centralize fallback sysroot search paths in get_tools_search_paths

compiler/rustc_codegen_ssa/src/back/link.rs

@@ -3117,16 +3117,12 @@ fn add_lld_args(
     let self_contained_linker = self_contained_cli || self_contained_target;
     if self_contained_linker && !sess.opts.cg.link_self_contained.is_linker_disabled() {
         // We tell `cc` where to find the lld wrappers, in a similar way to how we locate
-        // codegen-backends dylibs:
-        // - in the tool search paths: the sysroot's `rustlib` bin path,
-        // - and as a fallback for cases where `--sysroot` only contains a target std: in the
-        //   default host sysroot where rustc is currently located.
-        let fallback_sysroot_paths = filesearch::sysroot_candidates()
-            .into_iter()
-            .map(|sysroot| filesearch::make_target_bin_path(&sysroot, config::host_triple()));
-
+        // codegen-backends dylibs, in the tools search paths, which contain:
+        // - the sysroot's `rustlib` bin path,
+        // - and as a fallback for cases using `--sysroot` without tools: in the default host
+        //   sysroot where rustc is currently located.
         let mut linker_path_exists = false;
-        for path in sess.get_tools_search_paths(false).into_iter().chain(fallback_sysroot_paths) {
+        for path in sess.get_tools_search_paths(false) {
             let linker_path = path.join("gcc-ld");
             if linker_path.exists() {
                 linker_path_exists = true;

compiler/rustc_session/src/session.rs

@@ -449,15 +449,29 @@ impl Session {
         )
     }
 
-    /// Returns a list of directories where target-specific tool binaries are located.
+    /// Returns a list of directories where target-specific tool binaries are located. Some fallback
+    /// directories are also returned, for example if `--sysroot` is used but tools are missing
+    /// (#125246): we also add the bin directories to the sysroot where rustc is located.
     pub fn get_tools_search_paths(&self, self_contained: bool) -> Vec<PathBuf> {
         let rustlib_path = rustc_target::target_rustlib_path(&self.sysroot, config::host_triple());
         let p = PathBuf::from_iter([
             Path::new(&self.sysroot),
             Path::new(&rustlib_path),
             Path::new("bin"),
         ]);
-        if self_contained { vec![p.clone(), p.join("self-contained")] } else { vec![p] }
+
+        let fallback_sysroot_paths = filesearch::sysroot_candidates()
+            .into_iter()
+            .map(|sysroot| filesearch::make_target_bin_path(&sysroot, config::host_triple()));
+
+        if self_contained {
+            [p.clone(), p.join("self-contained")]
+                .into_iter()
+                .chain(fallback_sysroot_paths)
+                .collect()
+        } else {
+            std::iter::once(p).chain(fallback_sysroot_paths).collect()
+        }
     }
 
     pub fn init_incr_comp_session(&self, session_dir: PathBuf, lock_file: flock::Lock) {