Rollup merge of #125861 - name1e5s:fix/rpath_null_panic, r=michaelwoerister

rustc_codegen_ssa: fix `get_rpath_relative_to_output` panic when lib only contains file name

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When compiles program with `-C rpath=yes` but with no output filename specified, or with filename ONLY, we will get an ICE for now. Fix it by treat empty `output` path in `get_rpath_relative_to_output`  as current dir.

Before this patch:

```bash
rustc -C prefer_dynamic=yes -C rpath=yes -O h.rs  # ICE, no output filename specified
rustc -o hello -C prefer_dynamic=yes -C rpath=yes -O h.rs # ICE, output filename has no path
rustc -o ./hello -C prefer_dynamic=yes -C rpath=yes -O h.rs # Works
```

All those examples work after the patch.

Close #119571.
Close #125785.

compiler/rustc_codegen_ssa/src/back/rpath.rs

@@ -85,6 +85,11 @@ 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 or lib is empty, just assume it locates in current path
+    let lib = if lib == Path::new("") { Path::new(".") } else { lib };
+    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

@@ -57,6 +57,22 @@ fn test_rpath_relative() {
     }
 }
 
+#[test]
+fn test_rpath_relative_issue_119571() {
+    let config = &mut RPathConfig {
+        libs: &[],
+        out_filename: PathBuf::from("rustc"),
+        has_rpath: true,
+        is_like_osx: false,
+        linker_is_gnu: true,
+    };
+    // Should not panic when out_filename only contains filename.
+    // Issue 119571
+    let _ = get_rpath_relative_to_output(config, Path::new("lib/libstd.so"));
+    // Should not panic when lib only contains filename.
+    let _ = get_rpath_relative_to_output(config, Path::new("libstd.so"));
+}
+
 #[test]
 fn test_xlinker() {
     let args = rpaths_to_flags(vec!["a/normal/path".into(), "a,comma,path".into()]);