Why does U256::try_from(0u128) does not implement TryFrom<u128, Error=Infallible>?

[jmg-duarte]

As the question states, I'm curious as to why it is the case that U256::try_from(u128) -> Result<U256, Error=ToUintError<alloy::alloy_primitives::Uint<256, 4>>>. It feels to me like it should work

[prestwich]

This is blocked by specialization. Because rust does not yet support specialization of trait impls, we have to implement generically, which means the error type must be the same for all values of BITS and LIMBS. So even in the case where we can guarantee an Infallible conversion, the error unfortunately type cannot be Infallible

You can read about some related rough edges in the TryFrom impls here

It would be also great to do things like the following, but unfortunately also blocked by language features

impl<const BITS: usize, const LIMBS: usize> TryFrom<u128> for Uint<BITS, LIMBS> 
where 
    // blocked by const generics improvements
    BITS > 128
{
   ...
}

// blocked by specialization
impl TryFrom<u128> for Uint<8, 1> { ... }
impl TryFrom<u128> for Uint<7, 1> { ... }

[jmg-duarte]

TIL, thanks for the answer James!

[prestwich]

no problem :)