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 remove static_assert #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.

[Serentty]

@Centril It seems to me like it would still be useful to have a few macros in the standard library simply for easy of reading, and to make it look more like a “feature” instead of a “pattern”. It would be nice to have visual clarity for something like ensuring that a type implements a trait. I really like what static_assertions has done, and think that with const panicks, it could be cleaned up, improved, and standardized, all without adding any language features, just standard library ones.