Collapse bare ExpectedExpression to 'invalid syntax'; fix '<>' diagnostic offset by mumallaeng · Pull Request #8540 · RustPython/RustPython

mumallaeng · 2026-08-17T07:15:04Z

Closes #xxxx
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AI disclosure

This PR was implemented by Claude Code (Anthropic, Claude Sonnet 5), driven
interactively by a human maintainer of a downstream RustPython consumer
(Pyre) across a full session: the human directed the investigation, reviewed
and steered each step, and made the call to open this PR. The commit carries
an Assisted-by: Claude Code:claude-sonnet-5 trailer per policy. The change
itself is small and self-contained (~30 lines in one function), and has been
exercised against CPython's test_flufl.py via the downstream consumer
mentioned below, plus a spot-check regression pass over test_grammar,
test_syntax, test_tokenize, and test_compile showing no behavior change
outside the two fixed cases.

Summary

ParseErrorType::ExpectedExpression currently surfaces as the raw ruff
parser message (e.g. "Expected an expression") to callers that only depend
on rustpython-compiler (no rustpython-vm). rustpython-vm's
vm_new.rs already collapses this to CPython's generic "invalid syntax"
for its own callers; this mirrors that same collapse inside
cpython_parse_diagnostic_override so non-vm consumers get the same
CPython-compatible message.
A bare <> outside Barry-as-BDFL mode (2 <> 3) lexes as Less then an
unexpected Greater, so the resulting ExpectedExpression location points
at the > — one character past where CPython's tokenizer (which treats
<> as a single obsolete token) reports the error. Detect the <
immediately preceding the location and shift the reported range back over
it.

This is a companion to a RustPython/ruff PR implementing real
Barry-as-BDFL tokenizer support, which needs the offset fix here to make
CPython's test_flufl.py pass end to end. It's independently useful for any
caller hitting these two message/offset mismatches outside Barry mode too.

Test plan

Reproduced against CPython's Lib/test/test_flufl.py (via a downstream
consumer, Pyre) — test_guido_as_bdfl and
test_barry_as_bdfl_relative_import now pass with correct message text
and offset.
cargo check -p rustpython-compiler passes.

Summary by CodeRabbit

Bug Fixes
- Improved syntax error messages for obsolete <> usage.
- Standardized certain parser errors as “invalid syntax.”
- Adjusted error locations so diagnostics highlight the correct source text.

coderabbitai · 2026-08-17T07:15:49Z

No actionable comments were generated in the recent review. 🎉

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crates/compiler/src/lib.rs

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📝 Walkthrough

Walkthrough

The compiler now reports obsolete <> usage as invalid syntax across both characters. Bare ExpectedExpression errors also use invalid syntax with adjusted source locations.

Changes

Parse diagnostics

Layer / File(s)	Summary
Normalize obsolete syntax diagnostics `crates/compiler/src/lib.rs`	Parse diagnostics adjust ranges for obsolete `<>` operators and convert matching `ExpectedExpression` errors to `invalid syntax`.

Estimated code review effort: 3 (Moderate) | ~15–30 minutes

Merge Risk: ⚪ Minimal · up to 42900

The localized parser diagnostic change introduces no actionable merge-blocking risk; merge is ready after the routine cargo fmt and cargo clippy checks are run.

Possibly related PRs

RustPython/RustPython#7988: Both modify parse-error-to-diagnostic handling in crates/compiler/src/lib.rs.
RustPython/RustPython#8429: Both adjust parse-error diagnostics and source locations in crates/compiler/src/lib.rs.
RustPython/RustPython#8550: Both modify diagnostics for obsolete <> syntax in crates/compiler/src/lib.rs.

Suggested reviewers: shaharnaveh, youknowone

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly summarizes both main changes: normalizing bare ExpectedExpression errors and correcting the obsolete <> diagnostic offset.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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Actionable comments posted: 1

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Inline comments:
In `@crates/compiler/src/lib.rs`:
- Around line 391-398: Update the obsolete-token detection around the
ExpectedExpression check to require that the byte at start is an adjacent >
before returning the range; otherwise return None. Preserve the existing
preceding-< validation and only construct the range for the exact <> sequence.

