compiler/rustc_typeck/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::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, TypeFoldable};
  12 use rustc_span::Span;
  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 /// Obtains the span of just the impl header of `impl_def_id`.
  22 fn impl_header_span(tcx: TyCtxt<'_>, impl_def_id: LocalDefId) -> Span {
  23     tcx.sess.source_map().guess_head_span(tcx.span_of_impl(impl_def_id.to_def_id()).unwrap())
  24 }
  25
  26 fn check_impl(tcx: TyCtxt<'_>, impl_def_id: LocalDefId, trait_ref: ty::TraitRef<'_>) {
  27     debug!(
  28         "(checking implementation) adding impl for trait '{:?}', item '{}'",
  29         trait_ref,
  30         tcx.def_path_str(impl_def_id.to_def_id())
  31     );
  32
  33     // Skip impls where one of the self type is an error type.
  34     // This occurs with e.g., resolve failures (#30589).
  35     if trait_ref.references_error() {
  36         return;
  37     }
  38
  39     enforce_trait_manually_implementable(tcx, impl_def_id, trait_ref.def_id);
  40     enforce_empty_impls_for_marker_traits(tcx, impl_def_id, trait_ref.def_id);
  41 }
  42
  43 fn enforce_trait_manually_implementable(
  44     tcx: TyCtxt<'_>,
  45     impl_def_id: LocalDefId,
  46     trait_def_id: DefId,
  47 ) {
  48     let did = Some(trait_def_id);
  49     let li = tcx.lang_items();
  50
  51     // Disallow *all* explicit impls of `Pointee`, `DiscriminantKind`, `Sized` and `Unsize` for now.
  52     if did == li.pointee_trait() {
  53         let span = impl_header_span(tcx, impl_def_id);
  54         struct_span_err!(
  55             tcx.sess,
  56             span,
  57             E0322,
  58             "explicit impls for the `Pointee` trait are not permitted"
  59         )
  60         .span_label(span, "impl of 'Pointee' not allowed")
  61         .emit();
  62         return;
  63     }
  64
  65     if did == li.discriminant_kind_trait() {
  66         let span = impl_header_span(tcx, impl_def_id);
  67         struct_span_err!(
  68             tcx.sess,
  69             span,
  70             E0322,
  71             "explicit impls for the `DiscriminantKind` trait are not permitted"
  72         )
  73         .span_label(span, "impl of 'DiscriminantKind' not allowed")
  74         .emit();
  75         return;
  76     }
  77
  78     if did == li.sized_trait() {
  79         let span = impl_header_span(tcx, impl_def_id);
  80         struct_span_err!(
  81             tcx.sess,
  82             span,
  83             E0322,
  84             "explicit impls for the `Sized` trait are not permitted"
  85         )
  86         .span_label(span, "impl of 'Sized' not allowed")
  87         .emit();
  88         return;
  89     }
  90
  91     if did == li.unsize_trait() {
  92         let span = impl_header_span(tcx, impl_def_id);
  93         struct_span_err!(
  94             tcx.sess,
  95             span,
  96             E0328,
  97             "explicit impls for the `Unsize` trait are not permitted"
  98         )
  99         .span_label(span, "impl of `Unsize` not allowed")
 100         .emit();
 101         return;
 102     }
 103
 104     if tcx.features().unboxed_closures {
 105         // the feature gate allows all Fn traits
 106         return;
 107     }
 108
 109     if let ty::trait_def::TraitSpecializationKind::AlwaysApplicable =
 110         tcx.trait_def(trait_def_id).specialization_kind
 111     {
 112         if !tcx.features().specialization && !tcx.features().min_specialization {
 113             let span = impl_header_span(tcx, impl_def_id);
 114             tcx.sess
 115                 .struct_span_err(
 116                     span,
 117                     "implementing `rustc_specialization_trait` traits is unstable",
 118                 )
 119                 .help("add `#![feature(min_specialization)]` to the crate attributes to enable")
 120                 .emit();
 121             return;
 122         }
 123     }
 124 }
 125
 126 /// We allow impls of marker traits to overlap, so they can't override impls
 127 /// as that could make it ambiguous which associated item to use.
