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Summary

Improve selected CPython 3.15 parser diagnostics and error recovery on the RustPython 0.17.5 parser branch.

• Unpacking diagnostics distinguish invalid positions in comprehensions, dictionary keys and values, and conditional branches. Recovery preserves valid AST ranges, including else **value.
• Deferred conditional-unpacking errors preserve encounter order when an otherwise valid call expression is followed by a parse failure. Speculative diagnostic parsing restores parser recovery state.
• Parameter terminology and repeated-star wording match CPython.
• Standalone case clauses, assertion assignment/walrus syntax, mapping-rest ordering, and misplaced lazy imports receive specific diagnostics.
• Unclosed multiline f-string/t-string replacement fields identify the opening brace's line.
• Missing-comma hints after multiline literal concatenation point to the final literal line without changing original-expression error precedence.

Scope

This PR does not expand accepted PEP 798 operand grammar. General unparenthesized operands in dictionary comprehensions and parenthesized starred generators remain upstream work: astral-sh#30406 and astral-sh#30418. The current upstream PR addresses dictionary comprehensions, not the generator example reported in the issue. This diagnostic patch is not a substitute for those syntax fixes.

Parenthesized operands and already-supported call forms remain covered. Tests for deferred and speculative errors use grammar already accepted by the base parser. Existing upstream fixtures are retained; grammar-expansion-only fixtures introduced on this branch are excluded rather than recording the known upstream rejection as correct behavior.

The diagnostic implementation references CPython 3.15.0 at d220409a13b77de7a6c30abe16ea90db01528edc, including its grammar, replacement-field tests, and syntax tests. No new Cargo feature or diagnostic version gate is introduced. Complete CPython diagnostic parity is not claimed.

This builds on rustpython-0.17.5 at 043bd2bc4898a5348f66332c5cdd19de4fbecbe1. RustPython consumers still need the resulting revision and separate compiler/VM integration. No dependency pin or crates.io release is changed here.

Validation

At cd0390f5cb46db24d4d64d133c2155a05727af54:

• All-feature parser tests: 257 unit tests, 645 fixture tests, one inline-generation test and ten doctests passed, with snapshot updates disabled for final verification.
• Parser-only strict all-target/all-feature Clippy passed with --no-deps.
• Normal changed-file hooks passed.
• A focused scope check confirms unparenthesized dictionary/generator operand expansion remains absent, parenthesized forms and supported calls remain accepted, and conditional-branch unpacking retains its specific diagnostic and repaired recovery AST.

Dependency-wide strict Clippy remains blocked by nine existing disabled cache/salsa configuration warnings in the unchanged AST dependency. RustPython runtime/compiler integration and the earlier finite CPython comparison were not rerun for this reduced-scope head; earlier results do not establish full compatibility of this revision.

Assistance

Prepared and tested with OpenAI Codex at the maintainer's explicit request for the RustPython-managed fork. No claim of manual maintainer review is implied.

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