Make the parser TokenStream more resilient after mismatched delimiter recovery

src/libsyntax/parse/parser.rs

@@ -7670,14 +7670,26 @@ 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.get(prev).is_none() {
+ // This can happen due to a bad interaction of two unrelated recovery mechanisms with
+ // mismatched delimiters *and* recovery lookahead on `pub ident(` likely typo (#62881)
+ return Ok((ret?, TokenStream::new(vec![])));
  } else {
  &mut self.token_cursor.stack[prev].last_token
  };
 
  // Pull out the tokens that we've collected from the call to `f` above.
  let mut collected_tokens = match *last_token {
  LastToken::Collecting(ref mut v) => mem::take(v),
- LastToken::Was(_) => panic!("our vector went away?"),
+ LastToken::Was(ref was) => {
+ let msg = format!("our vector went away? - found Was({:?})", was);
+ debug!("collect_tokens: {}", msg);
+ self.sess.span_diagnostic.delay_span_bug(self.token.span, &msg);
+ // This can happen due to a bad interaction of two unrelated recovery mechanisms
+ // with mismatched delimiters *and* recovery lookahead on `pub ident(` likely typo
+ // (#62895, different but similar to the case above)
+ return Ok((ret?, TokenStream::new(vec![])));
+ }
  };
 
  // If we're not at EOF our current token wasn't actually consumed by

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`.

src/test/ui/issues/issue-62895.rs

@@ -0,0 +1,11 @@
+fn main() {}
+
+fn v() -> isize { //~ ERROR mismatched types
+mod _ { //~ ERROR expected identifier
+pub fn g() -> isizee { //~ ERROR cannot find type `isizee` in this scope
+mod _ { //~ ERROR expected identifier
+pub g() -> is //~ ERROR missing `fn` for function definition
+(), w20);
+}
+(), w20); //~ ERROR expected item, found `;`
+}

src/test/ui/issues/issue-62895.stderr

@@ -0,0 +1,49 @@
+error: expected identifier, found reserved identifier `_`
+ --> $DIR/issue-62895.rs:4:5
+ |
+LL | mod _ {
+ | ^ expected identifier, found reserved identifier
+
+error: expected identifier, found reserved identifier `_`
+ --> $DIR/issue-62895.rs:6:5
+ |
+LL | mod _ {
+ | ^ expected identifier, found reserved identifier
+
+error: missing `fn` for function definition
+ --> $DIR/issue-62895.rs:7:4
+ |
+LL | pub g() -> is
+ | ^^^^
+help: add `fn` here to parse `g` as a public function
+ |
+LL | pub fn g() -> is
+ | ^^
+
+error: expected item, found `;`
+ --> $DIR/issue-62895.rs:10:9
+ |
+LL | (), w20);
+ | ^ help: remove this semicolon
+
+error[E0412]: cannot find type `isizee` in this scope
+ --> $DIR/issue-62895.rs:5:15
+ |
+LL | pub fn g() -> isizee {
+ | ^^^^^^ help: a builtin type with a similar name exists: `isize`
+
+error[E0308]: mismatched types
+ --> $DIR/issue-62895.rs:3:11
+ |
+LL | fn v() -> isize {
+ | - ^^^^^ expected isize, found ()
+ | |
+ | this function's body doesn't return
+ |
+ = note: expected type `isize`
+ found type `()`
+
+error: aborting due to 6 previous errors
+
+Some errors have detailed explanations: E0308, E0412.
+For more information about an error, try `rustc --explain E0308`.