Rollup merge of #108376 - liushuyu:fix-sysroot-infer-103660, r=ozkanonur

compiler/rustc_session: fix sysroot detection logic

This pull request fixes the sysroot detection logic on systems where `/usr/lib` contains a multi-arch structure (e.g. installs `rustc_driver` into `/usr/lib/x86_64-linux-gnu` folder).

This fixes a regression for various Linux systems introduced in #103660. On Debian and Ubuntu systems, the logic in the pull request, as mentioned earlier, will resolve the sysroot to `/usr/lib`, making `rustc --print target-libdir` to return `/usr/lib/lib/rustlib/x86_64-unknown-linux-gnu/lib` (notice the extra `lib` at the beginning).

The fix is not very "clean" according to the standard. If you have any suggestions on improving the logic, you are more than welcome to comment below!

compiler/rustc_session/src/filesearch.rs

@@ -182,7 +182,17 @@ pub fn get_or_default_sysroot() -> Result<PathBuf, String> {
         if dir.ends_with(crate::config::host_triple()) {
             dir.parent() // chop off `$target`
                 .and_then(|p| p.parent()) // chop off `rustlib`
-                .and_then(|p| p.parent()) // chop off `lib`
+                .and_then(|p| {
+                    // chop off `lib` (this could be also $arch dir if the host sysroot uses a
+                    // multi-arch layout like Debian or Ubuntu)
+                    match p.parent() {
+                        Some(p) => match p.file_name() {
+                            Some(f) if f == "lib" => p.parent(), // first chop went for $arch, so chop again for `lib`
+                            _ => Some(p),
+                        },
+                        None => None,
+                    }
+                })
                 .map(|s| s.to_owned())
                 .ok_or(format!(
                     "Could not move 3 levels upper using `parent()` on {}",