Rollup merge of rust-lang#58120 - h-michael:build_helper-theme-2018, r=Centril

Transition build_helper to 2018 edition

Transitions build_helper to Rust 2018; cc rust-lang#58099
r? @Centril

Author: kennytm

src/build_helper/Cargo.toml

@@ -2,6 +2,7 @@
 name = "build_helper"
 version = "0.1.0"
 authors = ["The Rust Project Developers"]
+edition = "2018"
 
 [lib]
 name = "build_helper"

src/build_helper/lib.rs

@@ -1,3 +1,5 @@
+#![deny(rust_2018_idioms)]
+
 use std::fs::File;
 use std::path::{Path, PathBuf};
 use std::process::{Command, Stdio};