Immutable representation of compiled interfaces

[fhammerschmidt]

Summary

• Add Frozen_type_graph, an immutable indexed type arena with request-local views that materialize only reached mutable types while preserving graph sharing and cycles.
• Add Frozen_values, an immutable index for exported values, types, constructors, labels, modules, and module types. When REWATCH_FROZEN_VALUES=1, a project cache shares images across compiler domains and each request owns its materialized declarations.
• Use lazy opened-name sources in Env to avoid expanding an imported module just to look up selected members. Unsupported shapes retain request-owned fallback paths.
• Add graph and interface unit tests, compiler-driver coverage, a synthetic fixture generator, a microprobe, and design notes with measurements.

Status and measurements

This is an opt-in experiment; the flag is off by default and the CMI file format is unchanged. On five short synthetic fixtures with four compiler domains, median clean-build time fell from 184.54 to 165.14 ms for abstract types, 170.75 to 165.17 ms for values and records, 192.54 to 162.64 ms for variants, 169.22 to 144.67 ms for modules, and 174.07 to 151.84 ms for opened imports. These measurements are directional and do not establish a production-project speedup.

Selected generated artifacts matched byte for byte in the measured fixtures. Switching the flag can change an internal identifier stamp in a consumer CMI, and therefore its self CRC, even when exported declarations and JavaScript agree; this may cause downstream rebuilds. Larger projects, edit workloads, and fallback frequency still need validation before enabling the flag by default.

Stack

Depends on #8675.

Validation

• make checkformat
• make test
• make test-rewatch

[codecov]

Codecov Report

❌ Patch coverage is 67.79412% with 438 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.18%. Comparing base (2e2baf5) to head (ff965cc).

Files with missing lines	Patch %	Lines
compiler/ml/env.ml	34.72%	265 Missing ⚠️
compiler/ml/frozen_values.ml	74.38%	104 Missing ⚠️
tests/ounit_tests/ounit_frozen_values_tests.ml	88.30%	31 Missing ⚠️
compiler/ml/frozen_type_graph.ml	85.43%	22 Missing ⚠️
tests/ounit_tests/ounit_frozen_type_graph_tests.ml	87.87%	16 Missing ⚠️

Additional details and impacted files

@@                           Coverage Diff                            @@
##           faster-rewatch-ocaml-project-session    #8676      +/-   ##
========================================================================
- Coverage                                 78.40%   78.18%   -0.22%     
========================================================================
  Files                                       481      485       +4     
  Lines                                     65729    66992    +1263     
========================================================================
+ Hits                                      51537    52381     +844     
- Misses                                    14192    14611     +419     

Files with missing lines	Coverage Δ
tests/ounit_tests/ounit_tests_main.ml	100.00% <ø> (ø)
tests/ounit_tests/ounit_frozen_type_graph_tests.ml	87.87% <87.87%> (ø)
compiler/ml/frozen_type_graph.ml	85.43% <85.43%> (ø)
tests/ounit_tests/ounit_frozen_values_tests.ml	88.30% <88.30%> (ø)
compiler/ml/frozen_values.ml	74.38% <74.38%> (ø)
compiler/ml/env.ml	57.92% <34.72%> (-7.89%)	⬇️

🚀 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.

[fhammerschmidt]

Superseded by consolidated PR #8680, which preserves this work as one commit in the combined review branch.