 128 fn enforce_empty_impls_for_marker_traits(
 129     tcx: TyCtxt<'_>,
 130     impl_def_id: LocalDefId,
 131     trait_def_id: DefId,
 132 ) {
 133     if !tcx.trait_def(trait_def_id).is_marker {
 134         return;
 135     }
 136
 137     if tcx.associated_item_def_ids(trait_def_id).is_empty() {
 138         return;
 139     }
 140
 141     let span = impl_header_span(tcx, impl_def_id);
 142     struct_span_err!(tcx.sess, span, E0715, "impls for marker traits cannot contain items").emit();
 143 }
 144
 145 pub fn provide(providers: &mut Providers) {
 146     use self::builtin::coerce_unsized_info;
 147     use self::inherent_impls::{crate_incoherent_impls, crate_inherent_impls, inherent_impls};
 148     use self::inherent_impls_overlap::crate_inherent_impls_overlap_check;
 149     use self::orphan::orphan_check_crate;
 150
 151     *providers = Providers {
 152         coherent_trait,
 153         crate_inherent_impls,
 154         crate_incoherent_impls,
 155         inherent_impls,
 156         crate_inherent_impls_overlap_check,
 157         coerce_unsized_info,
 158         orphan_check_crate,
 159         ..*providers
 160     };
 161 }
 162
 163 fn coherent_trait(tcx: TyCtxt<'_>, def_id: DefId) {
 164     // Trigger building the specialization graph for the trait. This will detect and report any
 165     // overlap errors.
 166     tcx.ensure().specialization_graph_of(def_id);
 167
 168     let impls = tcx.hir().trait_impls(def_id);
 169     for &impl_def_id in impls {
 170         let trait_ref = tcx.impl_trait_ref(impl_def_id).unwrap();
 171
 172         check_impl(tcx, impl_def_id, trait_ref);
 173         check_object_overlap(tcx, impl_def_id, trait_ref);
 174     }
 175     builtin::check_trait(tcx, def_id);
 176 }
 177
 178 pub fn check_coherence(tcx: TyCtxt<'_>) {
 179     tcx.sess.time("unsafety_checking", || unsafety::check(tcx));
 180     tcx.ensure().orphan_check_crate(());
 181
 182     for &trait_def_id in tcx.all_local_trait_impls(()).keys() {
 183         tcx.ensure().coherent_trait(trait_def_id);
 184     }
 185
 186     // these queries are executed for side-effects (error reporting):
 187     tcx.ensure().crate_inherent_impls(());
 188     tcx.ensure().crate_inherent_impls_overlap_check(());
 189 }
 190
 191 /// Checks whether an impl overlaps with the automatic `impl Trait for dyn Trait`.
 192 fn check_object_overlap<'tcx>(
 193     tcx: TyCtxt<'tcx>,
 194     impl_def_id: LocalDefId,
 195     trait_ref: ty::TraitRef<'tcx>,
 196 ) {
 197     let trait_def_id = trait_ref.def_id;
 198
 199     if trait_ref.references_error() {
 200         debug!("coherence: skipping impl {:?} with error {:?}", impl_def_id, trait_ref);
 201         return;
 202     }
 203
 204     // check for overlap with the automatic `impl Trait for dyn Trait`
 205     if let ty::Dynamic(data, ..) = trait_ref.self_ty().kind() {
 206         // This is something like impl Trait1 for Trait2. Illegal
 207         // if Trait1 is a supertrait of Trait2 or Trait2 is not object safe.
 208
 209         let component_def_ids = data.iter().flat_map(|predicate| {
 210             match predicate.skip_binder() {
 211                 ty::ExistentialPredicate::Trait(tr) => Some(tr.def_id),
 212                 ty::ExistentialPredicate::AutoTrait(def_id) => Some(def_id),
 213                 // An associated type projection necessarily comes with
 214                 // an additional `Trait` requirement.
 215                 ty::ExistentialPredicate::Projection(..) => None,
 216             }
 217         });
 218
 219         for component_def_id in component_def_ids {
 220             if !tcx.is_object_safe(component_def_id) {
 221                 // Without the 'object_safe_for_dispatch' feature this is an error
 222                 // which will be reported by wfcheck.  Ignore it here.
 223                 // This is tested by `coherence-impl-trait-for-trait-object-safe.rs`.
 224                 // With the feature enabled, the trait is not implemented automatically,
 225                 // so this is valid.
 226             } else {
 227                 let mut supertrait_def_ids = traits::supertrait_def_ids(tcx, component_def_id);
 228                 if supertrait_def_ids.any(|d| d == trait_def_id) {
 229                     let span = impl_header_span(tcx, impl_def_id);
 230                     struct_span_err!(
 231                         tcx.sess,
 232                         span,
 233                         E0371,
 234                         "the object type `{}` automatically implements the trait `{}`",
 235                         trait_ref.self_ty(),
 236                         tcx.def_path_str(trait_def_id)
 237                     )
 238                     .span_label(
 239                         span,
 240                         format!(
 241                             "`{}` automatically implements trait `{}`",
 242                             trait_ref.self_ty(),
 243                             tcx.def_path_str(trait_def_id)
 244                         ),
 245                     )
 246                     .emit();
 247                 }
 248             }
 249         }
 250     }
 251 }