Tracking issue: Add back `Vec::from_elem` (RFC 832)

[aturon]

rust-lang/rfcs#832

[Gankra]

Super easy to implement for anyone interested. The implementation in the RFC is sound, although the new code should be abstracted out into a function that the macro calls for inline and macro-sanity reasons. The function needs to be marked as public/stable, but should otherwise be marked as #[doc(hidden)], as it is not intended to be called directly -- only by the macro.

[Gankra]

Willing to mentor on details.

[msiemens]

I'd like to give it a shot :)

[msiemens]

By the way: The documentation for vec! seems very sparse. If it's okay, I'd expand that a bit while I'm on it, or should that go into a seperate PR?

[Gankra]

Yeah, that's totally reasonable to do at the same time!

[msiemens]

Great! What would be the preferred place where to put the new function? I've added it to libcollections/vec.rs as pub fn from_elem<T: Clone>(elem: T, n: usize) -> Vec<T> for now (currently compiling it)

[Gankra]

Since it's supposed to only be used by the vec! macro (and will be doc(hidden)), it makes sense to me to put it in macros.rs. Putting it in vec.rs makes sense, too, though.

[msiemens]

vec.rs has a section labeled "Internal methods and functions" that seemed to fit well.