Type Parameter for Once

[Ericson2314]

When I made a spin lock version of Once for spin, I added a type parameter so this could act more like a promise of sorts. The is used in, e.g. lazy-static, store the result of the initialization function. It would be nice if Once became Once<T = ()> and offered the same expanded API.

One can always hand roll this with Cell, as lazy-static does when not using spin, but the composition of the Once and interior mutability is unsafe and thus requires extra care. This is exactly analogous to locks without the "slot", as POSIX threads defines the interface, also being unsafe/unergonomic.

Relatedly, there have long been murmers of moving lazy_static itself into std. I'm personally on the fence about that, but if we do so I see no reason why not to generalize Once as this issue proposes. There is also rust-lang-nursery/lazy-static.rs#111 advocating less macro magic by having the Lazy<T> type be exposed. Well, that Lazy<T> is basically Once<T>.

[Mark-Simulacrum]

I'm going to close this out in favor of moving once_cell/lazy_static into std. I suspect that we'll be doing that fairly soon, and I suspect that adding a type parameter to std::sync::Once is considerably more error prone (it would complicate the type quite a bit, at least).