I’m not getting it. What’s the point? It seems very much like a cpp-ism where you can put
constin so many places.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 constantThen 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.
Nope. This little neat feature mainly is just necessary part of bigger one - const-generics with const bounds.
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).
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?”
