[cknitt]

Step 1 of #9303.

The build system passed compiler-flags from rescript.json to the compiler unchecked:

• An unknown flag failed every module with the same "Unknown option" error.
• A flag the build system sets itself, such as -bs-ns, silently conflicted with its own.

A build now checks each package's flags once, before compiling, and reports a problem as an error in that rescript.json:

rescript.json: field "compiler-flags": unknown flag "-bs-fooo"
rescript.json: field "compiler-flags": flag "-open" needs a value
rescript.json: field "compiler-flags": "Belt" is not a flag
rescript.json: field "compiler-flags": flag "-bs-ns" is set by the build system
rescript.json: field "compiler-flags": flag "-v" does not compile a module

• Rescript_compiler_driver.check_flags walks the flags against bsc's own flag table, like the parser does, but doesn't apply them. Some flags act as soon as they're parsed: -e compiles a string, -v prints and exits, -format reads a file.
• Flag values, such as warning specs, are not checked yet. That comes with the typed request in the next step of Pass a typed compile request from the build system to the compiler, and validate user compiler flags #9303.
• The build system owns the reserved lists: the flags it passes itself, and the flags with which bsc doesn't compile.
• format, clean and compiler-args don't check the flags, so a bad flag doesn't stop formatting.

Tests

• rewatch/tests/compile/25-invalid-compiler-flags.sh: each kind of error. It also checks that formatting ignores the flags and that valid flags still build.
• Unit tests for check_flags, including values that start with a dash and a reserved flag's name used as a value.

🤖 Generated with Claude Code

https://claude.ai/code/session_01Kbf3gHAzsdStoCEpBgx2tS

[github-actions]

Changelog

💅 Polish

• Report an unknown flag, a flag without its value, or a flag the build system sets itself in compiler-flags once, as an error in rescript.json, instead of failing every module.

Semantic Diff

1 commit · Full page

Check compiler-flags when a build starts

_{84940e7 · cknitt + Claude · Nov 14, 2026, 09:37 UTC}

Why

The build system passed the entries of compiler-flags to the compiler unchecked. An unknown flag failed every module with the same error, and a flag that the build system sets itself, such as -bs-ns, silently conflicted with its own. A build now checks the flags of every package against the compiler's flag table once, without applying them. Format, clean and compiler-args don't compile, so they don't check them.

Diagnostic added (unknown compiler flag)

// rescript.json
{"name": "flags", "sources": ["src"], "compiler-flags": ["-bs-fooo"]}

diagnostic

-Unknown option "-bs-fooo".
-(for every module)
+rescript.json: field "compiler-flags": unknown flag "-bs-fooo"

The other new errors: a flag without its value, a value where a flag is expected, a flag the build system sets itself (-o, -bs-ns, -bs-package-name, -bs-package-output, -bs-project-root, -bs-read-cmi, -bs-ast, -runtime-path), and a flag with which the compiler does not compile (-e, -v, -version, -warn-help, -format, -reprint-source).

Part of #9303

Review

• Why not run the flag parser on the flags? Some flags act when parsed: -e compiles a string, -v prints and exits, -format reads a file. Values such as warning specs are checked once a typed request replaces the flag strings (Pass a typed compile request from the build system to the compiler, and validate user compiler flags #9303).

[codecov]

Codecov Report

❌ Patch coverage is 58.33333% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.06%. Comparing base (1efa212) to head (84940e7).

Files with missing lines	Patch %	Lines
compiler/bsc/rescript_compiler_driver.ml	58.33%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #9304      +/-   ##
==========================================
- Coverage   82.06%   82.06%   -0.01%     
==========================================
  Files         455      455              
  Lines       65399    65411      +12     
==========================================
+ Hits        53672    53679       +7     
- Misses      11727    11732       +5     

Files with missing lines	Coverage Δ
compiler/bsc/rescript_compiler_driver.ml	80.00% <58.33%> (-0.92%)	⬇️

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
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[github-actions]

✅ Compiler benchmarks

Allocated words

In total: 640,953,454 → 640,960,318 (±0.00%).

Generated projects: 920,897,697 → 920,902,093 (±0.00%).

None of the 265 benchmarks changed by 0.1% or more.

Instructions

In total: 9,209,713,949 → 9,209,427,884 (±0.00%).

None of the 33 benchmarks changed by 0.1% or more.

Output size

In total: 25,843,406 → 25,843,406 (±0.00%).

None of the 13 benchmarks changed by 0.1% or more.

Pull request 84940e7 compared with base 1efa212 in this run.

[pkg-pr-new]

Open in StackBlitz

rescript

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@rescript/belt

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@rescript/darwin-arm64

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@rescript/linux-arm64

npm i https://pkg.pr.new/@rescript/linux-arm64@9304

@rescript/linux-x64

npm i https://pkg.pr.new/@rescript/linux-x64@9304

@rescript/runtime

npm i https://pkg.pr.new/@rescript/runtime@9304

@rescript/win32-x64

npm i https://pkg.pr.new/@rescript/win32-x64@9304

commit: 84940e7

[github-actions]

Developer playground preview: https://rescript-lang.github.io/rescript/dev-playground/?version=pr-9304

[cknitt]

@codex review

[chatgpt-codex-connector]

Codex Review Summary: ✅ Completed

This comment shows the latest Codex review activity on this pull request.

Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-11-14T10:00:42.531707Z	84940e7	Manual request

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[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. 👍

Reviewed commit: 84940e72bc

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