rust-lld: fallback to the default default sysroot where rustc is currently located

Author: lqd

compiler/rustc_codegen_ssa/src/back/link.rs

@@ -3116,12 +3116,31 @@ 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() {
-        for path in sess.get_tools_search_paths(false) {
-            cmd.arg({
-                let mut arg = OsString::from("-B");
-                arg.push(path.join("gcc-ld"));
-                arg
-            });
+        // 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()));
+
+        let mut linker_path_exists = false;
+        for path in sess.get_tools_search_paths(false).into_iter().chain(fallback_sysroot_paths) {
+            let linker_path = path.join("gcc-ld");
+            if linker_path.exists() {
+                linker_path_exists = true;
+                cmd.arg({
+                    let mut arg = OsString::from("-B");
+                    arg.push(linker_path);
+                    arg
+                });
+            }
+        }
+        if !linker_path_exists {
+            // As an additional sanity check, we do nothing if the sysroot doesn't contain the
+            // linker path at all.
+            return;
         }
     }