Lossy type aliases in cast_lossless lint

[ijijn]

Summary

Hello team!

I've noticed that using type aliases for built-in arithmetic types doesn't play especially well with the cast_lossless lint when it comes to casting to more capacious siblings.

I would expect the type alias to be adopted for associated warnings and fixes, rather than the underlying type itself, producing a minor semantic disconnect, and failed compilations in a somewhat confusing place should the type alias ever change. See below for a very short example.

Thanks,
Iain

Reproducer

I tried this code:

#![warn(clippy::pedantic)]

fn twelve_gets_a_promotion() {
    type Big = u64;
    let u8_twelve = 12_u8;
    let big_twelve = u8_twelve as Big;
    dbg!(big_twelve);
}

I expected to see this happen:

(warning)

casting `u8` to `Big` may become silently lossy if you later change the type

(fix)

fn twelve_gets_a_promotion() {
    type Big = u64;
    let u8_twelve = 12_u8;
    let big_twelve = Big::from(u8_twelve);
    dbg!(big_twelve);
}

Instead, this happened:

(warning)

casting `u8` to `u64` may become silently lossy if you later change the type

(fix)

fn twelve_gets_a_promotion() {
    type Big = u64;
    let u8_twelve = 12_u8;
    let big_twelve = u64::from(u8_twelve);
    dbg!(big_twelve);
}

Version

rustc 1.73.0-nightly (d12c6e947 2023-08-01)
binary: rustc
commit-hash: d12c6e947ceacf3b22c154caf9532b390d8dc88a
commit-date: 2023-08-01
host: x86_64-pc-windows-msvc
release: 1.73.0-nightly
LLVM version: 16.0.5

Additional Labels

No response

[Centri3]

Type aliases are unfortunately entirely stripped when going from the HIR Ty to Ty, there's sadly nothing clippy can do without upstream changes.

There has been work on making them a "real type", though: rust-lang/rust#112792

[ijijn]

Ah, thank you very much, Catherine! Makes sense. Fingers crossed that we can get some improvements in that department soon. 👍🏾

[Centri3]

Hmm actually... I believe in this case we do have the HIR type, since it's an as cast, so we can probably fix this one by using that instead. I'll open a PR for that soon.

[ijijn]

Oh that’s wonderful! Thanks so much 🏆