Rollup merge of rust-lang#58115 - h-michael:rustdoc-2018, r=Centril

Transition rustdoc to 2018 edition

Transitions rustdoc to Rust 2018; cc rust-lang#58099

src/tools/rustdoc/Cargo.toml

@@ -2,6 +2,7 @@
 name = "rustdoc-tool"
 version = "0.0.0"
 authors = ["The Rust Project Developers"]
+edition = "2018"
 
 # Cargo adds a number of paths to the dylib search path on windows, which results in
 # the wrong rustdoc being executed. To avoid the conflicting rustdocs, we name the "tool"

src/tools/rustdoc/main.rs

@@ -1,3 +1,5 @@
+#![deny(rust_2018_idioms)]
+
 #![feature(link_args)]
 
 #[allow(unused_attributes)]
@@ -10,6 +12,4 @@
 // See src/rustc/rustc.rs for the corresponding rustc settings.
 extern {}
 
-extern crate rustdoc;
-
 fn main() { rustdoc::main() }