error: unused extern crate proc_macro when trying to run examples

[bio]

The examples fail to compile, could you fix? Steps to reproduce and error below.

git clone https://github.com/carllerche/tower-web.git
cd tower-web
cargo build --example hello_world

   Compiling tower-web-macros v0.3.5 (/tower-web/tower-web-macros)
warning: deny(rust_2018_idioms) is ignored unless specified at crate level
 --> tower-web-macros/src/lib.rs:2:8
  |
2 | #[deny(rust_2018_idioms)]
  |        ^^^^^^^^^^^^^^^^
  |
  = note: `#[warn(unused_attributes)]` on by default

warning: deny(rust_2018_idioms) is ignored unless specified at crate level
 --> tower-web-macros/src/lib.rs:2:8
  |
2 | #[deny(rust_2018_idioms)]
  |        ^^^^^^^^^^^^^^^^

error: unused extern crate
 --> tower-web-macros/src/lib.rs:4:1
  |
2 | / #[deny(rust_2018_idioms)]
3 | |
4 | | extern crate proc_macro;
  | | ^^^^^^^^^^^^^^^^^^^^^^^-
  | |________________________|
  |                          help: remove it
  |
note: the lint level is defined here
 --> tower-web-macros/src/lib.rs:2:8
  |
2 | #[deny(rust_2018_idioms)]
  |        ^^^^^^^^^^^^^^^^
  = note: `#[deny(unused_extern_crates)]` implied by `#[deny(rust_2018_idioms)]`

error: aborting due to previous error; 2 warnings emitted

error: could not compile `tower-web-macros`

OS: MacOS Big Sur 11.3.1 tower-web version: the latest one rustc 1.51.0 (2fd73fabe 2021-03-23) cargo 1.51.0 (43b129a20 2021-03-16)