Rustc_driver cann't correctly find codegen dirs with multiple subfolders in libdir.

[o01eg]

When rust compiled and installed with multiple subfolders in libdir it cann't correctly found directory with codegen-backends. See rust-lang/rust-clippy#3565

        // use `parent` twice to chop off the file name and then also the
        // directory containing the dll which should be either `lib` or `bin`.
        if let Some(path) = dll.parent().and_then(|p| p.parent()) {

https://github.com/rust-lang/rust/blob/master/src/librustc_driver/lib.rs#L302

It would be work if rustc_driver was placed in /usr/lib64/librustc_driver-62d9b292bf74f4a6.so so it get /usr as sysroot but it placed in /usr/lib64/rust-9999/librustc_driver-62d9b292bf74f4a6.so so it get incorrect /usr/lib64.

I suppose it could be two possible solutions: The first is about to iterate parents until root will be reached. The second is to calculate number of parents need to be removed with find_libdir from filesearch.rs. Which are better or there are some other ways to solve this?

P.S. Looks like it doesn't fail for system-wide installed rustc and clippy because filesearch::get_or_default_sysroot got correct /usr sysroot from binaries in /usr/bin. If them will be moved same way as gcc in Gentoo it will break as well:

$ echo `which gcc`
/usr/x86_64-pc-linux-gnu/gcc-bin/7.3.0/gcc

[o01eg]

Third solution I came up with is if librustc_driver*.so placed in the libdir why it's required to search sysroot if it possible to simply add $RUST_LIB_DIR/$target/codegen-backends to get correct codegen folder?

[o01eg]

The current algorithm was introduced in 884715c
cc @alexcrichton

[o01eg]

Could this issue be reopened because the PR was reverted?

[o01eg]

While working on #62497 I think, what if move codegen libraries from sysroot libdir to rustc libdir? I think it is more useful because codegen are libraries rustc loads and links itself.