Rust 1.38 can't bootstrap itself. It has some ridiculous warnings enabled
by default which break this (even though the compiler obviously needs to
be able to compile itself just fine, that's what stage1 is all about).

diff --git a/src/bootstrap/bin/main.rs b/src/bootstrap/bin/main.rs
index bd1a87c..138b7f4 100644
--- a/src/bootstrap/bin/main.rs
+++ b/src/bootstrap/bin/main.rs
@@ -5,9 +5,6 @@
 //! parent directory, and otherwise documentation can be found throughout the `build`
 //! directory in each respective module.
 
-// NO-RUSTC-WRAPPER
-#![deny(warnings, rust_2018_idioms, unused_lifetimes)]
-
 use std::env;
 
 use bootstrap::{Config, Build};
diff --git a/src/bootstrap/bin/rustc.rs b/src/bootstrap/bin/rustc.rs
index 81e7e46..d3d84ca 100644
--- a/src/bootstrap/bin/rustc.rs
+++ b/src/bootstrap/bin/rustc.rs
@@ -15,9 +15,6 @@
 //! switching compilers for the bootstrap and for build scripts will probably
 //! never get replaced.
 
-// NO-RUSTC-WRAPPER
-#![deny(warnings, rust_2018_idioms, unused_lifetimes)]
-
 use std::env;
 use std::ffi::OsString;
 use std::io;
diff --git a/src/bootstrap/bin/rustdoc.rs b/src/bootstrap/bin/rustdoc.rs
index 184c9b1..2a8397a 100644
--- a/src/bootstrap/bin/rustdoc.rs
+++ b/src/bootstrap/bin/rustdoc.rs
@@ -2,9 +2,6 @@
 //!
 //! See comments in `src/bootstrap/rustc.rs` for more information.
 
-// NO-RUSTC-WRAPPER
-#![deny(warnings, rust_2018_idioms, unused_lifetimes)]
-
 use std::env;
 use std::process::Command;
 use std::path::PathBuf;
diff --git a/src/bootstrap/lib.rs b/src/bootstrap/lib.rs
index 76cc0dd..5eeed7c 100644
--- a/src/bootstrap/lib.rs
+++ b/src/bootstrap/lib.rs
@@ -103,9 +103,6 @@
 //! More documentation can be found in each respective module below, and you can
 //! also check out the `src/bootstrap/README.md` file for more information.
 
-// NO-RUSTC-WRAPPER
-#![deny(warnings, rust_2018_idioms, unused_lifetimes)]
-
 #![feature(core_intrinsics)]
 #![feature(drain_filter)]
 
diff --git a/src/build_helper/lib.rs b/src/build_helper/lib.rs
index a1aa189..f3f5913 100644
--- a/src/build_helper/lib.rs
+++ b/src/build_helper/lib.rs
@@ -1,6 +1,3 @@
-// NO-RUSTC-WRAPPER
-#![deny(warnings, rust_2018_idioms, unused_lifetimes)]
-
 use std::fs::File;
 use std::path::{Path, PathBuf};
 use std::process::{Command, Stdio};