Compositional Coq builds depend on the current root

Expected Behavior

Build Coq dune project inner (stdpp) with caching enabled. Then place it inside project outer (iris), then build using outer as root. The code should not be rebuilt.

EDIT: this is important when sharing caches between builds of stdpp and compositional builds of stdpp + iris.

Actual Behavior

When using composition, and the root is outer, the code is built using paths relative to outer, not to outer/inner. That changes the command line and defeats caching.

Here's an example. Running from iris/stdpp with --root .:

Running[2]: (cd _build/default && /Users/pgiarrusso1/.opam/4.14.0+trunk/bin/coqc -q -w -deprecated-hint-rewrite-without-locality -w -deprecated-typeclasses-transparency-without-locality -w -deprecated-syntactic-definition -w -deprecated-native-compiler-option -w -native-compiler-disabled -native-compiler ondemand -R theories stdpp theories/options.v)
cache store success [442fc78b0dbcffaf142efd215cc5a689]

Running from iris with --root .

Running[2]: (cd _build/default && /Users/pgiarrusso1/.opam/4.14.0+trunk/bin/coqc -q -w -deprecated-hint-rewrite-without-locality -w -deprecated-typeclasses-transparency-without-locality -w -deprecated-syntactic-definition -w -deprecated-native-compiler-option -w -native-compiler-disabled -native-compiler ondemand -R theories stdpp theories/options.v)
cache store success [574d71dfc706ac57a5cf8f1f619fd297]

Changing current directory before the build step would fix this cache miss.

Workaround

Create a dummy folder for the outer project even to build only the inner project.

Reproduction

• PR with a reproducing test:

Specifications

• Version of dune (output of dune --version): dune 3.4.0
• Version of ocaml (output of ocamlc --version)
• Operating system (distribution and version):

Reproduce the cache miss by building stdpp as inner and then as part of iris, using the commands and dune 3.4.0 context in the report. Trace how compositional builds choose the current root for paths and cache keys; done when the inner build uses paths relative to outer/inner and reuses the existing cache entry.