"casting u32 to f64 may become silently lossy if types change" doesn't make sense

[hrydgard]

I'm pretty sure this warning should be removed. Same with f32 to f64, s32 to f64 and so on. All integer types with sizes <= u32 are trivially covered by f64, due to the 53 bit mantissa, and f32 should be too, right?

To me, having to do f64::from() is a lot visually noisier than "as f64" and I see no benefit in warning for something that will never occur in practice. I don't see any scenario in which the definition of f64 would change...

[oli-obk]

It's not that the definition of f64 will change. It's also not the fact that converting an u32 to a f64 is ever lossy. It's about the fact that your source type might change (e.g. if you change the argument type from u32 to u64 in the following function)

fn foo(u: u32) {
    // loads of code here
    let x = u as f64;
    // use x
}

The conversion of u32 to f64 is not lossy here, but if the type of u is changed later, then you can get a lossy cast without realizing it, because everything still compiles.

[hrydgard]

I see. Then I guess my issue is that the wording of the warning is unclear. "If types will change" suggests the agency of the compiler, "if you later change the type" or something like that would be better I think.