Reject _ as a match-pattern target

[charliermarsh]

Summary

Python reserves _ for wildcard patterns and does not allow it as the binding target in either of these forms:

match value:
    case 1 as _:
        pass
    case {**_}:
        pass

We currently recover an AST for both forms without reporting a parser error. Both are grammar errors in CPython, so this change validates the capture target while parsing as and mapping patterns and reports cannot use '_' as a target on the underscore.

We previously tried to address the mapping-pattern case in #6838 by representing the rest target as a general PatternMatchAs node. This keeps the existing Identifier representation and recovered AST, including statements after the invalid match, while rejecting the invalid target at the parser boundary.

[astral-sh-bot]

Memory usage report

Memory usage unchanged ✅

[astral-sh-bot]

ecosystem-analyzer results

No diagnostic changes detected ✅

Full report with detailed diff (timing results)

[astral-sh-bot]

ruff-ecosystem results

Linter (stable)

✅ ecosystem check detected no linter changes.

Linter (preview)

✅ ecosystem check detected no linter changes.

Formatter (stable)

✅ ecosystem check detected no format changes.

Formatter (preview)

✅ ecosystem check detected no format changes.

[MichaReiser]

I'm not sure using semantic syntax errors is appropriate here, given that ast.parse rejects these patterns too. We primarily use semantic syntax errors for what are compile errors in CPython.

[codspeed-hq]

Merging this PR will degrade performance by 4.52%

❌ 1 regressed benchmark
✅ 130 untouched benchmarks

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	WallTime	ty_module_resolver[25]	1.4 ms	1.4 ms	-4.52%

Tip

Investigate this regression with the CodSpeed MCP and your agent.

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_{Comparing charlie/reject-underscore-pattern-bindings (53bd691) with main (cfa4d72)}

[MichaReiser]

Thanks. Would you mind updating the summary before landing