Array type should say size expression is a *const* expression.

[ehuss]

The array size expression needs to be a const expression (I believe). Currently types/array.md only says that it needs to evaluate to usize.

[Centril]

Yep, that's right. Moreover, currently, the const expression acts as an anonymous constant, so:

fn foo<T>() { let x: [u8; core::mem::size_of::<T>()]; }

is not valid (but this may change...)

[ehuss]

cc rust-lang/rust#43408