trivial_regex prefers very slow code

Lint name: trivial_regex

#![feature(test)]

extern crate test;

use regex::Regex;
use test::{black_box, Bencher};

#[bench]
fn bench_contains(b: &mut Bencher) {
    let s = "0".repeat(100000);
    b.iter(|| black_box(&s).contains("asdf"));
}

#[bench]
fn bench_regex(b: &mut Bencher) {
    let s = "0".repeat(100000);
    let regex = Regex::new("asdf").unwrap();
    b.iter(|| regex.is_match(black_box(&s)));
}

$ cargo clippy --benches

warning: trivial regex
  --> src/main.rs:17:28
   |
17 |     let regex = Regex::new("asdf").unwrap();
   |                            ^^^^^^
   |
   = note: `#[warn(clippy::trivial_regex)]` on by default
   = help: consider using `str::contains`
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#trivial_regex

Clippy wants the regex replaced with str::contains, which is >10x slower.

$ cargo bench
     Running target/release/deps/testing-d8b23d81ce9bc37f

running 2 tests
test bench_contains ... bench:      12,031 ns/iter (+/- 101)
test bench_regex    ... bench:       1,126 ns/iter (+/- 8)

• cargo clippy -V: clippy 0.1.52 (a73c2e5 2021-02-06)
• rustc -Vv:

  rustc 1.52.0-nightly (a73c2e555 2021-02-06)
  binary: rustc
  commit-hash: a73c2e555c26ef0c8b98c91c97a7d24b7017267f
  commit-date: 2021-02-06
  host: x86_64-unknown-linux-gnu
  release: 1.52.0-nightly
  LLVM version: 11.0.1
  

Start by reproducing the reported trivial_regex warning with the src/main.rs benchmark and compare the str::contains and regex timings using cargo bench. Then trace the trivial_regex lint and its diagnostic guidance, and verify that the completed change no longer recommends the slower alternative for this case.