improve initial target libdir finding logic

Signed-off-by: onur-ozkan <work@onurozkan.dev>

Author: onur-ozkan

src/bootstrap/src/lib.rs

@@ -315,46 +315,19 @@ impl Build {
         let in_tree_llvm_info = config.in_tree_llvm_info.clone();
         let in_tree_gcc_info = config.in_tree_gcc_info.clone();
 
-        let initial_target_libdir_str = if config.dry_run() {
-            "/dummy/lib/path/to/lib/".to_string()
-        } else {
-            output(
-                Command::new(&config.initial_rustc)
-                    .arg("--target")
-                    .arg(config.build.rustc_target_arg())
-                    .arg("--print")
-                    .arg("target-libdir"),
-            )
-        };
+        let initial_target_libdir_str =
+            config.initial_sysroot.join("lib/rustlib").join(config.build).join("lib");
+
         let initial_target_dir = Path::new(&initial_target_libdir_str).parent().unwrap();
         let initial_lld = initial_target_dir.join("bin").join("rust-lld");
 
-        let initial_sysroot = if config.dry_run() {
-            "/dummy".to_string()
-        } else {
-            output(Command::new(&config.initial_rustc).arg("--print").arg("sysroot"))
-        }
-        .trim()
-        .to_string();
-
-        // FIXME(Zalathar): Determining this path occasionally fails locally for
-        // unknown reasons, so we print some extra context to help track down why.
-        let find_initial_libdir = || {
-            let initial_libdir =
-                initial_target_dir.parent()?.parent()?.strip_prefix(&initial_sysroot).ok()?;
-            Some(initial_libdir.to_path_buf())
-        };
-        let Some(initial_libdir) = find_initial_libdir() else {
-            panic!(
-                "couldn't determine `initial_libdir`:
-- config.initial_rustc:      {rustc:?}
-- initial_target_libdir_str: {initial_target_libdir_str:?}
-- initial_target_dir:        {initial_target_dir:?}
-- initial_sysroot:           {initial_sysroot:?}
-",
-                rustc = config.initial_rustc,
-            );
-        };
+        let initial_libdir = initial_target_dir
+            .ancestors()
+            .nth(2)
+            .unwrap()
+            .strip_prefix(&config.initial_sysroot)
+            .unwrap()
+            .to_path_buf();
 
         let version = std::fs::read_to_string(src.join("src").join("version"))
             .expect("failed to read src/version");
@@ -383,11 +356,11 @@ impl Build {
         }
 
         let mut build = Build {
-            initial_rustc: config.initial_rustc.clone(),
-            initial_cargo: config.initial_cargo.clone(),
             initial_lld,
             initial_libdir,
-            initial_sysroot: initial_sysroot.into(),
+            initial_rustc: config.initial_rustc.clone(),
+            initial_cargo: config.initial_cargo.clone(),
+            initial_sysroot: config.initial_sysroot.clone(),
             local_rebuild: config.local_rebuild,
             fail_fast: config.cmd.fail_fast(),
             doc_tests: config.cmd.doc_tests(),