A solution to have a completely safe casting API

[Kixunil]

Casting T into U is sound if they have the same type ID and at least one of these conditions hold:

• Both T and U are 'static
• U is guaranteed to not have lifetime parameters in itself or its type parameters recursively

The first one is already covered by Any, and second one can be implemented here. The crate just needs to define pub unsafe trait AlwaysStatic {} and bound U on that, then there are two ways to implement the trait soundly by the consumer crates without using the unsafe keyword:

• derive macro which forces the : AlwaysStatic bound on all parameters and forbids any lifetime parameters
• declarative macro that has its input syntax restricted to not have lifetime parameters - something like $(#[$($attr:tt)*])* $vis:$vis struct $name:ident$(<$(const $const_name:ident: $const_ty:ty),* $($param:ident $(= $default:ty))*> ...)? and emits both the type and the trait impl

Note that, as opposed to other derive macros, this has no problem with bounds on the type parameters rather than field types because the fields cannot have lifetime parameters if the parent doesn't and if the parent does but fields don't that needs to be forbidden anyway.

(Also the trait would be, obviously, implemented for all relevant std types.)

[Kixunil]

I found there's a crate called castaway that already does the trait thing but no derive/decl macro (yet).