`rustc` should prefer statically linking native dependencies with unspecified kind if `crt-static` is enabled

[petrochenkov]

The main goal of +crt-static (at least on non-Windows targets) is to produce self-contained statically linked executables.

With this goal in mind rustc switches linking of 1) Rust crates and 2) libc to static if possible. (The libc part is done in the libraries through lazy cfgs though.)
However, for native dependencies with unspecified kind rustc will still prefer the dynamic version of the library if both are available.

Unfortunately, doing this is not entirely trivial because -Bstatic -lmylib won't just prefer the static version (e.g. libmylib.a), it will require it.
So we need to check for the existence of libmylib.a first (in which directories exactly?) and then pass -l:libmylib.a instead of -lmylib if it exists.

cc #71586 #39998

[petrochenkov]

-Bstatic -lmylib won't just prefer the static version (e.g. libmylib.a), it will require it.

If #39998 is accepted as the way to go by default when crt-static is enabled (on non-Windows platforms), then we can just use -Bstatic.

[smaeul]

This was discussed at length in #55566, where I was told that NativeUnknown means dynamic, and changing the semantics of #[link] was unacceptable.

[petrochenkov]

Some history in links: #71586 (comment)

[petrochenkov]

@smaeul
I do not agree with what alexcrichton said #55566, it's mostly a "it mostly works, so don't touch it because you may break something" position motivated by lack of resources and people ready to take responsibilities for this work on the Rust side.

We can at least make an experiment (possibly using crater, possibly landing this on nightly and reverting) and verify what actually breaks in practice.

[nagisa]

I fear crater will likely have a pretty poor coverage as far as linkage is concerned as its environment won't generally have anything but the most common libraries installed on the system (and the "native" libraries built in build.rs are already almost universally statically linked).