Cannot put sized data into a StackBox / SmallBox with unsize feature

[RReverser]

Difference in definitions of new depending on a feature flag makes it impossible to create a simple sized box when needed. For example, the following code:

struct S { data: usize, }
let m = StackBox::<S, S1>::new(S { data: 10 }).unwrap();

compiles fine with default features, but fails with unsize with

the trait std::marker::Unsize<stackbox::tests::test_as_ptr::S> is not implemented for stackbox::tests::test_as_ptr::S

I'd suggest exposing same basic interface for sized values via new and create a separate feature-controlled new_unsized for when it's needed, so that compiler can distinguish and correctly resolve types in both cases.

[andylokandy]

Thanks for bringing this up!

This is one of the limitations of smallbox now. You can't have a sized SmallBox with unsize feature flag on. The only formal solution is using CoerceUnsize, which can automatically coerce a sized StackBox into unsized one, or just keep it sized if coercion is not required. There is a problem on SmallBox that CoerceUnsize can't be implement for an enum. I also have a solution for it: Turn SmallBox into a struct and then track the tag information by ourselves. But it has a trade off that we have to define our own Box type to control the heap pointer, which means we would not be able to get a standard Box<T> out of it again.

I'd suggest exposing same basic interface for sized values via new and create a separate feature-controlled new_unsized for when it's needed, so that compiler can distinguish and correctly resolve types in both cases.

It definitely is a solution for the problem, I'll take it into consideration.

[andylokandy]

CoerceUnsize is implemented for SmallBox and the new method only requires T: Sized now, so you can have either a sized SmallBox or an unsized SmallBox coerced from the sized one.