Clippy built from sources couldn't find codegen directory

[o01eg]

While looking for other #2874 solutions found another bug. Compiled with rustc clippy correctly finds codegen folder while built from sources doesn't:

$ RUST_LOG=debug rustc src/driver.rs
...
 INFO 2018-12-19T20:37:40Z: rustc_driver: codegen backend candidate: /usr/lib64/rust-9999/rustlib/x86_64-unknown-linux-gnu/codegen-backends
 INFO 2018-12-19T20:37:40Z: rustc_driver: probing /usr/lib64/rust-9999/rustlib/x86_64-unknown-linux-gnu/codegen-backends for a codegen backend
...
$ RUST_LOG=debug clippy-driver src/driver.rs
...
 INFO 2018-12-19T20:37:04Z: rustc_driver: codegen backend candidate: /usr/lib64/rust-9999/rustlib/x86_64-unknown-linux-gnu/codegen-backends
 INFO 2018-12-19T20:37:04Z: rustc_driver: probing /usr/lib64/rust-9999/rustlib/x86_64-unknown-linux-gnu/codegen-backends for a codegen backend
...
$ RUST_LOG=debug ./target/release/clippy-driver src/driver.rs
...
 INFO 2018-12-19T20:36:27Z: rustc_driver: codegen backend candidate: /tmp/rust-clippy/target/lib64/rust-9999/rustlib/x86_64-unknown-linux-gnu/codegen-backends
 INFO 2018-12-19T20:36:27Z: rustc_driver: codegen backend candidate: /usr/lib64/lib64/rust-9999/rustlib/x86_64-unknown-linux-gnu/codegen-backends
...

rustc config.toml:

[llvm]
optimize = true
release-debuginfo = false
assertions = false
targets = "X86;Mips;NVPTX;BPF;AArch64"
[build]
build = "x86_64-unknown-linux-gnu"
host = ["x86_64-unknown-linux-gnu"]
target = ["x86_64-unknown-linux-gnu"]
docs = false
submodules = false
python = "python2.7"
locked-deps = true
vendor = false
verbose = 2
sanitizers = false
extended = true
tools = ["clippy",]
[install]
prefix = "/usr"
libdir = "lib64/rust-9999"
docdir = "share/doc/rust-9999"
mandir = "share/rust-9999/man"
[rust]
optimize = true
debuginfo = false
debug-assertions = true # false
jemalloc = true
default-linker = "x86_64-pc-linux-gnu-gcc"
rpath = false
ignore-git = false
lld = false
llvm-tools = true
[target.x86_64-unknown-linux-gnu]
cc = "x86_64-pc-linux-gnu-gcc"
cxx = "x86_64-pc-linux-gnu-g++"
linker = "x86_64-pc-linux-gnu-gcc"
ar = "x86_64-pc-linux-gnu-ar"

OS: Gentoo Linux AMD64
Clippy from sources rev: 691e5e7
Rustc rev: rust-lang/rust@e7b4bc3

[o01eg]

When I launch system installed rustc or clippy-driver in the librustc_driver's get_codegen_sysroot happened next:

1. filesearch::get_or_default_sysroot gets current binary /usr/bin/clippy-driver-9999 and gets sysroot /usr from it.
2. then it concatenates sysroot with relative libdir from filesearch::relative_target_lib_path lib64/rust-9999/rustlib/x86_64-unknown-linux-gnu/lib and replace last element with codegen-backends
   Correct codegen /usr/lib64/rust-9999/rustlib/x86_64-unknown-linux-gnu/codegen-backends folder found.

When I launch ./target/release/clippy-driver from /tmp/rust-clippy in won't work:

1. filesearch::get_or_default_sysroot gets current binary /tmp/rust-clippy/target/release/clippy-driver and gets sysroot /tmp/rust-clippy/target from it.
2. then it concatenates sysroot with relative libdir from filesearch::relative_target_lib_path lib64/rust-9999/rustlib/x86_64-unknown-linux-gnu/lib and replace last element with codegen-backends
/tmp/rust-clippy/target/lib64/rust-9999/rustlib/x86_64-unknown-linux-gnu/codegen-backends isn't correct folder.
   But also it tries /usr/lib64 sysroot which also incorrect.

[o01eg]

Looks like it's a rustc issue: rust-lang/rust#57014