use strip_prefix over starts_with and manual slicing based on pattern length (clippy::manual_strip)

compiler/rustc_driver/src/args.rs

@@ -4,8 +4,7 @@ use std::fs;
 use std::io;
 
 pub fn arg_expand(arg: String) -> Result<Vec<String>, Error> {
-    if arg.starts_with('@') {
-        let path = &arg[1..];
+    if let Some(path) = arg.strip_prefix('@') {
         let file = match fs::read_to_string(path) {
             Ok(file) => file,
             Err(ref err) if err.kind() == io::ErrorKind::InvalidData => {