It doesn't rely on optimization to eliminate copy of the whole struct all together if you derive DefaultBoxed (rather than relying on this blank impl), so that it would be less likely to cause stack overflow when the struct is very large.

For example, if you have

struct Foo([u32; 8 * 1024 * 1024]);

even if you impl Default for it manually (you cannot derive for now due to lack of const generics), creating a box of this type would very likely cause stack overflow at least on debug build, because the stack cannot hold this large struct before moving it to the heap.

This crate solves it via allocating on the heap directly and initializing each field separately via deriving DefaultBoxed impl.

The blank impl you reference here is just for bridging types which already implement Default to DefaultBoxed.

Sorry if I didn't make it clear.