Auto merge of #45503 - thombles:tk/i44339-v5, r=petrochenkov

Improve diagnostics when list of tokens has incorrect separators

Make `parse_seq_to_before_tokens` more resilient to error conditions. Where possible it is better if it can consume up to the final bracket before returning. This change improves the diagnostics in a couple of situations:

```
struct S(pub () ()); // omitted separator
use std::{foo. bar}; // used a similar but wrong separator
```

Fixes #44339
r? @petrochenkov

src/libsyntax/parse/parser.rs

@@ -1033,7 +1033,23 @@ impl<'a> Parser<'a> {
                 } else {
                     if let Err(e) = self.expect(t) {
                         fe(e);
-                        break;
+                        // Attempt to keep parsing if it was a similar separator
+                        if let Some(ref tokens) = t.similar_tokens() {
+                            if tokens.contains(&self.token) {
+                                self.bump();
+                            }
+                        }
+                        // Attempt to keep parsing if it was an omitted separator
+                        match f(self) {
+                            Ok(t) => {
+                                v.push(t);
+                                continue;
+                            },
+                            Err(mut e) => {
+                                e.cancel();
+                                break;
+                            }
+                        }
                     }
                 }
             }