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::traits;
   9 use rustc::ty::query::Providers;
  10 use rustc::ty::{self, TyCtxt, TypeFoldable};
  11 use rustc_error_codes::*;
  12 use rustc_errors::struct_span_err;
  13 use rustc_hir::def_id::{DefId, LOCAL_CRATE};
  14 use rustc_hir::HirId;
  15
  16 mod builtin;
  17 mod inherent_impls;
  18 mod inherent_impls_overlap;
  19 mod orphan;
  20 mod unsafety;
  21
  22 fn check_impl(tcx: TyCtxt<'_>, hir_id: HirId) {
  23     let impl_def_id = tcx.hir().local_def_id(hir_id);
  24
  25     // If there are no traits, then this implementation must have a
  26     // base type.
  27
  28     if let Some(trait_ref) = tcx.impl_trait_ref(impl_def_id) {
  29         debug!(
  30             "(checking implementation) adding impl for trait '{:?}', item '{}'",
  31             trait_ref,
  32             tcx.def_path_str(impl_def_id)
  33         );
  34
  35         // Skip impls where one of the self type is an error type.
  36         // This occurs with e.g., resolve failures (#30589).
  37         if trait_ref.references_error() {
  38             return;
  39         }
  40
  41         enforce_trait_manually_implementable(tcx, impl_def_id, trait_ref.def_id);
  42         enforce_empty_impls_for_marker_traits(tcx, impl_def_id, trait_ref.def_id);
  43     }
  44 }
  45
  46 fn enforce_trait_manually_implementable(tcx: TyCtxt<'_>, impl_def_id: DefId, trait_def_id: DefId) {
  47     let did = Some(trait_def_id);
  48     let li = tcx.lang_items();
  49     let span = tcx.sess.source_map().def_span(tcx.span_of_impl(impl_def_id).unwrap());
  50
  51     // Disallow *all* explicit impls of `Sized` and `Unsize` for now.
  52     if did == li.sized_trait() {
  53         struct_span_err!(
  54             tcx.sess,
  55             span,
  56             E0322,
  57             "explicit impls for the `Sized` trait are not permitted"
  58         )
  59         .span_label(span, "impl of 'Sized' not allowed")
  60         .emit();
  61         return;
  62     }
  63
  64     if did == li.unsize_trait() {
  65         struct_span_err!(
  66             tcx.sess,
  67             span,
  68             E0328,
  69             "explicit impls for the `Unsize` trait are not permitted"
  70         )
  71         .span_label(span, "impl of `Unsize` not allowed")
  72         .emit();
  73         return;
  74     }
  75
  76     if tcx.features().unboxed_closures {
  77         // the feature gate allows all Fn traits
  78         return;
  79     }
  80
  81     let trait_name = if did == li.fn_trait() {
  82         "Fn"
  83     } else if did == li.fn_mut_trait() {
  84         "FnMut"
  85     } else if did == li.fn_once_trait() {
  86         "FnOnce"
  87     } else {
  88         return; // everything OK
  89     };
  90     struct_span_err!(
  91         tcx.sess,
  92         span,
  93         E0183,
  94         "manual implementations of `{}` are experimental",
  95         trait_name
  96     )
  97     .span_label(span, format!("manual implementations of `{}` are experimental", trait_name))
  98     .help("add `#![feature(unboxed_closures)]` to the crate attributes to enable")
  99     .emit();
 100 }
 101
 102 /// We allow impls of marker traits to overlap, so they can't override impls
 103 /// as that could make it ambiguous which associated item to use.
