-opaque option breaks equivalence between release and dev compilation in presence of [@inline always]

[craff]

Expected Behavior

dune build and dune build --release should both work or both fail

Actual Behavior

If the code contains [@inline always], dune build may fail with error File "test.ml", line 12, characters 18-52: 12 | module FloatMat = Mat.Make [@inlined always] (Float) ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Error (warning 55): Cannot inline: Unknown function

because of the -opaque option, while dune build --release succeed.

Reproduction

Easy!

• Proposed solution remove warn_error for warning 55 and maybe others when compiling in dev mode.
• Other possible solution: do not use -opaque is [@inline always] and similar appears in the code (if the user thinks inlining is really important, one should not use -opaque.

• PR with a reproducing test:

1. Create a first file with a functor or funciton
2. Ask to inline this in another file with [@inline always]

Specifications

• Version of dune (output of dune --version): 2.7.1
• Version of ocaml (output of ocamlc --version): 3.11.1 and 3.11.1+flambda
• Operating system (distribution and version): debian testing

Additional information

• Link to gist with verbose output (run dune with the --verbose flag):