js.rs | Javascript Interpreter , in Rust

Unfortunately, it looks like the answer is "you don't" for Rust Stable. Rust is very conservative in which implicit coercions it allows. In particular, from the docs, we see the rule that's kicking in for Box.

Coercion is allowed between the following types:

...

• TyCtor(T) to TyCtor(U), where TyCtor(T) is one of

  • &T
  • &mut T
  • *const T
  • *mut T
  • Box

and where U can be obtained from T by unsized coercion.

where the relevant unsized coercion rule is

• T to dyn U, when T implements U + Sized, and U is object safe.

There's not much room for special casing there, at least not in the current version of Rust.

However, if you're willing to dip into Nightly-only features, then you get the exciting CoerceUnsized trait, whose intended use case is... smart pointers to things which would coerce like a Box would.