Standardize on From/Into for all conversions

[Soni]

@ExpHP

My gut reaction is that needing to wrap something in a newtype to use some functionality can be unergonomic for consumers of a type—especially if these values may be inside a container. (e.g. changing Vec<T> to Vec<MyNewtype<T>> requires allocation, and changing &[T] to &[MyNewtype<T>] requires unsafe )

See Looking for RFC coauthors on "named impls"

@ExpHP

Clearly, we disagree on how big of a problem overlap will be in practice. You don’t believe it will happen often, whereas I fear it will happen all over the place. (As I see it, overlap happens quite often enough even when we aren’t overloading the meaning of traits!).

The only way to know for sure is to try implementing it at a large scale.

I would like the nightly compiler to have "A/B testing" mode. But I don't think that's implemented yet.