Prefer built-in sized impls for rigid types always

compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs

@@ -86,10 +86,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 // `Pointee` is automatically implemented for every type.
                 candidates.vec.push(BuiltinCandidate { has_nested: false });
             } else if tcx.is_lang_item(def_id, LangItem::Sized) {
-                // Sized is never implementable by end-users, it is
-                // always automatically computed.
-                let sized_conditions = self.sized_conditions(obligation);
-                self.assemble_builtin_bound_candidates(sized_conditions, &mut candidates);
+                self.assemble_builtin_sized_candidate(obligation, &mut candidates);
             } else if tcx.is_lang_item(def_id, LangItem::Unsize) {
                 self.assemble_candidates_for_unsizing(obligation, &mut candidates);
             } else if tcx.is_lang_item(def_id, LangItem::Destruct) {
@@ -1059,6 +1056,25 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
     /// Assembles the trait which are built-in to the language itself:
     /// `Copy`, `Clone` and `Sized`.
     #[instrument(level = "debug", skip(self, candidates))]
+    fn assemble_builtin_sized_candidate(
+        &mut self,
+        obligation: &PolyTraitObligation<'tcx>,
+        candidates: &mut SelectionCandidateSet<'tcx>,
+    ) {
+        match self.sized_conditions(obligation) {
+            BuiltinImplConditions::Where(_) => {
+                candidates.vec.push(SizedCandidate);
+            }
+            BuiltinImplConditions::None => {}
+            BuiltinImplConditions::Ambiguous => {
+                candidates.ambiguous = true;
+            }
+        }
+    }
+
+    /// Assembles the trait which are built-in to the language itself:
+    /// e.g. `Copy` and `Clone`.
+    #[instrument(level = "debug", skip(self, candidates))]
     fn assemble_builtin_bound_candidates(
         &mut self,
         conditions: BuiltinImplConditions<'tcx>,

compiler/rustc_trait_selection/src/traits/select/confirmation.rs

@@ -40,6 +40,11 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         candidate: SelectionCandidate<'tcx>,
     ) -> Result<Selection<'tcx>, SelectionError<'tcx>> {
         let mut impl_src = match candidate {
+            SizedCandidate => {
+                let data = self.confirm_builtin_candidate(obligation, true);
+                ImplSource::Builtin(BuiltinImplSource::Misc, data)
+            }
+
             BuiltinCandidate { has_nested } => {
                 let data = self.confirm_builtin_candidate(obligation, has_nested);
                 ImplSource::Builtin(BuiltinImplSource::Misc, data)

compiler/rustc_trait_selection/src/traits/select/mod.rs

@@ -1801,6 +1801,16 @@ impl<'tcx> SelectionContext<'_, 'tcx> {
             return Some(candidates.pop().unwrap().candidate);
         }
 
+        // We prefer `Sized` candidates over everything.
+        let mut sized_candidates =
+            candidates.iter().filter(|c| matches!(c.candidate, SizedCandidate));
+        if let Some(_sized_candidate) = sized_candidates.next() {
+            // There should only ever be a single sized candidate
+            // as they would otherwise overlap.
+            debug_assert_eq!(sized_candidates.next(), None);
+            return Some(SizedCandidate);
+        }
+
         // We prefer trivial builtin candidates, i.e. builtin impls without any nested
         // requirements, over all others. This is a fix for #53123 and prevents winnowing
         // from accidentally extending the lifetime of a variable.
@@ -1940,7 +1950,8 @@ impl<'tcx> SelectionContext<'_, 'tcx> {
             // Don't use impl candidates which overlap with other candidates.
             // This should pretty much only ever happen with malformed impls.
             if candidates.iter().all(|c| match c.candidate {
-                BuiltinCandidate { has_nested: _ }
+                SizedCandidate
+                | BuiltinCandidate { has_nested: _ }
                 | TransmutabilityCandidate
                 | AutoImplCandidate
                 | ClosureCandidate { .. }