Remove incorrect SYSROOT usage

[o01eg]

Fixes #2874
Temporary solution until rust-lang/rust#57117 would be fixed.
Make sys_root optional to use internal rustc_driver sysroot.

[o01eg]

If rust-lang/rust#57196 will be fixed it should be possible to remove rustc call.

[phansch]

@o01eg So I'm attempting to review this. The code changes look fine to me, but I have some general questions as I've never dealt with custom sysroots so far.

1. I assume the custom sysroot is needed if you want to do embedded development or cross compile to different targets? It's the root directory for locating headers and library files, right?

2. Unfortunately I can't find a place with documentation about rusts sysroot usage. Is there one?

3. Do you think it's possible to add regression tests for this particular usage of Clippy?

Thanks for digging into this!

[o01eg]

1. I assume the custom sysroot is needed if you want to do embedded development or cross compile to different targets? It's the root directory for locating headers and library files, right?

As I understand sysroot is a install.prefix where all binaries, libraries, and docs are placed. Rustc searches internal libraries in /$sysroot/$libdir.

2. Unfortunately I can't find a place with documentation about rusts sysroot usage. Is there one?

No, the best I found are commentaries in rustc sources or ask @alexcrichton. Maybe read related PRs and issues.

3. Do you think it's possible to add regression tests for this particular usage of Clippy?

No. There should be system-wide installed rustc with custom libdir. When I asked about to add tests for this case in rustc I was said they don't have enough CI resources.

[bors]

☔ The latest upstream changes (presumably #3665) made this pull request unmergeable. Please resolve the merge conflicts.

[oli-obk]

There have been some refactorings around our sysroot detection, especially when the user tries to overwrite them. Can you check if the current system does what you need?

[o01eg]

Currently I'm waiting for clippy to be fixed in rust's source tree.

[flip1995]

ping from triage @o01eg. Clippy is fixed in the current nightly.

[o01eg]

Now they broke rustdoc again.

[o01eg]

I have it fixed with patch on rust from rust-lang/rust#57101
Patch for clippy didn't required.

[phansch]

@o01eg hasn't that PR been reverted as part of rust-lang/rust#57303?

[o01eg]

I patch it in Gentoo build script.