Feature request: `assert_expr_impl!`

[scottjmaddox]

Over on the rust internals board, betamos noted that it would be nice to be able to annotate a type or value as sendable, and get a compile-time error if it is not. I thought this should be achievable using a macro, and that it would be a good fit for this crate. Maybe someone here can take a crack at it?

Edit: updated request to assert_expr_impl!

[nvzqz]

Is this not already achievable with assert_impl!(Type, Send)? However, reading the comment by @betamos, it seems they want to assert that an expression is sendable. In which case, I guess an assert_expr_impl! macro would be needed.

[scottjmaddox]

I actually didn't know about assert_impl!! So yeah, that covers the type assertion! But it would also be nice to have an expression assertion.

[leo60228]

macro_rules! assert_expr_impl_all {
    ($expr:expr => $($trait:path),+ $(,)?) => {
        const _: fn() = || {
            // Only callable when `$type` implements all traits in `$($trait)+`.
            fn assert_expr_impl_all<T: ?Sized $(+ $trait)+>(_arg: &T) {}
            assert_expr_impl_all(&$expr);
        };
    };
}