Rollup merge of #58117 - h-michael:rustdoc-theme-2018, r=Centril

Transition rustdoc-theme to 2018 edition

Transitions rustdoc-theme to Rust 2018; cc #58099

src/tools/rustdoc-themes/Cargo.toml

@@ -2,6 +2,7 @@
 name = "rustdoc-themes"
 version = "0.1.0"
 authors = ["Guillaume Gomez <guillaume1.gomez@gmail.com>"]
+edition = "2018"
 
 [[bin]]
 name = "rustdoc-themes"

src/tools/rustdoc-themes/main.rs

@@ -1,3 +1,5 @@
+#![deny(rust_2018_idioms)]
+
 use std::env::args;
 use std::fs::read_dir;
 use std::path::Path;