From c108451faf1fe034a09c02e8d5b217beafeddea6 Mon Sep 17 00:00:00 2001
From: lapla <me@lapla.dev>
Date: Thu, 20 Nov 2025 21:25:54 +0900
Subject: [PATCH] Reject `async fn` in `const impl` during AST validation

---
 compiler/rustc_ast_passes/messages.ftl                |  7 ++++---
 compiler/rustc_ast_passes/src/ast_validation.rs       | 11 +++++++++--
 compiler/rustc_ast_passes/src/errors.rs               |  2 +-
 .../const-traits/ice-149083-async-in-const-impl.rs    |  9 +++++++++
 .../ice-149083-async-in-const-impl.stderr             | 10 ++++++++++
 5 files changed, 33 insertions(+), 6 deletions(-)
 create mode 100644 tests/ui/traits/const-traits/ice-149083-async-in-const-impl.rs
 create mode 100644 tests/ui/traits/const-traits/ice-149083-async-in-const-impl.stderr

diff --git a/compiler/rustc_ast_passes/messages.ftl b/compiler/rustc_ast_passes/messages.ftl
index 3858bc80d4318..9bdbcf6ab9071 100644
--- a/compiler/rustc_ast_passes/messages.ftl
+++ b/compiler/rustc_ast_passes/messages.ftl
@@ -37,9 +37,10 @@ ast_passes_assoc_type_without_body =
     .suggestion = provide a definition for the type
 
 ast_passes_async_fn_in_const_trait_or_trait_impl =
-    async functions are not allowed in `const` {$in_impl ->
-        [true] trait impls
-        *[false] traits
+    async functions are not allowed in `const` {$context ->
+        [trait_impl] trait impls
+        [impl] impls
+        *[trait] traits
     }
     .label = associated functions of `const` cannot be declared `async`
 
diff --git a/compiler/rustc_ast_passes/src/ast_validation.rs b/compiler/rustc_ast_passes/src/ast_validation.rs
index 425ba407fd61e..163dbc3350ba2 100644
--- a/compiler/rustc_ast_passes/src/ast_validation.rs
+++ b/compiler/rustc_ast_passes/src/ast_validation.rs
@@ -312,9 +312,15 @@ impl<'a> AstValidator<'a> {
             return;
         };
 
+        let context = match parent {
+            TraitOrImpl::Trait { .. } => "trait",
+            TraitOrImpl::TraitImpl { .. } => "trait_impl",
+            TraitOrImpl::Impl { .. } => "impl",
+        };
+
         self.dcx().emit_err(errors::AsyncFnInConstTraitOrTraitImpl {
             async_keyword,
-            in_impl: matches!(parent, TraitOrImpl::TraitImpl { .. }),
+            context,
             const_keyword,
         });
     }
@@ -1714,9 +1720,10 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
                     self.check_async_fn_in_const_trait_or_impl(sig, parent);
                 }
             }
-            Some(TraitOrImpl::Impl { constness }) => {
+            Some(parent @ TraitOrImpl::Impl { constness }) => {
                 if let AssocItemKind::Fn(box Fn { sig, .. }) = &item.kind {
                     self.check_impl_fn_not_const(sig.header.constness, *constness);
+                    self.check_async_fn_in_const_trait_or_impl(sig, parent);
                 }
             }
             None => {}
diff --git a/compiler/rustc_ast_passes/src/errors.rs b/compiler/rustc_ast_passes/src/errors.rs
index 48fd6a7cdd5d0..02e6ecfbaee74 100644
--- a/compiler/rustc_ast_passes/src/errors.rs
+++ b/compiler/rustc_ast_passes/src/errors.rs
@@ -76,7 +76,7 @@ pub(crate) struct TraitFnConst {
 pub(crate) struct AsyncFnInConstTraitOrTraitImpl {
     #[primary_span]
     pub async_keyword: Span,
-    pub in_impl: bool,
+    pub context: &'static str,
     #[label]
     pub const_keyword: Span,
 }
diff --git a/tests/ui/traits/const-traits/ice-149083-async-in-const-impl.rs b/tests/ui/traits/const-traits/ice-149083-async-in-const-impl.rs
new file mode 100644
index 0000000000000..0880ae509111a
--- /dev/null
+++ b/tests/ui/traits/const-traits/ice-149083-async-in-const-impl.rs
@@ -0,0 +1,9 @@
+//@ edition:2024
+
+#![feature(const_trait_impl)]
+struct Foo;
+const impl Foo {
+    async fn e() {}
+    //~^ ERROR async functions are not allowed in `const` impls
+}
+fn main() {}
diff --git a/tests/ui/traits/const-traits/ice-149083-async-in-const-impl.stderr b/tests/ui/traits/const-traits/ice-149083-async-in-const-impl.stderr
new file mode 100644
index 0000000000000..447222ea4abbd
--- /dev/null
+++ b/tests/ui/traits/const-traits/ice-149083-async-in-const-impl.stderr
@@ -0,0 +1,10 @@
+error: async functions are not allowed in `const` impls
+  --> $DIR/ice-149083-async-in-const-impl.rs:6:5
+   |
+LL | const impl Foo {
+   | ----- associated functions of `const` cannot be declared `async`
+LL |     async fn e() {}
+   |     ^^^^^
+
+error: aborting due to 1 previous error
+