Sync platform npm binaries via dune promotion

[cristianoc]

Rationale

The compiler binaries in packages/@rescript/<platform>/bin are what actually runs in most workflows: cli/bsc.js and friends resolve them via the platform npm package, and so do the mocha/build tests (scripts/test.js), the tools tests, the rewatch integration tests, and the runtime build. But those binaries were only refreshed by copy steps — mtime-based Makefile rules locally, and a separate scripts/copyExes.js --compiler step in CI — which run only when make (or that CI step) runs.

The command sequence that gets into trouble

# edit a compiler source
$ dune build                # _build is now current
$ ./cli/bsc.js test.res     # ...runs the OLD compiler

Nothing updates the packages copy between those two commands. Worse, the tree becomes split-brained: scripts/test_syntax.sh and the analysis tests read _build/install/default/bin directly (new compiler), while everything routed through the cli reads the packages copy (old compiler) — in the same shell session, with no error anywhere. In practice this showed up as confusingly inconsistent results: syntax fixtures regenerated by the new compiler while mocha snapshots were regenerated by the old one, costing real debugging time before anyone suspects the binary.

A programmatic illustration (probe string compiled into the binaries, then one bare dune build):

binary                                               contains
_build/install/default/bin/bsc                       NEW
packages/@rescript/darwin-arm64/bin/bsc.exe          old   <- what cli/bsc.js runs

entry point                                          runs
cli/bsc.js, scripts/test.js, tools tests, rewatch    old
test_syntax.sh, analysis tests (read _build)         NEW

Why this resolves it

Dune promotion makes the packages binaries an output of the build itself: rules in compiler/sync/dune (one per platform, covering bsc, rescript-editor-analysis, rescript-tools) copy-and-strip each binary into the platform package on every dune build. There is no separate step to forget. Key properties:

• Content-compared: promotion only rewrites a file when its bytes changed, so no-op builds cause no timestamp churn and make's downstream stamps (runtime build, etc.) don't re-trigger spuriously.
• Sole producer: the Makefile copy rules, both mtime-compensation touch loops, and copyExes.js's compiler mode are deleted. If a platform's enabled_if predicate is wrong, the binary is missing on a fresh checkout and fails loudly and proximately — the "present but stale" failure mode no longer has a producer. (copyExes.js keeps its rewatch mode: that binary is cargo-built and stays make's responsibility.)
• Platform matrix: one rule per platform (%{system}/%{architecture} from ocamlc -config); Windows copies without strip, matching the historical packaging step.
• Playground-safe: all rules carry (<> %{profile} browser), so a make playground build (which compiles a playground-flavoured bsc) can never overwrite the native binaries.

Verification layers

• make compiler now verifies instead of assumes: each packages binary is cmp-checked against the dune build output (_build/default/compiler/sync/), with one forced re-promotion retry that also self-heals a damaged or deleted copy, and a lost executable bit is restored. If promotion genuinely didn't produce the binary (e.g. an uncovered platform), it fails with a message pointing at compiler/sync/dune instead of an ENOENT deep inside some downstream harness.
• CI's former copy step is replaced by scripts/checkCompilerExes.js, which asserts on every platform in the matrix that the promoted binaries byte-match the build (and are executable on Unix) — so the per-platform enabled_if predicates are actually tested rather than masked by a re-copy.

Verifications done (macOS/arm64)

• The illustration above re-run after the change: every entry point converges on the new compiler after a bare dune build.
• Fresh production: binaries deleted, bare dune build re-creates all three; fresh worktree (fresh-clone equivalent) produces them from nothing.
• No-op build leaves promoted files' mtimes untouched (no downstream churn).
• Failure injection: a deliberately wrong enabled_if makes both make compiler and checkCompilerExes.js fail with the intended messages; a corrupted or chmod 644 binary is detected and self-healed.
• Browser profile: dune rules --profile browser confirms the promotion targets don't exist there.
• make compiler, make lib, make test, Biome, and formatting all green.

Linux and Windows validation is exactly what this PR's CI run provides: with no fallback producer left, a wrong platform predicate means a missing binary and a loud, attributable failure in that job.

🤖 Generated with Claude Code

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commit: 3d23f5a

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[cknitt]

Nice improvement!