rustc: Loosen bootstrapping restrictions.

Newer nightlies check a new environment variable that if set will loosen
restrictions on which compiler version can be used for bootstrapping.

Upstream issue is at rust-lang/rust#37265

Author: the-kenny

pkgs/development/compilers/rust/rustc.nix

@@ -46,6 +46,12 @@ stdenv.mkDerivation {
 
   NIX_LDFLAGS = optionalString stdenv.isDarwin "-rpath ${llvmShared}/lib";
 
+  # Enable nightly features in stable compiles (used for
+  # bootstrapping, see https://github.com/rust-lang/rust/pull/37265).
+  # This loosens the hard restrictions on bootstrapping-compiler
+  # versions.
+  RUSTC_BOOTSTRAP = "1";
+
   src = fetchgit {
     url = https://github.com/rust-lang/rust;
     rev = srcRev;