Make the parser TokenStream more resilient after mismatched delimiter recovery

src/libsyntax/parse/parser.rs

@@ -7670,14 +7670,27 @@ 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 the likely typo `pub ident(`
+            // (#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 the likely typo
+                // `pub ident(` (#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/parser/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/parser/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/parser/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/parser/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`.