lint &PathBuf instead of &Path in PTR_ARG

- extract get_only_generic_arg_snippet to improve readability

clippy_dev/src/ra_setup.rs

@@ -3,7 +3,7 @@
 use std::fs;
 use std::fs::File;
 use std::io::prelude::*;
-use std::path::PathBuf;
+use std::path::{Path, PathBuf};
 
 // This module takes an absolute path to a rustc repo and alters the dependencies to point towards
 // the respective rustc subcrates instead of using extern crate xyz.
@@ -44,7 +44,7 @@ pub fn run(rustc_path: Option<&str>) {
 }
 
 fn inject_deps_into_manifest(
-    rustc_source_dir: &PathBuf,
+    rustc_source_dir: &Path,
     manifest_path: &str,
     cargo_toml: &str,
     lib_rs: &str,