Promotion and cross-compilation

[sir4ur0n]

Desired Behavior

Clarify and make deterministic the behavior of promotion with cross-compilation.

Currently, based on several tests, it seems that the file being promoted is not deterministic, and it is also not always the cross-compilation one being copied.

Example

Consider the following promotion rule:

(rule
 (mode
  (promote (until-clean)))
 (action
  (copy foobar/something.exe something)))

Now run dune build -x sometoolchain something.

Dune will build 2 binaries (as documented in https://dune.readthedocs.io/en/stable/cross-compilation.html):

• the native one (_build/default/foobar/something.exe)
• the cross-compilation one (_build/default.sometoolchain/foobar/something.exe)

But then Dune will copy one of those 2 (binary) files to ./something.

1. It is not clear which will be copied
2. It seems to change, sometimes it is the native one, sometimes the cross-compilation one

Note that since Dune supports passing several cross-compilation targets, I don't even know what should be the semantic of promotion with cross-compilation.

At least I would have liked an error/warning, e.g.,

⚠ Promotion is incompatible with more than 1 target, but you passed targets (native sometoolchain)

Kinda related question: is it possible to remove targets from the CLI? E.g. to only build for target sometoolchain without building the native target?