use char instead of &str for single char patterns

src/librustc_driver/args.rs

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

src/librustc_driver/lib.rs

@@ -521,7 +521,7 @@ fn stdout_isatty() -> bool {
 
 fn handle_explain(registry: Registry, code: &str, output: ErrorOutputType) {
     let normalised =
-        if code.starts_with("E") { code.to_string() } else { format!("E{0:0>4}", code) };
+        if code.starts_with('E') { code.to_string() } else { format!("E{0:0>4}", code) };
     match registry.find_description(&normalised) {
         Some(ref description) => {
             let mut is_in_code_block = false;