Static asserts

[nikomatsakis]

It would be useful to be able to specify static expressions which must be evaluated to true or else compilation will fail. This can be particularly useful in checking assumptions that are baked into unsafe code, such as that the size of two structs is equal.

Relevant RFCs:

• RFC #1096 removed the older #[static_assert] attribute.

[Centril]

The only thing needed to get this working is rust-lang/rust#51999 so I am closing this in favor of that issue.

[DoumanAsh]

@Centril Small question because I have use case for static_assert outside of const fn, there is no plan to have static assertion outside of const fn context?

[Centril]

@DoumanAsh With #2526 you can define const _ = $expr;. That way, you can statically assert things outside a const fn.