Add Ruby language support

[nsmmrs]

Summary

Adds Ruby support. Introduces a RubyLanguageEngine that parses and mutates Ruby code using the tree-sitter-ruby grammar.

Mutations

We mapped Mewt's standard mutation operators to Ruby AST nodes and created new operators to support Ruby-specific syntax and control flow.

1. Ruby-Specific Operators (New)

• UF / UT (Unless False / Unless True): Replaces unless conditions and unless_guard nodes with false or true.
• ULF / ULT (Until False / Until True): Replaces until loop conditions with false or true.
• UP (Unpin Pattern): Removes the ^ pin operator in pattern matching expressions (in ^var becomes in var).
• RBR (Range Bound Replacement): Swaps inclusive (..) and exclusive (...) range operators.
• SNR (Safe Navigation Removal): Removes the safe navigation operator (&. becomes .).
• LAOS (Logical Assignment Operator Swap): Swaps the ||= and &&= operators.
• RMOS (Regex Match Operator Swap): Swaps regex match operators (=~ and !~).
• CES (Case Equality Swap): Swaps the case equality operator (===) with the standard equality operator (==).
• EL (Empty Literals): Replaces populated strings, arrays, and hashes with empty equivalents ("", [], {}).

2. Core Operators (Mapped to Ruby)

• IF / IT (If False / If True): Targets if, elsif, if_guard, and ternary conditional expressions.
• CR / ER (Constant Replacement / Error Replacement): Replaces statements with nil or raise "mewt". Applies to top-level expressions, method calls, yield, super, redo, retry, break, next, rescue_modifier (inline rescue), and case_match (in clauses).
• NR (Negation Removal): Removes the ! operator and the not keyword (e.g., not valid?).
• AS (Argument Swap): Swaps arguments in method calls and named elements in array patterns (e.g., in [a, b, c]).
• LC (Loop Control): Swaps redo and retry keywords, and replaces break and next with redo or retry.
• (And all standard operators like arithmetic swaps, boolean replacements, and comparison swaps)

Testing

• Unit Tests: Added unit tests for each mutation operator in tests/ruby/mutations/ to verify mutant generation across different syntax scenarios.
• Stress Tests: Tested against open-source Ruby codebases (Discourse, Jekyll, and RuboCop). Testing against the Discourse codebase processed ~5,700 target files and generated ~464,000 mutants in under two minutes on a release build. No parser errors or infinite loops occurred.
• CI Checks: Code passes just pre-commit formatting and linter checks (cargo fmt, clippy, and typos).

[CLAassistant]

All committers have signed the CLA.

[nsmmrs]

We added a custom RBR (Range Bound Replacement) slug to swap Ruby's inclusive (..) and exclusive (...) range operators.

[nsmmrs]

We include nodes::CASE and nodes::CASE_MATCH but exclude top-level containers like method and class bodies. This creates isolated mutations for innermost statements rather than replacing the entire container.

[nsmmrs]

We added custom mutation slugs (UF/UT and ULF/ULT) for Ruby's unless and until statements. Standard IF/IT logic creates the opposite behavior because replacing an unless condition with false differs semantically from replacing an if condition with false.

[nsmmrs]

We created the UP (Unpin Pattern) slug to handle unpinning variables in pattern matching expressions. Removing the ^ pin operator (in ^var) changes the match from an exact-value match to a variable reassignment. We use a dedicated slug for this instead of reusing the NR (Negation Removal) slug, as the caret does not function as a logical negation in this context.

[nsmmrs]

We added this function to support mutating Ruby array patterns (e.g., in [a, b, c]). Tree-sitter treats array elements as standalone named children rather than wrapping them in an arguments list. This prevents the existing swap_args function from traversing them.

[nsmmrs]

Removed the unnecessary reference (&) here since .display() already returns a type that implements Display. This was done to resolve a clippy::useless_borrow warning.

[nsmmrs]

Refactored the if is_some() check that was followed by an unwrap() into an if let Some(id) binding to satisfy clippy::unnecessary_unwrap.

[nsmmrs]

Added varaint to the typos ignore list because this typo exists in a comment within the vendored tree-sitter-ruby grammar file, which we do not maintain.

[nsmmrs]

Just noticed it said I have an unfinished review, so I think my comments have been invisible this whole time.

[nsmmrs]

Not sure why, but I'm suddenly getting a bunch of notifications about failed CI runs on this PR.