Add fundamental to trait def

compiler/rustc_hir_analysis/src/collect.rs

@@ -1201,6 +1201,7 @@ fn trait_def(tcx: TyCtxt<'_>, def_id: LocalDefId) -> ty::TraitDef {
 
     let is_marker = tcx.has_attr(def_id, sym::marker);
     let rustc_coinductive = tcx.has_attr(def_id, sym::rustc_coinductive);
+    let is_fundamental = tcx.has_attr(def_id, sym::fundamental);
 
     // FIXME: We could probably do way better attribute validation here.
     let mut skip_array_during_method_dispatch = false;
@@ -1352,6 +1353,7 @@ fn trait_def(tcx: TyCtxt<'_>, def_id: LocalDefId) -> ty::TraitDef {
         has_auto_impl: is_auto,
         is_marker,
         is_coinductive: rustc_coinductive || is_auto,
+        is_fundamental,
         skip_array_during_method_dispatch,
         skip_boxed_slice_during_method_dispatch,
         specialization_kind,

compiler/rustc_middle/src/ty/trait_def.rs

@@ -31,7 +31,7 @@ pub struct TraitDef {
     /// and thus `impl`s of it are allowed to overlap.
     pub is_marker: bool,
 
-    /// If `true`, then this trait has to `#[rustc_coinductive]` attribute or
+    /// If `true`, then this trait has the `#[rustc_coinductive]` attribute or
     /// is an auto trait. This indicates that trait solver cycles involving an
     /// `X: ThisTrait` goal are accepted.
     ///
@@ -40,6 +40,11 @@ pub struct TraitDef {
     /// also have already switched to the new trait solver.
     pub is_coinductive: bool,
 
+    /// If `true`, then this trait has the `#[fundamental]` attribute. This
+    /// affects how conherence computes whether a trait may have trait implementations
+    /// added in the future.
+    pub is_fundamental: bool,
+
     /// If `true`, then this trait has the `#[rustc_skip_during_method_dispatch(array)]`
     /// attribute, indicating that editions before 2021 should not consider this trait
     /// during method dispatch if the receiver is an array.

compiler/rustc_trait_selection/src/traits/coherence.rs

@@ -666,7 +666,7 @@ pub fn trait_ref_is_local_or_fundamental<'tcx>(
     tcx: TyCtxt<'tcx>,
     trait_ref: ty::TraitRef<'tcx>,
 ) -> bool {
-    trait_ref.def_id.is_local() || tcx.has_attr(trait_ref.def_id, sym::fundamental)
+    trait_ref.def_id.is_local() || tcx.trait_def(trait_ref.def_id).is_fundamental
 }
 
 #[derive(Debug, Copy, Clone)]