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crates/compiler/src/lib.rs

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…stic offset `ParseErrorType::ExpectedExpression` currently surfaces as the raw ruff parser message (e.g. "Expected an expression") to callers that only depend on `rustpython-compiler` (no `rustpython-vm`). `rustpython-vm`'s `vm_new.rs` already collapses this to CPython's generic "invalid syntax" for its own callers; mirror that same collapse inside `cpython_parse_diagnostic_override` so non-vm consumers get the same CPython-compatible message. A bare `<>` outside Barry-as-BDFL mode (`2 <> 3`) lexes as `Less` then an unexpected `Greater`, so the resulting `ExpectedExpression` location points at the `>` -- one character past where CPython's tokenizer (which treats `<>` as a single obsolete token) reports the error. Detect the `<` immediately preceding the location and shift the reported range back over it. Assisted-by: Claude Code:claude-sonnet-5

youknowone · 2026-08-19T04:17:12Z

@mumallaeng you fixed more tests:

UNEXPECTED SUCCESS: test_barry_as_bdfl_relative_import (test.test_flufl.FLUFLTests.test_barry_as_bdfl_relative_import)
UNEXPECTED SUCCESS: test_guido_as_bdfl (test.test_flufl.FLUFLTests.test_guido_as_bdfl)

please remove expectedFailure marker from them

CodeRabbit review on RustPython#8540: the previous check only looked at the byte before the ExpectedExpression location for '<', without confirming an adjacent '>' really follows it. For inputs like a trailing '<' at EOF this could misclassify an unrelated ExpectedExpression as the bare '<>' case and report a bogus range. Require both bytes are present before constructing the diagnostic. Also remove the now-stale `@unittest.expectedFailure # TODO: RUSTPYTHON` markers on test_guido_as_bdfl and test_barry_as_bdfl_relative_import, which pass with this fix (test_barry_as_bdfl and test_barry_as_bdfl_look_ma_with_no_compiler_flags still need real Barry-as-BDFL tokenizer support and stay marked). Assisted-by: Claude Code:claude-sonnet-5

mumallaeng · 2026-08-19T10:19:43Z

@youknowone Removed the now-stale expectedFailure markers on test_guido_as_bdfl and test_barry_as_bdfl_relative_import. Also fixed the CodeRabbit finding — the obsolete-<> detection now requires an adjacent > in addition to the preceding <, instead of only checking the preceding byte.

Verified locally: test_flufl, test_grammar, test_syntax, test_tokenize, test_compile all pass (cargo run --release -- -m test test_grammar test_syntax test_tokenize test_compile test_flufl).

github-actions · 2026-08-19T10:20:31Z

📦 Library Dependencies

The following Lib/ modules were modified. Here are their dependencies:

[x] test: cpython/Lib/test/test_flufl.py (TODO: 2)

dependencies:

dependent tests: (no tests depend on flufl)

Legend:

[+] path exists in CPython
[x] up-to-date, [ ] outdated

youknowone · 2026-08-19T11:07:44Z

Thanks!

mumallaeng force-pushed the pyre-barry-as-bdfl branch from 0ada1a5 to bd7a45c Compare August 17, 2026 07:17

coderabbitai Bot reviewed Aug 17, 2026

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Comment thread crates/compiler/src/lib.rs

mumallaeng force-pushed the pyre-barry-as-bdfl branch from bd7a45c to 97f53b8 Compare August 17, 2026 07:28

youknowone mentioned this pull request Aug 18, 2026

Align compiler futures, annotations, and symtable with CPython #8550

Merged

youknowone merged commit ca907d1 into RustPython:main Aug 19, 2026
28 checks passed

mumallaeng deleted the pyre-barry-as-bdfl branch August 19, 2026 11:17

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Collapse bare ExpectedExpression to 'invalid syntax'; fix '<>' diagnostic offset - #8540

Collapse bare ExpectedExpression to 'invalid syntax'; fix '<>' diagnostic offset#8540
youknowone merged 2 commits into
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mumallaeng:pyre-barry-as-bdfl

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