I have a trait that look like this:
pub trait Buf<const N: usize> {
fn to_buf(&self) -> [u8; N];
fn from_buf(buf: [u8; N]) -> Self;
}
However I want to do something like this:
trait Buf {
const N: usize;
fn to_buf(&self) -> [u8; Self::N];
fn from_buf(buf: [u8; Self::N]) -> Self;
}
It give me this error (playground):
|
| fn to_buf(&self) -> [u8; Self::N];
| ^^^^^^^ cannot perform const operation using `Self`
|
= note: type parameters may not be used in const expressions
= help: use `#![feature(generic_const_exprs)]` to allow generic const expressions
Is it possible?
After some research, I found out that, In nightly, Rust allow us to use generic const expression, under this (#![feature(generic_const_exprs)]) feature flag,
However, This is still far from being stable.
#![allow(warnings)]
#![feature(generic_const_exprs)]
trait Buf {
const N: usize;
fn to_buf(&self) -> [u8; Self::N];
fn from_buf(buf: [u8; Self::N]) -> Self;
}
macro_rules! impl_trait {
($name:ident for $($t:ty:$N:literal)*) => ($(
impl $name for $t {
const N: usize = $N;
fn to_buf(&self) -> [u8; $N] {
self.to_le_bytes()
}
fn from_buf(bytes: [u8; $N]) -> Self {
Self::from_le_bytes(bytes)
}
}
)*)
}
impl_trait!(Buf for u8:1 u16:2 u32:4 i8:1 i16:2 i32:4);
fn test<T: Buf>(buf: T, rhs: [u8; T::N])
where
[u8; T::N]:,
{
assert_eq!(buf.to_buf(), rhs);
}
fn main() {
test(123_u8, [123]);
test(123_u16, [123, 0]);
test(123_u32, [123, 0, 0, 0]);
test(-123i8, [133]);
test(-123i16, [133, 255]);
test(-123i32, [133, 255, 255, 255]);
}