rustPackages.clippy fails to compile: can't find crate fmt_macros

[curiousleo]

Describe the bug

nix-build . -A rustPackages.clippy fails on master since e82fae1 with this error:

   Compiling clippy_lints v0.0.212 (/build/rustc-1.40.0-src/src/tools/clippy/clippy_lints)
error[E0463]: can't find crate for `fmt_macros`
  --> src/tools/clippy/clippy_lints/src/lib.rs:20:1
   |
20 | extern crate fmt_macros;
   | ^^^^^^^^^^^^^^^^^^^^^^^^ can't find crate

To Reproduce
Steps to reproduce the behavior:

1. Check out any commit between e82fae1 and master
2. nix-build . -A rustPackages.clippy
3. Observe the compilation failure pasted above.

Expected behavior
nix-build . -A rustPackages.clippy should succeed.

Additional context
I discovered this build failure in the context of #77327.

Metadata

$ nix-shell -p nix-info --run "nix-info -m"
 - system: `"x86_64-linux"`
 - host os: `Linux 4.19.93, NixOS, 19.09.1778.db3e8325a9b (Loris)`
 - multi-user?: `yes`
 - sandbox: `yes`
 - version: `nix-env (Nix) 2.3.1`
 - channels(root): `"nixos-19.09.1778.db3e8325a9b"`
 - 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:

[curiousleo]

rust-lang/rust-clippy#4651 points to rust-lang/rust#64823 being the root cause of this type of error - it looks like we'll need to pull in the rustc-dev component somehow and build clippy against that.

[curiousleo]

rust-lang/rust#64823 was reverted in rust-lang/rust#65342 due to rust-lang/rust#65335 ...

[curiousleo]

... and then re-applied with some changes: rust-lang/rust#65474.

[curiousleo]

... and then in rust-lang/rust#67469, rustc-dev (on which clippy_lints depends) was removed from the default set of components shipped with Rust nightly.

[curiousleo]

On the clippy side, this PR (re-)introduced the extern crate declarations that lead to the compilation error (though this is probably just a symptom, not the cause): rust-lang/rust-clippy#3189

[curiousleo]

(Very tangentially related: https://guix.gnu.org/blog/2018/bootstrapping-rust/, https://github.com/thepowersgang/mrustc)

[curiousleo]

There may be an alternative way of building clippy: together with rustc, via --tools=clippy in https://github.com/rust-lang/rust/blob/1.40.0/src/bootstrap/configure.py#L157, which is forwarded to the install script here: https://github.com/rust-lang/rust/blob/1.40.0/src/bootstrap/install.rs#L160

[Mic92]

How does upstream build clippy?

[mati865]

How does upstream build clippy?

By enabling this option.