rust: Allow building dependencies twice with `native: true` and `native: false`

[sdroege]

See #11702

This is required as proc-macro target must be built with native: true, but a proc-macro target might have the same dependencies as a non-proc-macro target. In that situation this dependency needs to be compiled twice, which is currently not possible.

[xclaesse]

wondering if proc-macro could be simple declare_dependency(sources: 'mymacro.rs') that we pass to different targets so it get rebuilt as part of each target that uses it. Unless you think that would increase significantly the build time?

[sdroege]

Apart from increasing build times, it also wouldn't solve the problem. mymacro.rs might use the same Rust dependency as some other non-native Rust target, so you would have to also handle that dependency in such a way.

[xclaesse]

Ah they can have dependencies other than proc-macro? I thought it was more like header files.

[sdroege]

No, they're normal Rust code that is compiled into a shared library that can then be loaded by the compiler :)

[sdroege]

@xclaesse Maybe another solution here would be to simply convert cargo projects to two dependencies (or maybe even two subprojects?). One would be compiled natively, the other not. And proc-macro targets would have to depend on the native ones while other targets would depend on the others.

[xclaesse]

Being able to build a subprojects twice for build and host machine is something we wanted for a while. I think that would be a solution here but would require more work.

[sdroege]

If there's more use-cases of this than Rust proc-macros that makes sense, sure :)

[xclaesse]

Another use case is glib, we could build tools like glib-compile-ressources for build machine, that would require building the libs too.