Type conversions

[nrc]

Casting and coercions. Adds custom DST coercions for smart pointers. Otherwise, mostly descriptive, with a few bits of tidying up.

Rendered

[Gankra]

rendered (draft)

[Gankra]

Finally! ❤️

[bstrie]

Is this the opportunity to discuss possibly allowing numeric types to implicitly coerce when there's no loss of precision, e.g. u8 -> u32? I'm not necessarily advocating for it myself, but it has been requested in the past.

[nrc]

@bstrie I think that is a big enough change that it deserves its own RFC

[comex]

I think it would be better to ban such implementations than ignore them.

[nrc]

That is a reasonable alternative. It would require an ad-hoc check in the type checker though

[Kimundi]

Does this make coercion an special case of casts? Eg, is is true that with the changes every implicit coercion can also be written as an explicit cast?

[nodakai]

Will generic integer literals be covered by this RFC? (Perhaps they aren't technically coercion...) I have a little frustration about generic literals with minus sign:

fn main() {
    for x in std::iter::range_step(3u32, 0, -1) {
        println!("{}", x);
    }
}

I don't like -1 silently evaluates to some 4 billion of type u32.

[ben0x539]

@nodakai fwiw that's unrelated to generics, a plain let x: u32 = -1; already silently evaluates to upwards of 4 billion. I don't think it's a coercion, it's just a legal value of unsigned integer literals.

[nodakai]

@ben0x539 When you say "generics," you mean the <T> things, right? I'm talking about those literals which rustc call "generic integer":

fn main() {
    0.f();
}

<anon>:2:7: 2:10 error: type `<generic integer #0>` does not implement any method in scope named `f`

• http://is.gd/8yIej1

[ben0x539]

@nodakai Yeah, I thought you were talking about the <T> in fn range_step<T: ...>. Disregard that, I must have been confused. :)

[abonander]

I would like &T to coerce to T where T is a primitive. It would eliminate so many unnecessary explicit derefs.

[aturon]

Merged. Discussion. Tracking.