Rollup merge of rust-lang#139264 - freyacodes:fix/bad-turbofish-hints, r=petrochenkov

Fix two incorrect turbofish suggestions

This fixes rust-lang#121901

This is my contribution to Rust, and my first contribution to a language parser that I didn't write myself.
I am a bit outside my depth here, so any constructive criticism is appreciated.

Author: matthiaskrgr

compiler/rustc_parse/src/parser/item.rs

@@ -2960,13 +2960,30 @@ impl<'a> Parser<'a> {
                 let parser_snapshot_before_ty = this.create_snapshot_for_diagnostic();
                 this.eat_incorrect_doc_comment_for_param_type();
                 let mut ty = this.parse_ty_for_param();
-                if ty.is_ok()
-                    && this.token != token::Comma
-                    && this.token != token::CloseDelim(Delimiter::Parenthesis)
-                {
-                    // This wasn't actually a type, but a pattern looking like a type,
-                    // so we are going to rollback and re-parse for recovery.
-                    ty = this.unexpected_any();
+
+                if let Ok(t) = &ty {
+                    // Check for trailing angle brackets
+                    if let TyKind::Path(_, Path { segments, .. }) = &t.kind {
+                        if let Some(segment) = segments.last() {
+                            if let Some(guar) =
+                                this.check_trailing_angle_brackets(segment, &[exp!(CloseParen)])
+                            {
+                                return Ok((
+                                    dummy_arg(segment.ident, guar),
+                                    Trailing::No,
+                                    UsePreAttrPos::No,
+                                ));
+                            }
+                        }
+                    }
+
+                    if this.token != token::Comma
+                        && this.token != token::CloseDelim(Delimiter::Parenthesis)
+                    {
+                        // This wasn't actually a type, but a pattern looking like a type,
+                        // so we are going to rollback and re-parse for recovery.
+                        ty = this.unexpected_any();
+                    }
                 }
                 match ty {
                     Ok(ty) => {
@@ -2977,6 +2994,7 @@ impl<'a> Parser<'a> {
                     }
                     // If this is a C-variadic argument and we hit an error, return the error.
                     Err(err) if this.token == token::DotDotDot => return Err(err),
+                    Err(err) if this.unmatched_angle_bracket_count > 0 => return Err(err),
                     // Recover from attempting to parse the argument as a type without pattern.
                     Err(err) => {
                         err.cancel();

tests/ui/fn/bad-turbofish-hints-issue-121901.rs

@@ -0,0 +1,8 @@
+// Regression test for the parser wrongfully suggesting turbofish syntax in below syntax errors
+
+type One = for<'a> fn(Box<dyn Send + 'a);
+//~^ ERROR: expected one of `+`, `,`, or `>`, found `)`
+type Two = for<'a> fn(Box<dyn Send + 'a>>);
+//~^ ERROR: unmatched angle bracket
+
+fn main() {}

tests/ui/fn/bad-turbofish-hints-issue-121901.stderr

@@ -0,0 +1,25 @@
+error: expected one of `+`, `,`, or `>`, found `)`
+  --> $DIR/bad-turbofish-hints-issue-121901.rs:3:40
+   |
+LL | type One = for<'a> fn(Box<dyn Send + 'a);
+   |                                        ^ expected one of `+`, `,`, or `>`
+   |
+help: you might have meant to end the type parameters here
+   |
+LL | type One = for<'a> fn(Box<dyn Send + 'a>);
+   |                                        +
+
+error: unmatched angle bracket
+  --> $DIR/bad-turbofish-hints-issue-121901.rs:5:41
+   |
+LL | type Two = for<'a> fn(Box<dyn Send + 'a>>);
+   |                                         ^
+   |
+help: remove extra angle bracket
+   |
+LL - type Two = for<'a> fn(Box<dyn Send + 'a>>);
+LL + type Two = for<'a> fn(Box<dyn Send + 'a>);
+   |
+
+error: aborting due to 2 previous errors
+