Infallible conversions from unsigned to larger-but-signed conversions

[morrisonlevi]

Specifically, I want the cases where unsigned value is one less than a platform fixed-width integer, eg:

impl From<u63> for i64 {} 
impl From<u31> for i32 {}  
impl From<u15> for i16 {}
impl From<u7> for i8 {}

But I suppose it can work for all width less than the ones listed too. Does this seem like a coherent feature that's general enough to add?

[JonathanWoollett-Light]

If you're interested, check out my overhaul project https://github.com/JonathanWoollett-Light/ux2 where this is implemented e.g. https://docs.rs/ux2/latest/ux2/struct.u2.html#impl-From%3Cu2%3E-for-i10.

[bbaldino]

Sounds reasonable to me. I'll try and take a look at adding this.