fix: Reject async assoc fns of #[const_trait] in ast_passes

compiler/rustc_ast_passes/messages.ftl

@@ -32,6 +32,10 @@ ast_passes_assoc_type_without_body =
     associated type in `impl` without body
     .suggestion = provide a definition for the type
 
+ast_passes_async_fn_in_const_trait =
+    async functions are not allowed in `#[const_trait]`
+    .label = associated functions of `#[const_trait]` cannot be declared async
+
 ast_passes_at_least_one_trait = at least one trait must be specified
 
 ast_passes_auto_generic = auto traits cannot have generic parameters

compiler/rustc_ast_passes/src/ast_validation.rs

@@ -294,6 +294,20 @@ impl<'a> AstValidator<'a> {
         });
     }
 
+    fn check_async_fn_in_const_trait(&self, sig: &FnSig, parent: &TraitOrTraitImpl) {
+        let TraitOrTraitImpl::Trait { constness: Const::Yes(const_trait), .. } = parent else {
+            return;
+        };
+
+        let Some(CoroutineKind::Async { span: async_keyword, .. }) = sig.header.coroutine_kind
+        else {
+            return;
+        };
+
+        self.dcx()
+            .emit_err(errors::AsyncFnInConstTrait { async_keyword, const_trait: *const_trait });
+    }
+
     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);
@@ -1586,6 +1600,7 @@ 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(sig, parent);
             }
         }
 

compiler/rustc_ast_passes/src/errors.rs

@@ -62,6 +62,15 @@ pub(crate) struct TraitFnConst {
     pub make_trait_const_sugg: Option<Span>,
 }
 
+#[derive(Diagnostic)]
+#[diag(ast_passes_async_fn_in_const_trait)]
+pub(crate) struct AsyncFnInConstTrait {
+    #[primary_span]
+    pub async_keyword: Span,
+    #[label]
+    pub const_trait: Span,
+}
+
 #[derive(Diagnostic)]
 #[diag(ast_passes_forbidden_bound)]
 pub(crate) struct ForbiddenBound {

tests/ui/traits/const-traits/const-trait-async-assoc-fn.rs

@@ -0,0 +1,10 @@
+//@ edition: 2021
+#![feature(const_trait_impl)]
+
+#[const_trait]
+trait Tr {
+    async fn ft1() {}
+//~^ ERROR async functions are not allowed in `#[const_trait]`
+}
+
+fn main() {}

tests/ui/traits/const-traits/const-trait-async-assoc-fn.stderr

@@ -0,0 +1,11 @@
+error: async functions are not allowed in `#[const_trait]`
+  --> $DIR/const-trait-async-assoc-fn.rs:6:5
+   |
+LL | #[const_trait]
+   | -------------- associated functions of `#[const_trait]` cannot be declared async
+LL | trait Tr {
+LL |     async fn ft1() {}
+   |     ^^^^^
+
+error: aborting due to 1 previous error
+