Fix directive `@warning("-102")` not working by cknitt · Pull Request #8322 · rescript-lang/rescript

cknitt · 2026-04-02T16:11:46Z

Symptom

The reported repro was:

let x = (a, b) => {
  @warning("-102") (a->Pair.second > b->Pair.second)
}

With warning 102 enabled, this still produced:

Warning number 102
Polymorphic comparison introduced (maybe unsafe)

Root Cause

Warning 102 (Bs_polymorphic_comparison) was emitted too late, in compiler/core/lam_compile_primitive.ml.

By the time that code ran, local warning scopes created by source attributes such as @warning("-102") had already been restored. As a result, file-wide warning settings still worked, but expression-level suppression did not affect warning 102.

Fix

The warning emission was moved into typedtree-to-lambda translation in compiler/ml/translcore.ml, where it still runs under the relevant source-level warning scopes.

Concretely:

warning 102 is emitted when the polymorphic comparison primitive is selected during translation
the late warning emission was removed from compiler/core/lam_compile_primitive.ml
translation re-enters warning scopes so local source attributes apply at the warning site

This keeps warning 102 enabled by default, while making local suppression behave correctly.

Tests

Regression tests were added under tests/build_tests/super_errors/:

warning102_expr_attribute.res
warning102_structure_attribute.res
warning102_value_attribute.res

These cover:

expression-level @warning("-102")
file-level @@warning("-102")
direct value use such as let v = @warning("-102") compare

All corresponding snapshots are empty, because successful suppression means there is no warning output.

Result

After the fix:

@warning("-102") works for local polymorphic comparison expressions
@@warning("-102") still works
warning 102 still appears normally when no suppression is present

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commit: c4f4a87

cknitt · 2026-04-02T17:55:30Z

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compiler/ml/translmod.ml

cknitt · 2026-04-02T18:34:11Z

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compiler/ml/translcore.ml

cknitt · 2026-04-03T06:45:14Z

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Codex Review: Didn't find any major issues. Breezy!

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cknitt · 2026-04-03T08:26:54Z

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compiler/ml/typemod.ml

cknitt · 2026-04-03T12:05:47Z

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chatgpt-codex-connector · 2026-04-03T12:10:02Z

compiler/frontend/bs_ast_invariant.ml

+let warning_attribute_iterator =
+  let structure_item self (structure_item : Parsetree.structure_item) =
+    (match structure_item.pstr_desc with
+    | Pstr_attribute attr -> Builtin_attributes.warning_attribute attr


Limit warning prepass to avoid leaking nested scopes

warning_attribute_iterator now applies every Pstr_attribute/Psig_attribute during a full AST walk, but this mutates the global warning state without any restore. Because this prepass runs before typing, non-leading or nested @@warning directives are effectively active from the start of compilation (and can leak outside their intended scope), so warnings in earlier or outer code can be incorrectly suppressed/enabled; for example, a nested module-level @@warning("-32") can silence outer unused-value warnings.

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Fix @warning("-102") not working

8f4f487

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fix

1654767

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Fix

f9e78f2

cknitt added 3 commits April 3, 2026 09:11

Fix

fdc694b

.resi tests

b738319

Simplify

a937c43

Update comment

ee289bc

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Fix

c4f4a87

cknitt force-pushed the warning-102 branch from aacb207 to c4f4a87 Compare April 3, 2026 12:04

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cknitt commented Apr 2, 2026

Symptom

Root Cause

Fix

Tests

Result

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