Reject async fn in const impl during AST validation

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`
 

compiler/rustc_ast_passes/src/ast_validation.rs

@@ -312,9 +312,15 @@ fn check_async_fn_in_const_trait_or_impl(&self, sig: &FnSig, parent: &TraitOrImp
             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 @@ fn visit_assoc_item(&mut self, item: &'a AssocItem, ctxt: AssocCtxt) {
                     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 => {}

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,
 }

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() {}

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
+