super_errors_multi: multi-file fixture harness for cross-module errors

[JonoPrest]

Follow-up to #8432 (jono/expand-coverage-2). Adds a new test directory
tests/build_tests/super_errors_multi/ that mirrors super_errors/ but for
errors/warnings that only surface across compilation-unit boundaries.

Why a new harness

The existing super_errors/ runner only feeds one .res file to bsc per
fixture. That covers everything reachable from a single source file but
leaves a class of compiler errors untested:

• Signature mismatch between .resi and .res (Includemod through
  compunit).
• Cross-module value / type / record / variant disambiguation.
• Cross-module abstract type construction, private-type guards.
• Cross-module deprecation warnings and shadowing warnings.
• Interface_not_compiled (missing .cmi when -bs-read-cmi is set).

This PR adds a parallel directory with its own runner that compiles each
fixture's files via bsc directly, ordering .resi ahead of .res per
module and tagging output by source file.

Harness design

• Fixture format: one subdirectory per fixture under fixtures/.
  Files inside are compiled alphabetically (with .resi ahead of the
  matching .res). Fixtures needing a specific cross-module order can
  prefix filenames with a numeric label (e.g. 01_Foo.res).
• bsc flags: -w +A -bs-jsx 4 -color always -bs-cmi-only, plus
  -bs-read-cmi per file when a corresponding .resi exists. -bs-cmi-only
  stops the pipeline after typechecking, avoiding the "Js_not_found" failure
  bsc otherwise raises when trying to resolve sibling .js for cross-module
  imports.
• Snapshots: one expected/<FixtureName>.expected per fixture with all
  files' stderr concatenated, tagged ===== Foo.res =====.
• Runner parallelism: worker pool over fixtures, mirroring the
  single-file runner (super_errors: run fixtures in parallel #8430).
• Artifact hygiene: .cmi/.cmj/.cmt/.cmti/.mjs/.js are cleaned per
  fixture before each run and globally gitignored.

Format / format-check scripts are extended to skip
fixtures/Iface_not_compiled/ since that fixture intentionally contains a
.resi with a parse error.

What's covered (multi-file fixtures by category)

Includemod via .resi / .res mismatch (through compunit):

• Iface_missing_value (Missing_field)
• Iface_value_descriptions
• Iface_type_decl_record / variant
• Iface_extension_constructors
• Iface_module_types
• Iface_modtype_infos
• Iface_not_compiled (Interface_not_compiled)
• Iface_privacy_mismatch (Includecore Privacy)
• Iface_field_mutable_mismatch (Includecore Field_mutable)
• Iface_field_optional_mismatch (Includecore Field_optional)
• Iface_unboxed_variant_mismatch (Includecore Unboxed_representation)
• Iface_tag_name_mismatch (Includecore Tag_name)
• Iface_variant_representation_mismatch (Includecore Variant_representation)

Cross-module type / record / constructor disambiguation:

• Smoke_cross_module_type_clash
• Smoke_missing_field
• Smoke_unbound_module_reference
• Cross_abstract_type_construction
• Cross_qualified_constructor_mismatch (Name_type_mismatch via cross-module
  constructor)
• Cross_module_record_disambiguation
• Cross_record_extra_field (Wrong_name via cross-module label)
• Cross_chain_of_aliases (module B = A.Inner, C types against
  B.Reexport.t)
• Cross_dependent_clash
• Cross_polyvariant_constraint
• Cross_module_alias_dot_access (module S = Lib.Settings; S.host)
• Cross_inline_record_constructor
• Cross_variant_spread (type b = ...A.t | C overlap)
• Cross_hidden_through_interface (record access on type abstracted by .resi)
• Cross_uncurried_arity (function arity mismatch via cross-module alias)
• Cross_module_type_only_no_impl (consumer typechecks against .resi alone)

Async / FFI / patterns / GADTs:

• Cross_async_promise_mismatch
• Cross_optional_label_omitted
• Cross_dict_pattern
• Cross_external_argument_clash
• Cross_private_constructor_pattern
• Cross_exception_arity_mismatch
• Cross_gadt_pattern (Unqualified_gadt_pattern — was reachable only here,
  not from a single file)
• Modules_not_allowed_toplevel (toplevel let module(M) = … pattern)
• Unexpected_existential_in_let (destructuring GADT constructor with existential
  in toplevel let)

Warnings across modules:

• Cross_unused_open (warning 33)
• Cross_deprecated_value (warning 3 via cross-module @deprecated @val
  external)
• Cross_deprecated_type (warning 3 on @deprecated type used cross-module)
• Cross_open_shadows_identifier (warning 44)
• Cross_module_open_shadow_label_constructor (warnings 45 + 41)

