compiler: avoid calls to env::current_dir

When these paths are already absolute the return value of
`env::current_dir` goes unused. Avoid possible panicks when the path is
already absolute.

Updates rust-lang#116426.

compiler/rustc_metadata/src/creader.rs

@@ -677,7 +677,15 @@ impl<'a, 'tcx> CrateLoader<'a, 'tcx> {
         stable_crate_id: StableCrateId,
     ) -> Result<&'static [ProcMacro], CrateError> {
         // Make sure the path contains a / or the linker will search for it.
-        let path = env::current_dir().unwrap().join(path);
+        //
+        // FIXME(https://github.com/rust-lang/rust/issues/92750): use std::path::absolute.
+        let buf;
+        let path = if path.is_absolute() {
+            path
+        } else {
+            buf = env::current_dir().unwrap().join(path);
+            &buf
+        };
         let lib = load_dylib(&path, 5).map_err(|err| CrateError::DlOpen(err))?;
 
         let sym_name = self.sess.generate_proc_macro_decls_symbol(stable_crate_id);

compiler/rustc_plugin_impl/src/load.rs

@@ -11,7 +11,7 @@ use rustc_span::symbol::{sym, Ident};
 
 use std::env;
 use std::mem;
-use std::path::PathBuf;
+use std::path::Path;
 
 /// Pointer to a registrar function.
 type PluginRegistrarFn = fn(&mut Registry<'_>);
@@ -51,7 +51,7 @@ fn load_plugin(
     ident: Ident,
 ) {
     let lib = locator::find_plugin_registrar(sess, metadata_loader, ident.span, ident.name);
-    let fun = dylink_registrar(lib).unwrap_or_else(|err| {
+    let fun = dylink_registrar(&lib).unwrap_or_else(|err| {
         // This is fatal: there are almost certainly macros we need inside this crate, so
         // continuing would spew "macro undefined" errors.
         sess.emit_fatal(LoadPluginError { span: ident.span, msg: err.to_string() });
@@ -60,11 +60,19 @@ fn load_plugin(
 }
 
 /// Dynamically link a registrar function into the compiler process.
-fn dylink_registrar(lib_path: PathBuf) -> Result<PluginRegistrarFn, libloading::Error> {
+fn dylink_registrar(lib_path: &Path) -> Result<PluginRegistrarFn, libloading::Error> {
     // Make sure the path contains a / or the linker will search for it.
-    let lib_path = env::current_dir().unwrap().join(&lib_path);
+    //
+    // FIXME(https://github.com/rust-lang/rust/issues/92750): use std::path::absolute.
+    let buf;
+    let lib_path = if lib_path.is_absolute() {
+        lib_path
+    } else {
+        buf = env::current_dir().unwrap().join(lib_path);
+        &buf
+    };
 
-    let lib = unsafe { Library::new(&lib_path) }?;
+    let lib = unsafe { Library::new(lib_path) }?;
 
     let registrar_sym = unsafe { lib.get::<PluginRegistrarFn>(b"__rustc_plugin_registrar") }?;