From c700d015b433024ab9123ad9a36f9af3a65d81f4 Mon Sep 17 00:00:00 2001
From: Shoyu Vanilla <modulo641@gmail.com>
Date: Mon, 4 Aug 2025 23:59:28 +0900
Subject: [PATCH] fix: Reject async assoc fns of `#[const_trait]` in ast_passes

---
 compiler/rustc_ast_passes/messages.ftl            |  4 ++++
 compiler/rustc_ast_passes/src/ast_validation.rs   | 15 +++++++++++++++
 compiler/rustc_ast_passes/src/errors.rs           |  9 +++++++++
 .../const-traits/const-trait-async-assoc-fn.rs}   |  5 ++---
 .../const-trait-async-assoc-fn.stderr             | 10 ++++++++++
 5 files changed, 40 insertions(+), 3 deletions(-)
 rename tests/{crashes/117629.rs => ui/traits/const-traits/const-trait-async-assoc-fn.rs} (52%)
 create mode 100644 tests/ui/traits/const-traits/const-trait-async-assoc-fn.stderr

diff --git a/compiler/rustc_ast_passes/messages.ftl b/compiler/rustc_ast_passes/messages.ftl
index af93d55c89826..0c2ca24f48a75 100644
--- a/compiler/rustc_ast_passes/messages.ftl
+++ b/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` traits
+    .label = associated functions of `const` 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
diff --git a/compiler/rustc_ast_passes/src/ast_validation.rs b/compiler/rustc_ast_passes/src/ast_validation.rs
index 895a457ec1d56..e8400bee979d0 100644
--- a/compiler/rustc_ast_passes/src/ast_validation.rs
+++ b/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);
             }
         }
 
diff --git a/compiler/rustc_ast_passes/src/errors.rs b/compiler/rustc_ast_passes/src/errors.rs
index fd4b2528541e3..2eb065d3436b6 100644
--- a/compiler/rustc_ast_passes/src/errors.rs
+++ b/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 {
diff --git a/tests/crashes/117629.rs b/tests/ui/traits/const-traits/const-trait-async-assoc-fn.rs
similarity index 52%
rename from tests/crashes/117629.rs
rename to tests/ui/traits/const-traits/const-trait-async-assoc-fn.rs
index f63365395c6b4..ccd5b1ae8c01f 100644
--- a/tests/crashes/117629.rs
+++ b/tests/ui/traits/const-traits/const-trait-async-assoc-fn.rs
@@ -1,10 +1,9 @@
-//@ known-bug: #117629
-//@ edition:2021
-
+//@ edition: 2021
 #![feature(const_trait_impl)]
 
 const trait Tr {
     async fn ft1() {}
+//~^ ERROR async functions are not allowed in `const` traits
 }
 
 fn main() {}
diff --git a/tests/ui/traits/const-traits/const-trait-async-assoc-fn.stderr b/tests/ui/traits/const-traits/const-trait-async-assoc-fn.stderr
new file mode 100644
index 0000000000000..48f0ff92a9e85
--- /dev/null
+++ b/tests/ui/traits/const-traits/const-trait-async-assoc-fn.stderr
@@ -0,0 +1,10 @@
+error: async functions are not allowed in `const` traits
+  --> $DIR/const-trait-async-assoc-fn.rs:5:5
+   |
+LL | const trait Tr {
+   | ----- associated functions of `const` cannot be declared `async`
+LL |     async fn ft1() {}
+   |     ^^^^^
+
+error: aborting due to 1 previous error
+