Make Array.slice(start) JS-compatible by defaulting end

[ATX24]

Pull Request Template

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Issue Reference

Please link to any related issues

• This PR fixes/closes #N/A

Changes

Please describe the changes proposed in this pull request

• Restored JS-compatible Array.slice(start) behavior while preserving existing slice(start, end) calls.
• Added a BAML wrapper for Array.slice(self, start, end = self.length()) and moved native execution to _slice(self, start, end).
  • This avoids native default-argument omission issues (native functions do not run default-argument prologues).
• Updated VM native implementation hook from slice to _slice.
• Updated call binding behavior to allow positional arguments for defaulted parameters.
  • This is required so existing positional 2-arg calls remain valid when a trailing defaulted arg exists.
• Added/updated array tests for one-arg slice behavior.
• Updated affected snapshots for:
  • baml_src/arrays
  • compiler2_tir::phase3a::optional_param_call_binding_diagnostics
  • diagnostic_errors/optional_parameter_defaults

Testing

Please describe how you tested these changes

• Unit tests added/updated
• Manual testing performed
• Tested in local Rust test environment

Commands run:

• cargo test --package baml_tests --test baml_src ✅
• cargo test --package baml_tests optional_param_call_binding_diagnostics ✅
• cargo test --package baml_tests optional_parameter_defaults ✅
• cargo test --lib ⚠️ fails in this cloud environment due missing system link/runtime deps for unrelated targets (bridge_typescript N-API linker symbols / bridge_python python3.12 system lib).

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PR Checklist

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• I have read and followed the contributing guidelines
• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings

Additional Notes

Add any other context about the PR here

• This change intentionally keeps compatibility for both slice(start) and existing positional slice(start, end) call sites.

  

Summary by CodeRabbit

• New Features
  • Array slicing now supports calling slice(start) (end defaults to the array length) to return items from the specified start through the end.
  • Start-only slicing supports negative indexes for JavaScript-style behavior.
• Bug Fixes
  • Optional/default parameters can now be passed positionally without incorrectly producing a “must be passed by name” diagnostic.
• Tests
  • Added/updated array slice and type-checking test coverage for the new slice behavior and diagnostic expectations.

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Project	Deployment	Actions	Updated (UTC)
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✨ Finishing Touches

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• Create PR with unit tests
• Commit unit tests in branch cursor/array-slice-js-compatibility-b003

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[github-actions]

⏭️ Performance benchmarks were skipped

Perf benchmarks (CodSpeed) are opt-in on pull requests — they no longer run on every push. They always run automatically after merge to canary/main.

To run them on this PR, do any of the following, then push a commit (or re-run CI):

• Add RUN_CODSPEED=1 to the PR description, or
• Include run-perf or /perf in the PR title or any commit message.

[github-actions]

Binary size checks passed

✅ 7 passed

	Artifact	Platform	File	Gzip	Gated on	Baseline	Delta	Status
✅	baml-cli	Linux	🔒 23.1 MB	9.8 MB	file	22.8 MB	+304.8 KB (+1.3%)	OK
✅	packed-program	Linux	🔒 16.4 MB	6.9 MB	file	16.3 MB	+154.7 KB (+0.9%)	OK
✅	baml-cli	macOS	🔒 17.8 MB	8.6 MB	file	17.6 MB	+252.5 KB (+1.4%)	OK
✅	packed-program	macOS	🔒 12.8 MB	6.0 MB	file	12.6 MB	+116.3 KB (+0.9%)	OK
✅	baml-cli	Windows	🔒 19.3 MB	8.7 MB	file	19.2 MB	+113.7 KB (+0.6%)	OK
✅	packed-program	Windows	🔒 13.7 MB	6.1 MB	file	13.5 MB	+134.7 KB (+1.0%)	OK
✅	bridge_wasm	WASM	15.3 MB	🔒 4.3 MB	gzip	4.3 MB	+25.7 KB (+0.6%)	OK

🔒 = the size this artifact is GATED on (ceiling + delta). Binaries gate on file size (installed binary); WASM gates on gzip (download size). The other size is shown for information only.

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Generated by cargo size-gate · workflow run

[cursor]

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