Make the project—not the module—the compiler’s lifetime boundary

[fhammerschmidt]

Summary

• Give each OCaml rewatch project a compiler dependency session that survives module jobs and watch edits, including full graph rebuilds. Each job still receives fresh inference, diagnostics, and environment state.
• Lease decoded small CMI tables and an expanded signature graph to one job at a time. Check load paths and file identity before reuse, verify graph integrity afterward, and restore changed graphs from a saved image.
• Capture ordinary compiler text output in memory, creating a temporary file only for channel-based or binary output.
• Add tests for project-session lifetime, reuse across worker batches, mutation recovery, load-path changes, and compiler output behavior.

Measurements

On a synthetic 1,201-module project, the project cache did not show a separate clean-build wall-time gain over the runtime-only cache: nine interleaved pairs measured 839.4 ms versus 843.4 ms. A seven-edit retained-watch gate measured 77 ms in each mode. Moving text capture to memory reduced summed parse-request setup time from 1,211 ms to 11 ms across 1,202 requests; typed CMI checks reduced summed verification time from about 170 ms to 8 ms across 1,201 implementation jobs. See the benchmark README for scope and method.

The available testrepo dependencies came from a Linux container, so the larger testrepo gate was not run on macOS. Its cross-edit performance on larger projects remains unmeasured.

Stack

Depends on #8673.

Validation

• make checkformat
• make test
• make test-rewatch
• sh rewatch-ocaml/tests/run.sh _build/default/rewatch-ocaml/rescript_ocaml.exe (passed with filesystem event access)

[codecov]

Codecov Report

❌ Patch coverage is 72.80702% with 31 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.40%. Comparing base (5d68298) to head (2e2baf5).

Files with missing lines	Patch %	Lines
compiler/ml/env.ml	70.37%	16 Missing ⚠️
compiler/ext/compiler_request_output.ml	72.72%	15 Missing ⚠️

Additional details and impacted files

@@                           Coverage Diff                            @@
##           faster-rewatch-ocaml-signature-cache    #8675      +/-   ##
========================================================================
+ Coverage                                 78.27%   78.40%   +0.13%     
========================================================================
  Files                                       481      481              
  Lines                                     65663    65729      +66     
========================================================================
+ Hits                                      51396    51537     +141     
+ Misses                                    14267    14192      -75     

Files with missing lines	Coverage Δ
compiler/bsc/rescript_compiler_driver.ml	75.96% <100.00%> (+0.31%)	⬆️
compiler/ext/compiler_request_output.ml	74.64% <72.72%> (-3.13%)	⬇️
compiler/ml/env.ml	65.80% <70.37%> (+6.64%)	⬆️

🚀 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.