Make bootstrap build on stable

This is generally a good idea, and will help with being able to build bootstrap
without Python over time as it means we can "just" build with cargo +beta build
rather than needing the user to set environment variables. This is a minor step,
but a necessary one on that road.

src/bootstrap/bootstrap.py

@@ -714,7 +714,6 @@ class RustBuild(object):
         # See also: <https://github.com/rust-lang/rust/issues/70208>.
         if "CARGO_BUILD_TARGET" in env:
             del env["CARGO_BUILD_TARGET"]
-        env["RUSTC_BOOTSTRAP"] = '1'
         env["CARGO_TARGET_DIR"] = build_dir
         env["RUSTC"] = self.rustc()
         env["LD_LIBRARY_PATH"] = os.path.join(self.bin_root(), "lib") + \

src/bootstrap/lib.rs

@@ -103,8 +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.
 
-#![feature(drain_filter)]
-
 use std::cell::{Cell, RefCell};
 use std::collections::{HashMap, HashSet};
 use std::env;

src/bootstrap/tool.rs

@@ -162,13 +162,15 @@ impl Step for ToolBuild {
                 "the following dependencies are duplicated although they \
                       have the same features enabled:"
             );
-            for (id, cur, prev) in duplicates.drain_filter(|(_, cur, prev)| cur.2 == prev.2) {
+            let (same, different): (Vec<_>, Vec<_>) =
+                duplicates.into_iter().partition(|(_, cur, prev)| cur.2 == prev.2);
+            for (id, cur, prev) in same {
                 println!("  {}", id);
                 // same features
                 println!("    `{}` ({:?})\n    `{}` ({:?})", cur.0, cur.1, prev.0, prev.1);
             }
             println!("the following dependencies have different features:");
-            for (id, cur, prev) in duplicates {
+            for (id, cur, prev) in different {
                 println!("  {}", id);
                 let cur_features: HashSet<_> = cur.2.into_iter().collect();
                 let prev_features: HashSet<_> = prev.2.into_iter().collect();