 104 fn enforce_empty_impls_for_marker_traits(tcx: TyCtxt<'_>, impl_def_id: DefId, trait_def_id: DefId) {
 105     if !tcx.trait_def(trait_def_id).is_marker {
 106         return;
 107     }
 108
 109     if tcx.associated_item_def_ids(trait_def_id).is_empty() {
 110         return;
 111     }
 112
 113     let span = tcx.sess.source_map().def_span(tcx.span_of_impl(impl_def_id).unwrap());
 114     struct_span_err!(tcx.sess, span, E0715, "impls for marker traits cannot contain items").emit();
 115 }
 116
 117 pub fn provide(providers: &mut Providers<'_>) {
 118     use self::builtin::coerce_unsized_info;
 119     use self::inherent_impls::{crate_inherent_impls, inherent_impls};
 120     use self::inherent_impls_overlap::crate_inherent_impls_overlap_check;
 121
 122     *providers = Providers {
 123         coherent_trait,
 124         crate_inherent_impls,
 125         inherent_impls,
 126         crate_inherent_impls_overlap_check,
 127         coerce_unsized_info,
 128         ..*providers
 129     };
 130 }
 131
 132 fn coherent_trait(tcx: TyCtxt<'_>, def_id: DefId) {
 133     let impls = tcx.hir().trait_impls(def_id);
 134     for &impl_id in impls {
 135         check_impl(tcx, impl_id);
 136     }
 137     for &impl_id in impls {
 138         check_impl_overlap(tcx, impl_id);
 139     }
 140     builtin::check_trait(tcx, def_id);
 141 }
 142
 143 pub fn check_coherence(tcx: TyCtxt<'_>) {
 144     for &trait_def_id in tcx.hir().krate().trait_impls.keys() {
 145         tcx.ensure().coherent_trait(trait_def_id);
 146     }
 147
 148     tcx.sess.time("unsafety_checking", || unsafety::check(tcx));
 149     tcx.sess.time("orphan_checking", || orphan::check(tcx));
 150
 151     // these queries are executed for side-effects (error reporting):
 152     tcx.ensure().crate_inherent_impls(LOCAL_CRATE);
 153     tcx.ensure().crate_inherent_impls_overlap_check(LOCAL_CRATE);
 154 }
 155
 156 /// Overlap: no two impls for the same trait are implemented for the
 157 /// same type. Likewise, no two inherent impls for a given type
 158 /// constructor provide a method with the same name.
 159 fn check_impl_overlap<'tcx>(tcx: TyCtxt<'tcx>, hir_id: HirId) {
 160     let impl_def_id = tcx.hir().local_def_id(hir_id);
 161     let trait_ref = tcx.impl_trait_ref(impl_def_id).unwrap();
 162     let trait_def_id = trait_ref.def_id;
 163
 164     if trait_ref.references_error() {
 165         debug!("coherence: skipping impl {:?} with error {:?}", impl_def_id, trait_ref);
 166         return;
 167     }
 168
 169     // Trigger building the specialization graph for the trait of this impl.
 170     // This will detect any overlap errors.
 171     tcx.specialization_graph_of(trait_def_id);
 172
 173     // check for overlap with the automatic `impl Trait for Trait`
 174     if let ty::Dynamic(ref data, ..) = trait_ref.self_ty().kind {
 175         // This is something like impl Trait1 for Trait2. Illegal
 176         // if Trait1 is a supertrait of Trait2 or Trait2 is not object safe.
 177
 178         let component_def_ids = data.iter().flat_map(|predicate| {
 179             match predicate.skip_binder() {
 180                 ty::ExistentialPredicate::Trait(tr) => Some(tr.def_id),
 181                 ty::ExistentialPredicate::AutoTrait(def_id) => Some(*def_id),
 182                 // An associated type projection necessarily comes with
 183                 // an additional `Trait` requirement.
 184                 ty::ExistentialPredicate::Projection(..) => None,
 185             }
 186         });
 187
 188         for component_def_id in component_def_ids {
 189             if !tcx.is_object_safe(component_def_id) {
 190                 // Without the 'object_safe_for_dispatch' feature this is an error
 191                 // which will be reported by wfcheck.  Ignore it here.
 192                 // This is tested by `coherence-impl-trait-for-trait-object-safe.rs`.
 193                 // With the feature enabled, the trait is not implemented automatically,
 194                 // so this is valid.
 195             } else {
 196                 let mut supertrait_def_ids = traits::supertrait_def_ids(tcx, component_def_id);
 197                 if supertrait_def_ids.any(|d| d == trait_def_id) {
 198                     let sp = tcx.sess.source_map().def_span(tcx.span_of_impl(impl_def_id).unwrap());
 199                     struct_span_err!(
 200                         tcx.sess,
 201                         sp,
 202                         E0371,
 203                         "the object type `{}` automatically implements the trait `{}`",
 204                         trait_ref.self_ty(),
 205                         tcx.def_path_str(trait_def_id)
 206                     )
 207                     .span_label(
 208                         sp,
 209                         format!(
 210                             "`{}` automatically implements trait `{}`",
 211                             trait_ref.self_ty(),
 212                             tcx.def_path_str(trait_def_id)
 213                         ),
 214                     )
 215                     .emit();
 216                 }
 217             }
 218         }
 219     }
 220 }