Fix standalone constraints in multi_grep

[Kh05ifr4nD]

Fixes #738.

Supersedes #739 with a configuration-level fix.

multi_grep receives patterns and constraints separately, but its adapters parsed the constraint field as an ordinary grep query. A lone path could become search text and disappear before the search. Exact-file misses could then discard the file constraint during prefiltering and search the wider index.

Parse the dedicated constraint field directly and keep multi-pattern prefiltering strict. Ordinary grep retains its existing query parsing and filename retry. The unsupported plain-grep fallback is removed.

Verified with the Rust workspace tests, Clippy, rustfmt, a release build, and MCP stdio cases covering directory, glob, exact and missing files, exclusions, and OR matching.

Summary by CodeRabbit

• Bug Fixes

  • Improved multi-pattern searches with file path and directory constraints.
  • Searches now correctly return no matches when constraints match no indexed files.
  • Prevented unmatched constraints from broadening search results.
  • Standardized empty multi-pattern results to display “0 matches.”
  • Applied consistent constraint handling across supported search interfaces.

• Tests

  • Added regression coverage for unmatched file path constraints and multiple standalone constraints.

[coderabbitai]

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ℹ️ Recent review info

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📥 Commits

Reviewing files that changed from the base of the PR and between 33b8cc2 and 1fbb431.

📒 Files selected for processing (1)

• crates/fff-core/src/grep/prefilter.rs

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📝 Walkthrough

Walkthrough

The PR adds direct standalone constraint parsing, applies constraints through the multi-pattern prefilter, removes MCP fallback searching, updates C and Python integrations, and adds parser and path-filter regression tests.

Changes

Constraint Parsing and Multi-Grep Integration

Layer / File(s)	Summary
Standalone constraint parser crates/fff-query-parser/src/parser.rs	Adds QueryParser::parse_constraints and tests directory, file-path, and multiple standalone constraint handling across parser configurations.
Constraint-aware prefiltering crates/fff-core/src/grep/multi_pattern.rs, crates/fff-core/src/grep/prefilter.rs, crates/fff-core/tests/path_separator_constraint_test.rs	Uses prefilter_files for multi-pattern constraint filtering, exposes it to the parent module, and tests that missing FilePath constraints return no matches.
Frontend constraint integration crates/fff-c/src/lib.rs, crates/fff-mcp/src/server.rs, crates/fff-python/src/finder.rs	Passes directly parsed constraints to multi_grep and removes the MCP individual-pattern fallback and special empty-result handling.

Estimated code review effort: 3 (Moderate) | ~20 minutes

Mergeability Score: ⚪ Minimal · up to 1fbb4

The PR updates multi-pattern constraint handling and removes an unsupported fallback; no actionable merge-blocking risk remains.

Suggested reviewers: dmtrkovalenko

🚥 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 describes the main fix for standalone constraints in multi_grep.
Linked Issues check	✅ Passed	The changes parse standalone constraints directly, enforce strict filtering, preserve grep behavior, and add regression coverage for issue #738.
Out of Scope Changes check	✅ Passed	All changes support standalone constraint parsing, strict multi-pattern filtering, preserved grep behavior, or regression coverage.
Docstring Coverage	✅ Passed	Docstring coverage is 90.00% which is sufficient. The required threshold is 80.00%.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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