Inline const expressions have been stabilised
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...
This is a nice small feature. I'm curious about the commit description:
rust
foo(const { 1 + 1 })
which is roughly desugared into
rust
struct Foo;
impl Foo {
const FOO: i32 = 1 + 1;
}
foo(Foo::FOO)
I would have expected it to desugar to something like:
rust
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.
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).
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.
Wouldn't this just prevent you from allocating more memory (than zero)?
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?"
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.
I'm not getting it. What's the point? It seems very much like a cpp-ism where you can put const in so many places.
cpp
const int n2 = 0; // const object int const n3 = 0; // const object (same as n2) // https://learn.microsoft.com/en-us/cpp/cpp/const-and-volatile-pointers?view=msvc-170 const char *cpch; // const variable cannot point to another pointer char * const pchc; // value of pointer is constant int f() const; // members cannot be modified in this, only read std::string const f(); // returns a constant
Then there are constant expressions.
Can anybody look at that and tell me it's readable with a straight face? I hope they don't start adding all this stuff to rust.
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).
Nope. This little neat feature mainly is just necessary part of bigger one - const-generics with const bounds.