fix: Reject async assoc fns of const traits/impls in ast_passes

compiler/rustc_ast_passes/src/ast_validation.rs

@@ -293,6 +293,21 @@ impl<'a> AstValidator<'a> {
         });
     }
 
+    fn check_async_fn_in_const_trait_or_impl(&self, sig: &FnSig, parent: &TraitOrTraitImpl) {
+        let Some(const_keyword) = parent.constness() else { return };
+
+        let Some(CoroutineKind::Async { span: async_keyword, .. }) = sig.header.coroutine_kind
+        else {
+            return;
+        };
+
+        self.dcx().emit_err(errors::AsyncFnInConstTraitOrTraitImpl {
+            async_keyword,
+            in_impl: matches!(parent, TraitOrTraitImpl::TraitImpl { .. }),
+            const_keyword,
+        });
+    }
+
     fn check_fn_decl(&self, fn_decl: &FnDecl, self_semantic: SelfSemantic) {
         self.check_decl_num_args(fn_decl);
         self.check_decl_cvariadic_pos(fn_decl);
@@ -1566,6 +1581,7 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
             self.visibility_not_permitted(&item.vis, errors::VisibilityNotPermittedNote::TraitImpl);
             if let AssocItemKind::Fn(box Fn { sig, .. }) = &item.kind {
                 self.check_trait_fn_not_const(sig.header.constness, parent);
+                self.check_async_fn_in_const_trait_or_impl(sig, parent);
             }
         }