`--cache=disabled` is triggering some re-compilation

[Khady]

Expected Behavior

running with --cache=disabled after a successful build shouldn't trigger recompilation, as _build contains all the up to date artifacts.

Actual Behavior

Depending on the configuration of the shared cache, building with --cache=disabled can trigger some recompilation.

Reproduction

• PR with a reproducing test: test: showcase impact of cache disabled #12308

The flow can be shorten to:

1. dune build
2. dune clean
3. dune build
4. dune build --cache=disabled
5. dune build --cache=disabled

Specifications

• Version of dune (output of dune --version): 3.19.1
• Version of ocaml (output of ocamlc --version): 4.14
• Operating system (distribution and version): debian

[rgrinberg]

I explained in the repro why this behavior is actually intentional. What are your use cases for switching the cache back and forth? Perhaps we could support them better.

[Khady]

Build speed measurements and debugging broken builds are the two immediate cases I can think about.