Improve [..] and as_slice documentation

[rseymour]

The section of the Rust book on Slices is nice (hard to find in primitive types) but it gives more information on [..](and it's origins in [1..] and [3..5] [..3] type indexing):

https://github.com/rust-lang/rust/blob/master/src/doc/trpl/primitive-types.md#slices

The actual docs for slice just intro it out of nowhere with no explanation of the fact that it's syntactic sugar for as_slice.

rust/src/libcollections/slice.rs

Line 19 in 9165a4e

//! let int_slice = &vec[..];

This is a subtle thing, but since googling for [..] is hard and github searching for "[..]" is basically impossible, it's a tough thing to have to search out, let alone explain on the web.

Thanks! Anyway, great documentation, hard to search for without single page, but I would want to fix the docs in libcollections if possible since ideally it would have more information and detail there.

-Rich

[steveklabnik]

hard to search for without single page

cc #22786

I completely agree that this is a bit weird right now. I'd like to add something, but I'm not sure what form it should take.

[alexispurslane]

It doesn't seem like as_slice is defined in rust nightly. Playground link: https://play.rust-lang.org/?gist=b220989e7c850403dd7b&version=stable

[steveklabnik]

In general, as_slice() is behind the convert feature gate, and &foo[..] should be used instead.

http://is.gd/srNESj

[alexispurslane]

Ah. That explains it. So it seems like as_slice and [..] are actually different things, because one is behind a feature gate, and one is not. Am I right about this?

[steveklabnik]

They do the same thing. The syntax was added later, one of the motivations was that .as_slice() is common and quite long, as well as an inexpensive operation.

[leoyvens]

👍 to this, it's totally ungoogleable, from the reference I concluded it's an use of the range operator, the book section could mention it.