Stop writing a copy of the .cmi into .cmt and .cmti files

[cknitt]

Stacked on #8773.

Like OCaml, the compiler wrote a full copy of the module's .cmi in front of the cmt infos in every .cmti, and in every .cmt of a module without an interface. This PR stops writing it.

Why the copy was there

This comes from upstream OCaml's -bin-annot design. A .cmt/.cmti pair is meant to describe a compilation unit on its own: the format is "a .cmi, then the cmt infos", so a tool that only has the annotation files (documentation generators, indexers) can get both the typed tree and the exact signature with its CRCs from one file. That is why .cmti files always embed the .cmi (it comes from the .mli), .cmt files embed it only when there is no .mli, and Cmt_format.read accepts .cmi, .cmt and .cmti alike. cmt_interface_digest records the CRC of the embedded .cmi, so that a tool can check that an annotation file matches the interface it is used with.

Why ReScript doesn't need it

• The .cmi is always available next to the .cmt: bsc writes it to lib/bs and rewatch copies it to lib/ocaml.
• Nothing reads the copy. Analysis, reanalyze, gentype and rescript-tools use Cmt_format.read_cmt and only look at the cmt infos. The one tool that needs a signature, analysis' createInterface, reads the .cmi file.
• Nothing reads cmt_interface_digest (the playground already sets it to None).

So the copy was pure cost: an extra marshalled signature in every .cmti and interface-less .cmt, a re-read of the partially written file to compute its digest, and an unmarshal of the copy on every read_cmt. The only thing given up is self-containment for an external tool that reads just .cmt/.cmti and wants the signature from them; nothing in this repository or the editor tooling does that.

Compatibility

There is no format change: Cmt_format.read already accepts files with and without the copy (it dispatches on the leading magic number), so files from older compilers keep working, and no magic number bump is needed. cmt_interface_digest is now always None; removing the field would change the marshalled record. save_cmt no longer takes the cmi.

Measurements

Runtime and Belt lib/ocaml:

	Before	After
.cmt (147 files)	7.18 MB	6.63 MB (−7.6%)
.cmti (85 files)	3.85 MB	2.53 MB (−34%)

.cmt files shrink less because modules with an interface never had the copy there.

rescript-tools reanalyze -dce on the runtime package (lib/ contains the lib/bs copies too: 12.2 → 9.5 MB of .cmt/.cmti), same binary, old vs. new files: identical output, major-heap words −6.7% (the unmarshalled copies), CPU time within noise since the analysis itself dominates. Compile time and allocation are unchanged (same method as #8769).

🤖 Generated with Claude Code

[cknitt]

@codex review

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Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-10-10T10:49:23.075246Z	3dccd48	Manual request

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Reviewed commit: 23e70a181e

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.64%. Comparing base (36d26f2) to head (09b9a2a).

Additional details and impacted files

@@                Coverage Diff                @@
##           perf-js-shake    #8774      +/-   ##
=================================================
- Coverage          80.66%   80.64%   -0.02%     
=================================================
  Files                463      463              
  Lines              62755    62755              
=================================================
- Hits               50619    50609      -10     
- Misses             12136    12146      +10     

Files with missing lines	Coverage Δ
compiler/core/js_implementation.ml	84.40% <100.00%> (ø)
compiler/ml/cmt_format.ml	100.00% <ø> (ø)
compiler/ml/cmt_format_common.ml	72.72% <ø> (-5.46%)	⬇️
compiler/ml/typemod.ml	73.76% <100.00%> (ø)

... and 1 file with indirect coverage changes

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[cknitt]

@codex review

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

Reviewed commit: 3dccd48822

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