rustc_codegen_ssa: fix get_rpath_relative_to_output panic when lib only contains file name

Author: name1e5s

compiler/rustc_codegen_ssa/src/back/rpath.rs

@@ -85,6 +85,10 @@ fn get_rpath_relative_to_output(config: &RPathConfig<'_>, lib: &Path) -> OsStrin
     // Strip filenames
     let lib = lib.parent().unwrap();
     let output = config.out_filename.parent().unwrap();
+
+    // If output is empty, just assume it locates in current path
+    let output = if output == Path::new("") { Path::new(".") } else { output };
+
     let lib = try_canonicalize(lib).unwrap();
     let output = try_canonicalize(output).unwrap();
     let relative = path_relative_from(&lib, &output)

compiler/rustc_codegen_ssa/src/back/rpath/tests.rs

@@ -44,6 +44,9 @@ fn test_rpath_relative() {
         };
         let res = get_rpath_relative_to_output(config, Path::new("lib/libstd.so"));
         assert_eq!(res, "@loader_path/../lib");
+        // Should not panic when lib only contains filename.
+        // Issue 119571
+        let _ = get_rpath_relative_to_output(config, Path::new("libstd.so"));
     } else {
         let config = &mut RPathConfig {
             libs: &[],
@@ -54,6 +57,9 @@ fn test_rpath_relative() {
         };
         let res = get_rpath_relative_to_output(config, Path::new("lib/libstd.so"));
         assert_eq!(res, "$ORIGIN/../lib");
+        // Should not panic when lib only contains filename.
+        // Issue 119571
+        let _ = get_rpath_relative_to_output(config, Path::new("libstd.so"));
     }
 }