Fix corrosion_set_linker_language

[jschwe]

PR #126 brought up the topic that corrosion_set_linker_language currently does not do anything, since the used property LINKER_LANGUAGE is apparently special and can't be used in the way that it was.
Therefore the current behaviour of corrosion_set_linker_language is that nothing happens, and the choice of the linker is not influenced by using this function. E.g. adding corrosion_set_linker_language(rust-exe C) to cpp2rust/CMakeLists.txt does not break the build, since still the C++ linker is invoked.

As far as I can see this topic is only relevant for x2rust style projects, where linking is invoked via cargo.

#126 proposed to fix this by renaming the used property. Due to other corrosion code, the proposed change has the effect that the language specified in corrosion_set_linker_language is appended to a list of linker_languages and build_crate.rs will choose the language from the list with the highest CMAKE_<lang>_LINKER_PREFERENCE. This is probably not what a user would expect from the name of the function (In the case of cpp2rust, again still the C++ linker is invoked).

We could of course also fix that, but I would instead propose to just deprecate corrosion_set_linker_language (which should not be a breaking change, since it effectively doesn't do anything anyway). Instead I think it would make more sense to add a function corrosion_set_linker(<target> <linker>), which allows the user to directly specify the linker it wants to use. If you want to explicitely use the compiler/linker CMake chose for a certain language, you could still do corrosion_set_linker(my_target ${CMAKE_C_COMPILER}) instead of explicitely specifiying a linker.

The new behaviour would then be that if a linker was explictly set via corrosion_set_linker, then the linker_languages and CMAKE_<lang>_LINKER_PREFERENCE is just completely ignored and the linker specified by the user is passed to cargo via CARGO_TARGET_<target_triple>_LINKER

Any thoughts on this?

[jschwe]

When adding a test for #139, I noticed that corrosion_set_linker_language apparently does have an effect. I'm not sure what exactly changed in comparison to my previous tests, so I'll need to investigate that further.