Int::{min,max} in const fn

language design
1

Right now, if you want to compute the maximum of two numbers in const context, you have to do an if a < b { b } else { a } manually. This is because usize::max() evaluates to <usize as Ord>::max which is a trait method and hence can't be const.

Could we add inherent methods to the integer types to work around this?

impl usize {
    pub const fn min(self, rhs: Self) -> Self {
        if self > rhs { rhs } else { self }
    }
    pub const fn max(self, rhs: Self) -> Self {
        if self < rhs { rhs } else { self }
    }
}

This way, whenever anyone writes usize::max(a,b) it will evaluate to this method instead, which is const. It shouldn't be a breaking change since this does not change the behavior of anyone calling the trait method.

2

We could, but I do expect that we'll make Ord a const trait soon.

9 Likes
3

Maybe is it reasonable to add 2 new methods const_max and const_min to impl usize (and other integer types)?

4

A lot of trait methods could be useful in const context. Since it's supposed to be a temporary workaround for a missing language feature, it's better to create a crate, say const_trait_workarounds, and later once the feature lands users can drop the dependency. Putting it in std would mean the deprecated workaround stays around forever.

15 Likes
5

Are const traits actually nearing stabilization? I know there's been a ton of activity on that front but haven't been following it too closely.

8 Likes
6

Part of the reason I suggest it is that const traits did not seem near stabilization to me. But I have not followed it that closely. But I do not think it's a particularly big technical debt to have a max method on integers. We already have them for f32/f64.

@VitWW I would not call them const_max/const_min. Just call them min/max and let them shadow the trait method.

6 Likes
7

It looks like there's tracking issues for making specific traits const-callable for specific std-lib types, which would be enough for this and I think will land well before support for const traits on arbitrary library traits/types. Though admittedly I haven't been following the most closely with that effort

3 Likes
8

In the meantime, you can use const_utils.

2 Likes
9

The konst crate has more stuff in it.

1 Like