Trait core_extensions::ConstVal

source · [−]

pub trait ConstVal {
    type Ty;

    const VAL: Self::Ty;

    fn const_val(&self) -> Self::Ty { ... }
}

Available on crate feature const_val only.

Expand description

For types that represent constants.

Examples

Quasiconstants

Using the quasiconst macro to declare types that implement this trait, and emulate generic constants.

This example requires Rust 1.46.0, because it uses a while loop in a const context.

use core_extensions::{ConstVal, getconst, quasiconst};

const LEN: usize = 8;

quasiconst!{
   const FIBNUMS[T: ConstVal<Ty = u128>]: &'static [u128; LEN] = {
       let mut ret = [T::VAL; LEN];
       let mut i = 2;
       while i < LEN {
           ret[i] = ret[i - 1] + ret[i - 2];
           i += 1;
       }
       &{ret}
   };
    
   const ONE: u128 = 1;
   const FOUR: u128 = 4;
   const SEVEN: u128 = 7;
}

assert_eq!(getconst!(FIBNUMS<ONE>), &[1, 1, 2, 3, 5, 8, 13, 21]);
assert_eq!(getconst!(FIBNUMS<FOUR>), &[4, 4, 8, 12, 20, 32, 52, 84]);
assert_eq!(getconst!(FIBNUMS<SEVEN>), &[7, 7, 14, 21, 35, 56, 91, 147]);

Manual impl

use core_extensions::{ConstVal, getconst};
 
struct Foo;

struct Bar<T>(std::marker::PhantomData<T>);
 
impl ConstVal for Foo {
    type Ty = u32;
    const VAL: Self::Ty = 3;
}
 
impl<T> ConstVal for Bar<T> 
where
    T: ConstVal<Ty = u32>,
{
    type Ty = u32;

    const VAL: Self::Ty = T::VAL * 3 / 2;
}
 
assert_eq!(getconst!(Foo), 3);
assert_eq!(getconst!(Bar<Foo>), 4);
assert_eq!(getconst!(Bar<Bar<Foo>>), 6);
assert_eq!(getconst!(Bar<Bar<Bar<Foo>>>), 9);