src/librustc_typeck/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, LOCAL_CRATE};
  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 `DiscriminantKind`, `Sized` and `Unsize` for now.
  52     if did == li.discriminant_kind_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 `DiscriminantKind` trait are not permitted"
  59         )
  60         .span_label(span, "impl of 'DiscriminantKind' not allowed")
  61         .emit();
  62         return;
  63     }
  64
  65     if did == li.sized_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 `Sized` trait are not permitted"
  72         )
  73         .span_label(span, "impl of 'Sized' not allowed")
  74         .emit();
  75         return;
  76     }
  77
  78     if did == li.unsize_trait() {
  79         let span = impl_header_span(tcx, impl_def_id);
  80         struct_span_err!(
  81             tcx.sess,
  82             span,
  83             E0328,
  84             "explicit impls for the `Unsize` trait are not permitted"
  85         )
  86         .span_label(span, "impl of `Unsize` not allowed")
  87         .emit();
  88         return;
  89     }
  90
  91     if tcx.features().unboxed_closures {
  92         // the feature gate allows all Fn traits
  93         return;
  94     }
  95
  96     if let ty::trait_def::TraitSpecializationKind::AlwaysApplicable =
  97         tcx.trait_def(trait_def_id).specialization_kind
  98     {
  99         if !tcx.features().specialization && !tcx.features().min_specialization {
 100             let span = impl_header_span(tcx, impl_def_id);
 101             tcx.sess
 102                 .struct_span_err(
 103                     span,
 104                     "implementing `rustc_specialization_trait` traits is unstable",
 105                 )
 106                 .help("add `#![feature(min_specialization)]` to the crate attributes to enable")
 107                 .emit();
 108             return;
 109         }
 110     }
 111
 112     let trait_name = if did == li.fn_trait() {
 113         "Fn"
 114     } else if did == li.fn_mut_trait() {
 115         "FnMut"
 116     } else if did == li.fn_once_trait() {
 117         "FnOnce"
 118     } else {
 119         return; // everything OK
 120     };
 121
 122     let span = impl_header_span(tcx, impl_def_id);
 123     struct_span_err!(
 124         tcx.sess,
 125         span,
 126         E0183,
 127         "manual implementations of `{}` are experimental",
 128         trait_name
 129     )
 130     .span_label(span, format!("manual implementations of `{}` are experimental", trait_name))
 131     .help("add `#![feature(unboxed_closures)]` to the crate attributes to enable")
 132     .emit();
 133 }
 134
 135 /// We allow impls of marker traits to overlap, so they can't override impls
 136 /// as that could make it ambiguous which associated item to use.
 137 fn enforce_empty_impls_for_marker_traits(
 138     tcx: TyCtxt<'_>,
 139     impl_def_id: LocalDefId,
 140     trait_def_id: DefId,
 141 ) {
 142     if !tcx.trait_def(trait_def_id).is_marker {
 143         return;
 144     }
 145
 146     if tcx.associated_item_def_ids(trait_def_id).is_empty() {
 147         return;
 148     }
 149
 150     let span = impl_header_span(tcx, impl_def_id);
 151     struct_span_err!(tcx.sess, span, E0715, "impls for marker traits cannot contain items").emit();
 152 }
 153
 154 pub fn provide(providers: &mut Providers<'_>) {
 155     use self::builtin::coerce_unsized_info;
 156     use self::inherent_impls::{crate_inherent_impls, inherent_impls};
 157     use self::inherent_impls_overlap::crate_inherent_impls_overlap_check;
 158
 159     *providers = Providers {
 160         coherent_trait,
 161         crate_inherent_impls,
 162         inherent_impls,
 163         crate_inherent_impls_overlap_check,
 164         coerce_unsized_info,
 165         ..*providers
 166     };
 167 }
 168
 169 fn coherent_trait(tcx: TyCtxt<'_>, def_id: DefId) {
 170     // Trigger building the specialization graph for the trait. This will detect and report any
 171     // overlap errors.
