gcd: Update for Rust 2018.

gcd/Cargo.toml

@@ -2,5 +2,6 @@
 name = "gcd"
 version = "0.1.0"
 authors = ["You <you@example.com>"]
+edition = "2018"
 
 [dependencies]

gcd/src/main.rs

@@ -1,3 +1,6 @@
+#![warn(rust_2018_idioms)]
+#![allow(elided_lifetimes_in_paths)]
+
 fn gcd(mut n: u64, mut m: u64) -> u64 {
     assert!(n != 0 && m != 0);
     while m != 0 {
@@ -20,19 +23,19 @@ fn test_gcd() {
                3 * 11);
 }
 
-use std::io::Write;
 use std::str::FromStr;
+use std::env;
 
 fn main() {
     let mut numbers = Vec::new();
 
-    for arg in std::env::args().skip(1) {
+    for arg in env::args().skip(1) {
         numbers.push(u64::from_str(&arg)
                      .expect("error parsing argument"));
     }
 
     if numbers.len() == 0 {
-        writeln!(std::io::stderr(), "Usage: gcd NUMBER ...").unwrap();
+        eprintln!("Usage: gcd NUMBER ...");
         std::process::exit(1);
     }