Reject CVarArgs in parse_ty_for_where_clause

compiler/rustc_parse/src/parser/ty.rs

@@ -194,7 +194,7 @@ impl<'a> Parser<'a> {
  pub(super) fn parse_ty_for_where_clause(&mut self) -> PResult<'a, P<Ty>> {
  self.parse_ty_common(
  AllowPlus::Yes,
- AllowCVariadic::Yes,
+ AllowCVariadic::No,
  RecoverQPath::Yes,
  RecoverReturnSign::OnlyFatArrow,
  None,
@@ -344,8 +344,9 @@ impl<'a> Parser<'a> {
  match allow_c_variadic {
  AllowCVariadic::Yes => TyKind::CVarArgs,
  AllowCVariadic::No => {
- // FIXME(Centril): Should we just allow `...` syntactically
+ // FIXME(c_variadic): Should we just allow `...` syntactically
  // anywhere in a type and use semantic restrictions instead?
+ // NOTE: This may regress certain MBE calls if done incorrectly.
  let guar = self
  .dcx()
  .emit_err(NestedCVariadicType { span: lo.to(self.prev_token.span) });

tests/ui/parser/macro/mbe-dotdotdot-may-not-begin-a-type.rs

@@ -0,0 +1,11 @@
+// A bare `...` represents `CVarArgs` (`VaListImpl<'_>`) in function argument type
+// position without being a proper type syntactically.
+// This test ensures that we do not regress certain MBE calls would we ever promote
+// `...` to a proper type syntactically.
+
+//@ check-pass
+
+macro_rules! ck { ($ty:ty) => { compile_error!(""); }; (...) => {}; }
+ck!(...);
+
+fn main() {}

tests/ui/parser/variadic-ffi-nested-syntactic-fail.rs

@@ -4,6 +4,10 @@ fn f1<'a>(x: u8, y: &'a ...) {}
 fn f2<'a>(x: u8, y: Vec<&'a ...>) {}
 //~^ ERROR C-variadic type `...` may not be nested inside another type
 
+// Regression test for issue #125847.
+fn f3() where for<> ...: {}
+//~^ ERROR C-variadic type `...` may not be nested inside another type
+
 fn main() {
  let _recovery_witness: () = 0;
  //~^ ERROR: mismatched types

tests/ui/parser/variadic-ffi-nested-syntactic-fail.stderr

@@ -10,15 +10,21 @@ error[E0743]: C-variadic type `...` may not be nested inside another type
 LL | fn f2<'a>(x: u8, y: Vec<&'a ...>) {}
  | ^^^
 
+error[E0743]: C-variadic type `...` may not be nested inside another type
+ --> $DIR/variadic-ffi-nested-syntactic-fail.rs:8:21
+ |
+LL | fn f3() where for<> ...: {}
+ | ^^^
+
 error[E0308]: mismatched types
- --> $DIR/variadic-ffi-nested-syntactic-fail.rs:8:33
+ --> $DIR/variadic-ffi-nested-syntactic-fail.rs:12:33
  |
 LL | let _recovery_witness: () = 0;
  | -- ^ expected `()`, found integer
  | |
  | expected due to this
 
-error: aborting due to 3 previous errors
+error: aborting due to 4 previous errors
 
 Some errors have detailed explanations: E0308, E0743.
 For more information about an error, try `rustc --explain E0308`.