Use [x; n] syntax for non-Copy types

[steveklabnik]

Originally at rust-lang/rust#5244

[Gankra]

The title of this issue implies making this work for all arbitrary non-Copy types. Is clone a lang item yet? If not, this would necessitate that. Unless the desugar is to textually duplicate x and evaluate it n times, but that would be contrary to how vec! desugars.

However the original issue referenced was just about making enum variants that only contain Copy data behaving as Copy types for the purposes of array literals. The canonical example being None, which is always producable, no matter what type the Option wraps.

[Kimundi]

I think the intention was "make the [x; n] syntax accept either Copyable types or constexprs".

This would never run user code, but still enable the usecase of, for example, initializing a array of Options to non-copyable data.

(static X: Option<Box<u32>> = None; compiles today for the same reasoning)

[ticki]

Big 👍 here. I've needed this many times.

[ticki]

Dup of #1109?

[steveklabnik]

yes, it is.