Make sure we consume a generic arg when checking mistyped turbofish

compiler/rustc_parse/src/parser/diagnostics.rs

@@ -1224,7 +1224,11 @@ impl<'a> Parser<'a> {
             let x = self.parse_seq_to_before_end(
                 &token::Gt,
                 SeqSep::trailing_allowed(token::Comma),
-                |p| p.parse_generic_arg(None),
+                |p| match p.parse_generic_arg(None)? {
+                    Some(arg) => Ok(arg),
+                    // If we didn't eat a generic arg, then we should error.
+                    None => p.unexpected_any(),
+                },
             );
             match x {
                 Ok((_, _, Recovered::No)) => {

tests/ui/parser/recover/turbofish-arg-with-stray-colon.rs

@@ -0,0 +1,6 @@
+fn foo() {
+    let x = Tr<A, A:>;
+    //~^ ERROR expected one of `!`, `.`, `::`, `;`, `?`, `else`, `{`, or an operator, found `,`
+}
+
+fn main() {}

tests/ui/parser/recover/turbofish-arg-with-stray-colon.stderr

@@ -0,0 +1,14 @@
+error: expected one of `!`, `.`, `::`, `;`, `?`, `else`, `{`, or an operator, found `,`
+  --> $DIR/turbofish-arg-with-stray-colon.rs:2:17
+   |
+LL |     let x = Tr<A, A:>;
+   |                 ^ expected one of 8 possible tokens
+   |
+   = note: type ascription syntax has been removed, see issue #101728 <https://github.com/rust-lang/rust/issues/101728>
+help: maybe write a path separator here
+   |
+LL |     let x = Tr<A, A::>;
+   |                    ~~
+
+error: aborting due to 1 previous error
+