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Inline const expressions have been stabilised (github.com)

submitted 4 months ago by [email protected] to c/[email protected]

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[–] [email protected] 5 points 4 months ago (2 children)

Huh, this is awesome! From the link, this now works:

let v: Vec<i32> = const { Vec::new() };

I'm going to have to play with this to see how far it goes.

[–] [email protected] 2 points 3 months ago (1 children)

Wouldn't this just prevent you from allocating more memory (than zero)?

[–] [email protected] 2 points 3 months ago* (last edited 3 months ago)

Ah, apparently for now you're not allowed to allocate. But vec::new_in(allocator) looks interesting. This works in nightly today:

#![feature(allocator_api)]

use std::alloc::Global;

fn main() {
    const MY_VEC: Vec<i32> = const {
        Vec::new_in(Global)
    };
    println!("{:?}", MY_VEC);
}

Maybe at some point I can append to it at compile time too. I'd love to be able to put a const {} and have allocations that resolve down to a 'static, and this seems to be a step toward that.

I guess I'm just excited that Vec::new() is the example they picked, since the next obvious question is, "can I push?"

[–] [email protected] 2 points 3 months ago* (last edited 3 months ago)

What I'm curious about - this function was already const, so for stuff like this, I'd think there's basically a 100% chance the compiler would optimize this too, just implicitly.

AFAIK this new feature is just for times when it isn't an optimization, but more your own domain invariants. E.g. assertions.

But I could be wrong. I wonder if this can be used for actual optimizations in some places that the compiler couldn't figure out by itself.