feat(nix): add Nix list support

[bennypowers]

Summary

• Custom split/join handler for Nix list_expression nodes, which use space-delimited items instead of commas
• Handles function calls with nested braces ((callPackage ./foo.nix {})), string items, path identifiers
• Single-element lists are correctly skipped (no-op)

Test plan

• Basic list split/rejoin (3 items)
• Single element list (no-op, round-trips unchanged)
• List with path-like identifiers (pkg-config)
• List with parenthesized function calls and nested braces
• List with string items
• Indented list (inside attrset)
• Full suite passes (250 tests)

Assisted-By: Claude Opus 4.6 (1M context) noreply@anthropic.com

Summary by CodeRabbit

New Features

• Added Nix language support with automatic list expression formatting capabilities. Seamlessly converts between inline and multiline formats for Nix lists. Handles various list contents including single-element items, attribute paths, function calls, quoted string values, and deeply nested structures with consistent indentation. Includes comprehensive test coverage for all real-world formatting scenarios.

[coderabbitai]

📝 Walkthrough

Walkthrough

This pull request adds support for the Nix language to the splitjoin plugin. It introduces Nix-specific list split/join functions, configuration bindings, Tree-sitter query rules, and comprehensive test coverage. The implementation enables transforming between single-line and multi-line Nix list expressions.

Changes

Cohort / File(s)	Summary
Core Implementation lua/splitjoin/languages/nix/functions.lua, lua/splitjoin/languages/nix/defaults.lua	Added Nix language module with two functions: split_list converts inline lists to multi-line format using configurable indentation, and join_list collapses multi-line lists to single-line format. Configuration module registers these functions for list_expression nodes.
Tree-sitter Integration queries/nix/splitjoin.scm	Added query rule capturing list_expression nodes as split/join targets.
Test Infrastructure test/bootstrap.lua, test/helpers.lua	Extended Tree-sitter language installation list to include nix and added nix file extension mapping for test buffer setup.
Test Suite test/nix_spec.lua	Added comprehensive test suite with 6 test cases covering inline-to-multi-line conversion, single-element lists, nested structures, quoted strings, attribute names, and function call expressions within lists.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~20 minutes

Poem

🐰 A fluffy hop through Nix we go,
Lists splitting high and joining low,
With brackets neat and indent bright,
We made the code flow just right! ✨

🚥 Pre-merge checks | ✅ 2 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (2 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title 'feat(nix): add Nix list support' clearly and specifically describes the main change: adding support for splitting and joining Nix lists.

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✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feat/nix-support

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[coderabbitai]

Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

Verify each finding against the current code and only fix it if needed.

Inline comments:
In `@lua/splitjoin/languages/nix/functions.lua`:
- Around line 5-25: The Nix.split_list function currently always prefixes items
with options.default_indent and writes the closing ']' at column 0, and it
doesn't handle default_indent when provided as a function; update Nix.split_list
to detect and preserve the node's base indentation (e.g., read indentation of
the node's starting line or previous sibling), normalize options.default_indent
if it's a function (call it to get a string), then build lines that insert the
base indentation before each item indent and emit the closing ']' aligned with
the base indentation; finally call Node.replace and Node.goto_node as before so
the replaced text keeps the surrounding indentation.

🪄 Autofix (Beta)

Fix all unresolved CodeRabbit comments on this PR:

• Push a commit to this branch (recommended)
• Create a new PR with the fixes

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 99fab636-c0d1-441c-a2d0-c92884bd6628

📥 Commits

Reviewing files that changed from the base of the PR and between f8459af and 8eb89cd.

📒 Files selected for processing (6)

• lua/splitjoin/languages/nix/defaults.lua
• lua/splitjoin/languages/nix/functions.lua
• queries/nix/splitjoin.scm
• test/bootstrap.lua
• test/helpers.lua
• test/nix_spec.lua