trait(tcx, def_id);
 141 }
 142
 143 /// Checks whether an impl overlaps with the automatic `impl Trait for dyn Trait`.
 144 fn check_object_overlap<'tcx>(
 145     tcx: TyCtxt<'tcx>,
 146     impl_def_id: LocalDefId,
 147     trait_ref: ty::TraitRef<'tcx>,
 148 ) {
 149     let trait_def_id = trait_ref.def_id;
 150
 151     if trait_ref.references_error() {
 152         debug!("coherence: skipping impl {:?} with error {:?}", impl_def_id, trait_ref);
 153         return;
 154     }
 155
 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.
 160
 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,
 168             }
 169         });
 170
 171         for component_def_id in component_def_ids {
 172             if !tcx.check_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,
 177                 // so this is valid.
 178             } else {
 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);
 182                     struct_span_err!(
 183                         tcx.sess,
 184                         span,
 185                         E0371,
 186                         "the object type `{}` automatically implements the trait `{}`",
 187                         trait_ref.self_ty(),
 188                         tcx.def_path_str(trait_def_id)
 189                     )
 190                     .span_label(
 191                         span,
 192                         format!(
 193                             "`{}` automatically implements trait `{}`",
 194                             trait_ref.self_ty(),
 195                             tcx.def_path_str(trait_def_id)
 196                         ),
 197                     )
 198                     .emit();
 199                 }
 200             }
 201         }
 202     }
 203 }