test: document single-module library recompilation behavior (#4572)#14103
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robinbb wants to merge 1 commit intoocaml:mainfrom
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test: document single-module library recompilation behavior (#4572)#14103robinbb wants to merge 1 commit intoocaml:mainfrom
robinbb wants to merge 1 commit intoocaml:mainfrom
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Add baseline test showing that single-module library consumers are recompiled when an unused module in a dependency changes. This happens because dune skips ocamldep for singleton stanzas, preventing future per-module filtering from determining which libraries the module actually references. Signed-off-by: Robin Bate Boerop <me@robinbb.com>
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@rgrinberg Thanks for your comments on #14079. In the end, I think that only this test can merge in isolation, so I closed that PR and made this one. |
rgrinberg
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Apr 9, 2026
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| uses_alpha is recompiled even though it only references Alpha, not Unused. | ||
| This is because dune skips ocamldep for single-module stanzas: |
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This explanation is not clear and possibly incomplete. The executable would still need to be rebuilt regardless of what modules are referenced by main.ml. This is because you still need to relink to produce the final executable and thus maintain all transitive cmx dependencies up to date.
The only thing you can save here perhaps not recompiling dune__exe__Main.cmx.
More details should be produced in the trace events to demonstrate which events you expect to see and which events will be dropped.
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Summary
Add baseline test documenting that single-module library consumers are
unnecessarily recompiled when an unused dependency module changes. This
happens because dune skips ocamldep for singleton stanzas, so per-module
filtering cannot determine which libraries the module actually references.
The code change to remove the singleton skip is deferred to the per-module
filtering PR (#14021) where it is actually needed and can be tested
end-to-end.
Ref: #4572
Test plan
dune runtest test/blackbox-tests/test-cases/per-module-lib-deps/single-module-lib.tpasses