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