Implement #[rustc_skip_array_during_method_dispatch]

compiler/rustc_feature/src/builtin_attrs.rs

@@ -540,6 +540,11 @@ pub const BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
         rustc_main, Normal, template!(Word),
         "the `#[rustc_main]` attribute is used internally to specify test entry point function",
     ),
+    rustc_attr!(
+        rustc_skip_array_during_method_dispatch, Normal, template!(Word),
+        "the `#[rustc_skip_array_during_method_dispatch]` attribute is used to exclude a trait \
+        from method dispatch when the receiver is an array, for compatibility in editions < 2021."
+    ),
 
     // ==========================================================================
     // Internal attributes, Testing:

compiler/rustc_metadata/src/rmeta/decoder.rs

@@ -757,6 +757,7 @@ impl<'a, 'tcx> CrateMetadataRef<'a> {
                     data.paren_sugar,
                     data.has_auto_impl,
                     data.is_marker,
+                    data.skip_array_during_method_dispatch,
                     data.specialization_kind,
                     self.def_path_hash(item_id),
                 )
@@ -767,6 +768,7 @@ impl<'a, 'tcx> CrateMetadataRef<'a> {
                 false,
                 false,
                 false,
+                false,
                 ty::trait_def::TraitSpecializationKind::None,
                 self.def_path_hash(item_id),
             ),

compiler/rustc_metadata/src/rmeta/encoder.rs

@@ -1422,6 +1422,7 @@ impl EncodeContext<'a, 'tcx> {
                     paren_sugar: trait_def.paren_sugar,
                     has_auto_impl: self.tcx.trait_is_auto(def_id),
                     is_marker: trait_def.is_marker,
+                    skip_array_during_method_dispatch: trait_def.skip_array_during_method_dispatch,
                     specialization_kind: trait_def.specialization_kind,
                 };
 

compiler/rustc_metadata/src/rmeta/mod.rs

@@ -385,6 +385,7 @@ struct TraitData {
     paren_sugar: bool,
     has_auto_impl: bool,
     is_marker: bool,
+    skip_array_during_method_dispatch: bool,
     specialization_kind: ty::trait_def::TraitSpecializationKind,
 }
 

compiler/rustc_middle/src/ty/trait_def.rs

@@ -35,6 +35,11 @@ pub struct TraitDef {
     /// and thus `impl`s of it are allowed to overlap.
     pub is_marker: bool,
 
+    /// If `true`, then this trait has the `#[rustc_skip_array_during_method_dispatch]`
+    /// attribute, indicating that editions before 2021 should not consider this trait
+    /// during method dispatch if the receiver is an array.
+    pub skip_array_during_method_dispatch: bool,
+
     /// Used to determine whether the standard library is allowed to specialize
     /// on this trait.
     pub specialization_kind: TraitSpecializationKind,
@@ -82,6 +87,7 @@ impl<'tcx> TraitDef {
         paren_sugar: bool,
         has_auto_impl: bool,
         is_marker: bool,
+        skip_array_during_method_dispatch: bool,
         specialization_kind: TraitSpecializationKind,
         def_path_hash: DefPathHash,
     ) -> TraitDef {
@@ -91,6 +97,7 @@ impl<'tcx> TraitDef {
             paren_sugar,
             has_auto_impl,
             is_marker,
+            skip_array_during_method_dispatch,
             specialization_kind,
             def_path_hash,
         }

compiler/rustc_span/src/symbol.rs

@@ -1030,6 +1030,7 @@ symbols! {
         rustc_regions,
         rustc_reservation_impl,
         rustc_serialize,
+        rustc_skip_array_during_method_dispatch,
         rustc_specialization_trait,
         rustc_stable,
         rustc_std_internal_symbol,

compiler/rustc_typeck/src/check/method/probe.rs

@@ -1461,6 +1461,15 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
                 }
 
                 TraitCandidate(trait_ref) => {
+                    if let Some(method_name) = self.method_name {
+                        // Some trait methods are excluded for arrays before 2021.
+                        if self_ty.is_array() && !method_name.span.rust_2021() {
+                            let trait_def = self.tcx.trait_def(trait_ref.def_id);
+                            if trait_def.skip_array_during_method_dispatch {
+                                return ProbeResult::NoMatch;
+                            }
+                        }
+                    }
                     let predicate = trait_ref.without_const().to_predicate(self.tcx);
                     let obligation = traits::Obligation::new(cause, self.param_env, predicate);
                     if !self.predicate_may_hold(&obligation) {

compiler/rustc_typeck/src/collect.rs

@@ -1191,6 +1191,8 @@ fn trait_def(tcx: TyCtxt<'_>, def_id: DefId) -> ty::TraitDef {
     }
 
     let is_marker = tcx.has_attr(def_id, sym::marker);
+    let skip_array_during_method_dispatch =
+        tcx.has_attr(def_id, sym::rustc_skip_array_during_method_dispatch);
     let spec_kind = if tcx.has_attr(def_id, sym::rustc_unsafe_specialization_marker) {
         ty::trait_def::TraitSpecializationKind::Marker
     } else if tcx.has_attr(def_id, sym::rustc_specialization_trait) {
@@ -1199,7 +1201,16 @@ fn trait_def(tcx: TyCtxt<'_>, def_id: DefId) -> ty::TraitDef {
         ty::trait_def::TraitSpecializationKind::None
     };
     let def_path_hash = tcx.def_path_hash(def_id);
-    ty::TraitDef::new(def_id, unsafety, paren_sugar, is_auto, is_marker, spec_kind, def_path_hash)
+    ty::TraitDef::new(
+        def_id,
+        unsafety,
+        paren_sugar,
+        is_auto,
+        is_marker,
+        skip_array_during_method_dispatch,
+        spec_kind,
+        def_path_hash,
+    )
 }
 
 fn has_late_bound_regions<'tcx>(tcx: TyCtxt<'tcx>, node: Node<'tcx>) -> Option<Span> {