Don't emit `clippy::useless_conversion` on type aliases

[Centri3]

Description

Currently, clippy::useless_conversion will still be emitted even on type aliases, for example:

HWND::from(0isize)

Clippy will suggest:

0isize

This conveys the type worse, but looking over rustc_hir, it doesn't seem whether it's a type alias is available so I don't know if this is possible to implement. I saw a few things relating to whether it's a trait alias but not whether it's a type alias. If this is possible, I'm happy to implement this 🙂

Version

rustc 1.71.0-nightly (9ecda8de8 2023-04-30)
binary: rustc
commit-hash: 9ecda8de85ce893cc3fc748ab06be0b8250147a7
commit-date: 2023-04-30
host: x86_64-unknown-linux-gnu
release: 1.71.0-nightly
LLVM version: 16.0.2

Additional Labels

@rustbot label +C-enhancement

[samueltardieu]

IMO, From::from() conveys the idea of a conversion, I'm not surprised that it warns even in the case of a type alias.

Note that 0isize as HWND would not trigger a warning, and for me it better conveys the idea of a representation rather than a conversion.

[Centri3]

That's fair enough, though 0isize as HWND also warns if you have clippy::as_conversions on warn (which is actually really handy, so I don't feel like just disabling that would be wise). And I also agree with this since a cast can silently change types.

I feel HWND::from(0isize) would be better though, since if HWND in some world changed which primitive type it was an alias of, 0isize as HWND would still compile; even if it could suddenly wrap and cause issues. Along with disallowing clippy::default_numeric_fallback and you know exactly where issues may arise when updating.

0isize would function the same, but also wouldn't convey its purpose very well.

[Centri3]

Created #10778 which implements this.