Rollup merge of rust-lang#58113 - h-michael:build-manifest-2018, r=alexcrichton

Transition build-manifest to 2018 edition

rust-lang#58099

Author: kennytm

src/tools/build-manifest/Cargo.toml

@@ -2,8 +2,9 @@
 name = "build-manifest"
 version = "0.1.0"
 authors = ["Alex Crichton <alex@alexcrichton.com>"]
+edition = "2018"
 
 [dependencies]
 toml = "0.4"
-serde = "1.0"
+serde = { version = "1.0", features = ["derive"] }
 serde_derive = "1.0"

src/tools/build-manifest/src/main.rs

@@ -1,6 +1,7 @@
-extern crate toml;
-#[macro_use]
-extern crate serde_derive;
+#![deny(rust_2018_idioms)]
+
+use toml;
+use serde::Serialize;
 
 use std::collections::BTreeMap;
 use std::env;
@@ -469,7 +470,7 @@ impl Builder {
         }
         manifest.pkg.insert("rust".to_string(), pkg);
 
-        return manifest;
+        manifest
     }
 
     fn profile(&mut self,