Clippy does not run on proc macro crates

[dtolnay]

#!/bin/sh

cargo new repro
echo >repro/src/lib.rs '
#[allow(dead_code)]
fn f() {
    let _x = 3.14;
}
'

echo >>repro/Cargo.toml '
[lib]
proc-macro = true
'

cargo clippy --manifest-path=repro/Cargo.toml

The same script without proc-macro = true does catch the approx_constant lint.