Stabilise inline_const by nbdd0121 · Pull Request #104087 · rust-lang/rust
github.com
Stabilisation Report Summary This PR will stabilise inline_const feature in expression position. inline_const_pat is still unstable and will not be stabilised. The feature will allow code like this...
  • kevincox@lemmy.ml
    6·
    1 year ago

    This is a nice small feature. I’m curious about the commit description:

    foo(const { 1 + 1 })

    which is roughly desugared into

    struct Foo;
impl Foo {
    const FOO: i32 = 1 + 1;
}
foo(Foo::FOO)

    I would have expected it to desugar to something like:

    foo({
  const TMP: i32 = 1 + 1;
  TMP
})

    But I can’t seem an explanation why the struct with impl is used. I wonder if it has something to do with propagating generics.

    • Giooschi@lemmy.worldEnglish
      7·
      1 year ago

      They tested the same strings on that implementation

      The code they were looking at was used for writing the table, but they were testing the one that read it (which is instead correct).

      though judging by the recent comments someone’s found something.

      Yeah that’s me :)The translation using an associated const also works when the const block uses generic parameters. For example:

      fn require_zst<T>() {
    const { assert!(std::mem::size_of::<T>() == 0) }
}

      This can be written as:

      fn require_zst<T>() {
    struct Foo<T>(PhantomData<T>);
    impl<T> Foo<T> {
        const FOO: () = assert!(std::mem::size_of::<T>() == 0);
    }
    Foo::<T>::FOO
}

      However it cannot be written as:

      fn require_zst<T>() {
    const FOO: () = assert!(std::mem::size_of::<T>() == 0);
    FOO
}

      Because const FOO: () is an item, thus it is only lexically scoped (i.e. visible) inside require_zst, but does not inherit its generics (thus it cannot use T).

  • sugar_in_your_tea@sh.itjust.works
    61·
    1 year ago

    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.

    • asdfasdfasdf@lemmy.world
      2·
      1 year 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.

      • sugar_in_your_tea@sh.itjust.works
        2·
        1 year 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?”

    • Miaou@jlai.lu
      4·
      1 year ago

      It can be used for producing const values in arbitrary context. Can basically be swapped for c++'s constexpr.

      C++'s const does not exist in rust (values are const by default).

    • Alex@programming.dev
      3·
      1 year ago

      Nope. This little neat feature mainly is just necessary part of bigger one - const-generics with const bounds.