https://doc.rust-lang.org/src/core/convert/num.rs.html#49-54 code here makes u32::from(u8) to require non-constness, while all it does is (u8 as u32) which compiles fine in const fn.
Shouldn't these blanket impls be marked const? Maybe I'm missing some flag that enables it?
The problem is that we currently have no way at all in Rust to mark methods/functions in traits as const.
Well, there is, it's just unstable.
There is indeed impl const Trait for Ty, the unstable implementation of RFC2632. However, that RFC has not been accepted yet, and is implemented purely to explore any issues that might come up with that approach.
Could it be enabled behind a feature gate then?