remove certain array elem moves for non-zeroing drop

[pnkfelix]

Summary

In order to prepare for an expected future implementation of
non-zeroing dynamic drop, remove support for:

• moving individual elements into an uninitialized fixed-sized array, and
• moving individual elements out of fixed-sized arrays [T, ..n],
  (copying and borrowing such elements is still permitted).

(rendered)

[reem]

My only worry here is that I'd like there to be a way to get a by-value Iterator over a small, constant number of items without allocating (right now it requires Vec), and I worry these changes may prevent that.

[arielb1]

@reem

This version of array indexing isn't useful for iterators, as it prevents access to all elements of the array except the one accessed. The problem with array iterators is that we can't have impls generic over integers.

[thestinger]

@reem: It definitely doesn't prevent that. Look at how the by-value vector iterator is implemented.

[thestinger]

It's not going to work well though, because you need to take the fixed-size array by-value... which is not going to scale at all.

[reem]

I guess it is orthogonal to this RFC, but I imagined the implementation would take some form of &move [T] or otherwise use a reference under the hood.

[pnkfelix]

@reem removing language features such as this now does not preclude making other independent additions in the future that would enable useful functionality.

The only issue I could see would be if there is some useful pattern that is allowed today that would be made invalid by changes in this RFC, but as far as I can tell (and as mentioned in the comments above) the example you describe cannot be expressed today.

[pnkfelix]

@nikomatsakis can we get discussion of this at a meeting?

[pnkfelix]

@nikomatsakis please do bring this up with core team when you can.

[nikomatsakis]

We've decided to accept this RFC. The precautions here are sensible and unlikely to have wide-spread impact on Rust code. If necessary, they can be lifted later, at the cost of more complex codegen.

[nikomatsakis]

Tracking issue rust-lang/rust#21963.