Limitations

• The harness uses raw bsc rather than rescript build, so fixtures can't
  rely on rescript.json, namespacing, or third-party packages. JSX fixtures
  that import React were ruled out for this reason.
• Cannot_scrape_alias (typetexp / typemod) requires a broken alias chain
  in .cmi data that a hand-written fixture can't produce.
• Scoping_pack, With_makes_applicative_functor_ill_typed,
  With_changes_module_alias, Cannot_eliminate_dependency all need
  applicative-functor or destructive-substitution constructions that didn't
  fire from any fixture I tried. Likely dead in current ReScript syntax;
  flagged for a separate audit.
• Includemod.Unbound_module_path / Unbound_modtype_path /
  Invalid_module_alias require signature comparisons whose triggers couldn't
  be reproduced from the fixture format. Also flagged.

Coverage impact

Measured locally with make coverage (bisect_ppx point coverage). Baseline
is upstream/master (commit at the time of this PR).

Metric	Upstream/master	This branch	Δ
Overall	52.11% (38049 / 73012)	52.63% (38428 / 73012)	+0.52 pp / +379 points

Per-file moves on the modules this branch targets:

File	Upstream	This branch	Δ pp	Points
compiler/ml/includecore.ml	54.70%	59.54%	+4.84	+17
compiler/ml/includemod.ml	63.17%	67.99%	+4.82	+17
compiler/ml/typemod.ml	58.58%	60.17%	+1.59	+15
compiler/ml/typedecl.ml	68.62%	70.08%	+1.46	+19
compiler/ml/translmod.ml	71.56%	72.94%	+1.38	+3
compiler/ml/typecore.ml	75.53%	76.81%	+1.28	+37
compiler/ml/typetexp.ml	74.38%	75.62%	+1.24	+7
compiler/ml/env.ml	68.16%	69.24%	+1.08	+13
compiler/ml/translcore.ml	71.45%	71.82%	+0.37	+2

The two ~+5 pp jumps on includecore.ml and includemod.ml are the
headline — those are precisely the modules that the .resi/.res mismatch
fixtures (Iface_*) exercise via the Includemod.compunit entry point,
which has no equivalent in single-file work. Privacy, mutable-field,
optional-field, unboxed-rep, tag-name, and variant-representation paths in
includecore are now hit. The +37 absolute points on typecore.ml come
from cross-module GADT disambiguation (Unqualified_gadt_pattern),
Modules_not_allowed, Unexpected_existential, Cannot_infer_signature,
and Undefined_method.

Modules where the single-file suite already topped out the reachable paths
(rec_check.ml, transl_recmodule.ml, bs_syntaxerr.ml, ast_attributes.ml,
ast_external_process.ml, cmi_format.ml) show 0 movement — multi-file
doesn't unlock anything new in those.

[JonoPrest]

@codex review

[chatgpt-codex-connector]

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

ℹ️ About Codex in GitHub

Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review".

If Codex has suggestions, it will comment; otherwise it will react with 👍.

Codex can also answer questions or update the PR. Try commenting "@codex address that feedback".

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 60.10%. Comparing base (4f27edd) to head (a7e68e5).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8433      +/-   ##
==========================================
+ Coverage   59.90%   60.10%   +0.19%     
==========================================
  Files         373      373              
  Lines       54231    54231              
==========================================
+ Hits        32489    32593     +104     
+ Misses      21742    21638     -104     

see 19 files with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[pkg-pr-new]

Open in StackBlitz

rescript

npm i https://pkg.pr.new/rescript@8433

@rescript/darwin-arm64

npm i https://pkg.pr.new/@rescript/darwin-arm64@8433

@rescript/darwin-x64

npm i https://pkg.pr.new/@rescript/darwin-x64@8433

@rescript/linux-arm64

npm i https://pkg.pr.new/@rescript/linux-arm64@8433

@rescript/linux-x64

npm i https://pkg.pr.new/@rescript/linux-x64@8433

@rescript/runtime

npm i https://pkg.pr.new/@rescript/runtime@8433

@rescript/win32-x64

npm i https://pkg.pr.new/@rescript/win32-x64@8433

commit: a7e68e5

[cknitt]

Nice work! 🎉

[JonoPrest]

Nice work! 🎉

Thanks, I have 2 PRs ready after this is merged but I don't know how best to stack/stage them for review without making a huge PR.

Next 2 are:

• last sweep of reachable variants with this single and multi file harness
• removing dead/unreachable errors

I will then do a once over to try and improve error messages that aren't so good right now (which these tests have surfaced)

And that should conclude this work on error coverage. We can squeeze out more sub cases but I don't think its worth much.