rls doesn't build

[bbigras]

Describe the bug
A clear and concise description of what the bug is.

on master (254d6e5)

   Compiling rls-rustc v0.6.0 (/build/rustc-1.40.0-src/src/tools/rls/rls-rustc)
error[E0463]: can't find crate for `rustc`
 --> src/tools/rls/rls-rustc/src/lib.rs:4:1
  |
4 | extern crate rustc;
  | ^^^^^^^^^^^^^^^^^^^ can't find crate

error: aborting due to previous error

For more information about this error, try `rustc --explain E0463`.
error: could not compile `rls-rustc`.
warning: build failed, waiting for other jobs to finish...
error: build failed
builder for '/nix/store/bbjm5mgxyfmxj72ypnv7x65588icm9hh-rls-1.40.0.drv' failed with exit code 101
error: build of '/nix/store/bbjm5mgxyfmxj72ypnv7x65588icm9hh-rls-1.40.0.drv' failed

To Reproduce
Steps to reproduce the behavior:

1. cd nixpkgs
2. nix-build -A rls

Expected behavior

Screenshots

Additional context

Metadata

• system: "x86_64-linux"
• host os: Linux 4.19.92, NixOS, 20.03pre208171.2e8fc97dbfa (Markhor)
• multi-user?: yes
• sandbox: yes
• version: nix-env (Nix) 2.3.1
• channels(root): "nixos-20.03pre208171.2e8fc97dbfa"
• channels(bbigras2): "home-manager, nixos-19.09-19.09.1776.b926503738c"
• nixpkgs: /nix/var/nix/profiles/per-user/root/channels/nixos

Maintainer information:

# a list of nixpkgs attributes affected by the problem
attribute:
# a list of nixos modules affected by the problem
module:

[bbigras]

cc @symphorien

[symphorien]

It seems like it has been caused by rust-lang/rust#64823
I will have a look next week-end.

[bjornfor]

Fixed on master (tested at 13a4b48).

[bbigras]

thanks!