Make the parser TokenStream more resilient after mismatched delimiter recovery

Author: estebank

src/libsyntax/parse/parser.rs

@@ -7670,6 +7670,9 @@ impl<'a> Parser<'a> {
         let ret = f(self);
         let last_token = if self.token_cursor.stack.len() == prev {
             &mut self.token_cursor.frame.last_token
+        } else if self.token_cursor.stack.is_empty() {//&& !self.unclosed_delims.is_empty() {
+            // This can happen with mismatched delimiters (#62881)
+            return Ok((ret?, TokenStream::new(vec![])));
         } else {
             &mut self.token_cursor.stack[prev].last_token
         };

src/test/ui/issues/issue-62881.rs

@@ -0,0 +1,6 @@
+fn main() {}
+
+fn f() -> isize { fn f() -> isize {} pub f<
+//~^ ERROR missing `fn` or `struct` for function or struct definition
+//~| ERROR mismatched types
+//~ ERROR this file contains an un-closed delimiter

src/test/ui/issues/issue-62881.stderr

@@ -0,0 +1,29 @@
+error: this file contains an un-closed delimiter
+  --> $DIR/issue-62881.rs:6:53
+   |
+LL | fn f() -> isize { fn f() -> isize {} pub f<
+   |                 - un-closed delimiter
+...
+LL |
+   |                                                     ^
+
+error: missing `fn` or `struct` for function or struct definition
+  --> $DIR/issue-62881.rs:3:41
+   |
+LL | fn f() -> isize { fn f() -> isize {} pub f<
+   |                                         ^
+
+error[E0308]: mismatched types
+  --> $DIR/issue-62881.rs:3:29
+   |
+LL | fn f() -> isize { fn f() -> isize {} pub f<
+   |                      -      ^^^^^ expected isize, found ()
+   |                      |
+   |                      this function's body doesn't return
+   |
+   = note: expected type `isize`
+              found type `()`
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0308`.