Detect top-level `...` in argument type

When writing something like the expression `|_: ...| {}`, we now detect the `...` during parsing explicitly instead of relying on the detection in `parse_ty_common` so that we don't talk about "nested `...` are not supported".

```
error: unexpected `...`
  --> $DIR/no-closure.rs:6:35
   |
LL | const F: extern "C" fn(...) = |_: ...| {};
   |                                   ^^^
   |
   = note: only `extern "C"` and `extern "C-unwind"` functions may have a C variable argument list
```

1 files changed, 8 insertions, 0 deletions

diff --git a/compiler/rustc_parse/src/errors.rs b/compiler/rustc_parse/src/errors.rs
index 2b107fe35d5..1abeee6fe43 100644
--- a/compiler/rustc_parse/src/errors.rs
+++ b/compiler/rustc_parse/src/errors.rs
@@ -3033,6 +3033,14 @@ pub(crate) struct NestedCVariadicType {
 }
 
 #[derive(Diagnostic)]
+#[diag(parse_dotdotdot_rest_type)]
+#[note]
+pub(crate) struct InvalidCVariadicType {
+    #[primary_span]
+    pub span: Span,
+}
+
+#[derive(Diagnostic)]
 #[diag(parse_invalid_dyn_keyword)]
 #[help]
 pub(crate) struct InvalidDynKeyword {