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Inline const expressions have been stabilised (github.com)

submitted 6 months ago by SorteKanin@feddit.dk to c/rust@programming.dev

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[–] kevincox@lemmy.ml 6 points 6 months ago (2 children)

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.

[–] 0v0@sopuli.xyz 10 points 6 months ago (1 children)

It's because it has to work in pattern contexts as well, which are not expressions.

[–] SorteKanin@feddit.dk 2 points 6 months ago (1 children)

Wait, in pattern context? How? Can you give an example?

[–] 0v0@sopuli.xyz 10 points 6 months ago

fn foo(x: i32) {
    match x {
        const { 3.pow(3) } => println!("three cubed"),
        _ => {}
    }
}

But it looks like inline_const_pat is still unstable, only inline_const in expression position is now stabilized.

[–] Giooschi@lemmy.world 7 points 6 months 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).