`unused_io_amount` fails to fire on offending code

Summary

Code on playground:

https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=3deb8813d6956a51bdffe787d721c7b8

Code was found in a bigger codebase where the same lint previously fired on some other patterns. The example is obviously incorrect as it does not care about how many bytes were written in any of the write calls. Considering how close this code is to the documented example, I'm not entirely sure whether it's an actual bug or a PEBKAC.

Lint Name

unused_io_amount

Reproducer

I tried this code:

use std::io::Write;

const END: u8 = 0xC0;
const ESC: u8 = 0xDB;

const ESC_END: u8 = 0xDC;
const ESC_ESC: u8 = 0xDD;

pub fn encode_byte(writer: &mut impl Write, byte: u8) -> std::io::Result<()> {
    match byte {
        END => writer.write(&[ESC, ESC_END])?,
        ESC => writer.write(&[ESC, ESC_ESC])?,
        byte => writer.write(&[byte])?,
    };

    Ok(())
}

I expected to see this happen:

Clippy should have reported errors.

Instead, this happened:

Nothing.

Version

rustc 1.61.0-nightly (335ffbfa5 2022-03-11)
binary: rustc
commit-hash: 335ffbfa547df94ac236f5c56130cecf99c8d82b
commit-date: 2022-03-11
host: x86_64-pc-windows-msvc
release: 1.61.0-nightly
LLVM version: 14.0.0

Start at the unused_io_amount lint entry point and run the supplied playground reproducer with the listed Rust version. Compare its behavior with the documented example and verify whether the three write calls should produce lint errors; done means the reported behavior is explained and covered by an appropriate regression test.