feat: per-module [-I]/[-H] include flags via [kept_libs]

[robinbb]

Layer 6 of 9 of #14492.

Compilation_context.filtered_include_flags: new function returning -I / -H flags restricted to kept_libs. The cctx's requires_compile and requires_hidden are each filtered by Lib.Set.mem kept_libs. No caching yet — layer 7 adds it.

Module_compilation.lib_deps_for_module: the tight branch was already producing kept_libs (unused at layers 4–5); now wired into filtered_include_flags. Returns the filtered include args instead of the cctx-wide Includes.

Effect: a consumer's compile command sees -I / -H only for libraries its ocamldep reference set actually reaches. Adding an unreferenced sibling to the cctx's (libraries ...) no longer changes the compile command, so the rule does not re-execute.

Promotes per-module-include-flags.t and add-unreferenced-sibling-lib.t — both deferred from layer 4 because their assertions depend on this filter.

Stack: rebases on #14517. Next: #14519.

Part of #14492. Related to #4572.

[Copilot]

Pull request overview

Adds per-module filtering of OCaml include flags so compile rules only see -I/-H for libraries actually reached by that module’s ocamldep reference set (via kept_libs), reducing unnecessary rule cache invalidations/rebuilds when unrelated sibling libraries are present in (libraries ...).

Changes:

• Introduce Compilation_context.filtered_include_flags to compute -I/-H restricted to a Lib.Set.t of “kept” libraries.
• Wire Module_compilation.lib_deps_for_module to use kept_libs to produce per-module include args (instead of cctx-wide includes) alongside the deps set.
• Promote/adjust blackbox tests to assert the new per-module include-flag behavior and the resulting no-rebuild trace when adding an unreferenced sibling lib.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
test/blackbox-tests/test-cases/per-module-lib-deps/per-module-include-flags.t	Updates expectations so only the referenced dep library’s objdir appears in -I flags.
test/blackbox-tests/test-cases/per-module-lib-deps/add-unreferenced-sibling-lib.t	Updates expectations to assert no rebuild occurs when adding an unreferenced library to (libraries ...).
src/dune_rules/module_compilation.ml	Uses kept_libs to request per-module include flags via Compilation_context.filtered_include_flags.
src/dune_rules/compilation_context.mli	Exposes the new filtered_include_flags API with documentation describing kept_libs semantics.
src/dune_rules/compilation_context.ml	Implements filtered_include_flags by filtering requires_compile/requires_hidden against kept_libs and generating -I/-H.