 172     tcx.ensure().specialization_graph_of(def_id);
 173
 174     let impls = tcx.hir().trait_impls(def_id);
 175     for &hir_id in impls {
 176         let impl_def_id = tcx.hir().local_def_id(hir_id);
 177         let trait_ref = tcx.impl_trait_ref(impl_def_id).unwrap();
 178
 179         check_impl(tcx, impl_def_id, trait_ref);
 180         check_object_overlap(tcx, impl_def_id, trait_ref);
 181     }
 182     builtin::check_trait(tcx, def_id);
 183 }
 184
 185 pub fn check_coherence(tcx: TyCtxt<'_>) {
 186     for &trait_def_id in tcx.hir().krate().trait_impls.keys() {
 187         tcx.ensure().coherent_trait(trait_def_id);
 188     }
 189
 190     tcx.sess.time("unsafety_checking", || unsafety::check(tcx));
 191     tcx.sess.time("orphan_checking", || orphan::check(tcx));
 192
 193     // these queries are executed for side-effects (error reporting):
 194     tcx.ensure().crate_inherent_impls(LOCAL_CRATE);
 195     tcx.ensure().crate_inherent_impls_overlap_check(LOCAL_CRATE);
 196 }
 197
 198 /// Checks whether an impl overlaps with the automatic `impl Trait for dyn Trait`.
 199 fn check_object_overlap<'tcx>(
 200     tcx: TyCtxt<'tcx>,
 201     impl_def_id: LocalDefId,
 202     trait_ref: ty::TraitRef<'tcx>,
 203 ) {
 204     let trait_def_id = trait_ref.def_id;
 205
 206     if trait_ref.references_error() {
 207         debug!("coherence: skipping impl {:?} with error {:?}", impl_def_id, trait_ref);
 208         return;
 209     }
 210
 211     // check for overlap with the automatic `impl Trait for dyn Trait`
 212     if let ty::Dynamic(ref data, ..) = trait_ref.self_ty().kind {
 213         // This is something like impl Trait1 for Trait2. Illegal
 214         // if Trait1 is a supertrait of Trait2 or Trait2 is not object safe.
 215
 216         let component_def_ids = data.iter().flat_map(|predicate| {
 217             match predicate.skip_binder() {
 218                 ty::ExistentialPredicate::Trait(tr) => Some(tr.def_id),
 219                 ty::ExistentialPredicate::AutoTrait(def_id) => Some(*def_id),
 220                 // An associated type projection necessarily comes with
 221                 // an additional `Trait` requirement.
 222                 ty::ExistentialPredicate::Projection(..) => None,
 223             }
 224         });
 225
 226         for component_def_id in component_def_ids {
 227             if !tcx.is_object_safe(component_def_id) {
 228                 // Without the 'object_safe_for_dispatch' feature this is an error
 229                 // which will be reported by wfcheck.  Ignore it here.
 230                 // This is tested by `coherence-impl-trait-for-trait-object-safe.rs`.
 231                 // With the feature enabled, the trait is not implemented automatically,
 232                 // so this is valid.
 233             } else {
 234                 let mut supertrait_def_ids = traits::supertrait_def_ids(tcx, component_def_id);
 235                 if supertrait_def_ids.any(|d| d == trait_def_id) {
 236                     let span = impl_header_span(tcx, impl_def_id);
 237                     struct_span_err!(
 238                         tcx.sess,
 239                         span,
 240                         E0371,
 241                         "the object type `{}` automatically implements the trait `{}`",
 242                         trait_ref.self_ty(),
 243                         tcx.def_path_str(trait_def_id)
 244                     )
 245                     .span_label(
 246                         span,
 247                         format!(
 248                             "`{}` automatically implements trait `{}`",
 249                             trait_ref.self_ty(),
 250                             tcx.def_path_str(trait_def_id)
 251                         ),
 252                     )
 253                     .emit();
 254                 }
 255             }
 256         }
 257     }
 258 }