While reviewing #20682 today, I wanted to suggest emitting a diagnostic about alternate match patterns binding different variables just like rustc does:

error[E0408]: variable `a` is not bound in all patterns
 --> src/main.rs:4:5
  |
4 |     [] | [a, b, c] | [d, e, f]=> {}
  |     ^^    -          ^^^^^^^^^ pattern doesn't bind `a`
  |     |     |
  |     |     variable not in all patterns
  |     pattern doesn't bind `a`

However, we don't have direct access to the Diagnostic in the SemanticSyntaxChecker. Instead, we can currently only report_semantic_error to the parent visitor. I don't think we can simply replace report_semantic_error with report_diagnostic, for example, because Ruff's implementation of report_semantic_error needs to convert some semantic errors into rule violations, unlike ty's implementation.

Another wrinkle is that Ruff and ty use a different helper function to convert errors into diagnostics, create_syntax_error_diagnostic vs Diagnostic::invalid_syntax. The main difference is where the error message ends up:

> ruff check try.py
invalid-syntax: name capture `x` makes remaining patterns unreachable
 --> try.py:2:10
  |
1 | match 42:
2 |     case x: ...
  |          ^
3 |     case y: ...
  |

Found 1 error.
> uvx ty check try.py
WARN ty is pre-release software and not ready for production use. Expect to encounter bugs, missing features, and fatal errors.
Checking ------------------------------------------------------------ 1/1 files                                                                                                 error[invalid-syntax]
 --> try.py:2:10
  |
1 | match 42:
2 |     case x: ...
  |          ^ name capture `x` makes remaining patterns unreachable
3 |     case y: ...
  |

Found 1 diagnostic

This is a bit of a separate issue, and I kind of think we should standardize on the Ruff version anyway.

But regardless, it seems useful to get access directly to a Diagnostic for semantic syntax errors and possibly for other types of syntax errors too so that we can take advantage of the new functionality instead of converting the various simplified representations into diagnostics later.