test: cross-library .cmi regression baselines (4 fixtures)

[robinbb]

Four fixtures pinning the consumer-compile cmi-dep behaviour today, when the cctx-wide cmi glob over a dep lib's objdir covers the consumer's transitive .cmi needs regardless of whether ocamldep can see the chain syntactically. Each fixture hides the leaf's name from ocamldep through a different mechanism:

• cross-lib-preprocess-barrier.t — (preprocess (action ...))
• cross-lib-pps-runtime-no-ocamldep-barrier.t — (preprocess (pps X)) + ppx_runtime_libraries
• cross-lib-instrumentation-barrier.t — (instrumentation (backend X))
• cross-lib-open-flag-barrier.t — (flags (-open M))

Three of the four assert build success under --sandbox=copy. The instrumentation case additionally pins today's wide cmi glob over middle's objdir in dune rules output.

The forthcoming per-module narrowing work (#14492) will validate that each construct continues to work soundly under tighter per-module dep tracking, and will flip the instrumentation case's jq expected output from "glob present" to "glob absent".

[Copilot]

Pull request overview

Adds blackbox regression fixtures under per-module-lib-deps/ to pin today’s “wide .cmi glob” behavior for cross-library compilation dependencies in cases where ocamldep can’t see the full transitive .cmi chain (preprocess/action, ppx runtime deps, instrumentation backend, and -open flags). These baselines are intended to guard upcoming per-module dependency narrowing work (#14492).

Changes:

• Add a preprocess (action ...) fixture ensuring a consumer build succeeds under --sandbox=copy when only transitive .cmi loads make the leaf needed.
• Add a ppx runtime-libraries fixture ensuring the consumer build succeeds under --sandbox=copy even when the runtime library is never named in consumer source.
• Add an instrumentation-backend fixture that snapshots the current .cmi glob present in dune rules JSON output.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
test/blackbox-tests/test-cases/per-module-lib-deps/cross-lib-preprocess-barrier.t	New blackbox test for .cmi dep tracking across an action-preprocessed library under sandbox copy.
test/blackbox-tests/test-cases/per-module-lib-deps/cross-lib-pps-runtime-no-ocamldep-barrier.t/run.t	New blackbox test exercising ppx_runtime_libraries-induced compile-time .cmi needs under sandbox copy.
test/blackbox-tests/test-cases/per-module-lib-deps/cross-lib-instrumentation-barrier.t/run.t	New blackbox test pinning current instrumentation-disabled behavior and .cmi glob presence in dune rules output.
test/blackbox-tests/test-cases/per-module-lib-deps/cross-lib-instrumentation-barrier.t/ppx/hello.ml	Support library for the instrumentation backend fixture.
test/blackbox-tests/test-cases/per-module-lib-deps/cross-lib-instrumentation-barrier.t/ppx/hello_ppx.ml	Support ppx rewriter for the instrumentation backend fixture.
test/blackbox-tests/test-cases/per-module-lib-deps/cross-lib-instrumentation-barrier.t/ppx/dune-project	Defines the nested hello package for the instrumentation backend fixture.
test/blackbox-tests/test-cases/per-module-lib-deps/cross-lib-instrumentation-barrier.t/ppx/dune	Declares the instrumentation backend and its ppx/runtime library.
test/blackbox-tests/test-cases/per-module-lib-deps/cross-lib-open-flag-barrier.t	New blackbox test intended to cover -open-hidden transitive .cmi needs under sandbox copy.

[robinbb]

@Alizter This is more code that I have factored out of the 9-layer PR 14492.