If you have T and box it, a new type Box<T> is created. The compiler not only has to do typeck to make sure that you're using Box<T> in both sides of an assignm

If you have T and box it, a new type Box<T> is created. The compiler not only has to do typeck to make sure that you're using Box<T> in both sides of an assignment, it also has to evaluate the bound on Box (in this case it's only that it is Well-Formed, an internal concept, because Box accepts unsized types, struct Box<A: ?Sized>), but also evaluate all the bounds on T. If T: Foo, and the type you used for T is Bar<X: Qux>, then by the time rustc actually has to confirm the type Box<Bar<X>> is correct, it has to check that Bar<X>: Foo and X: Qux, but you never wrote those close to the Box, these flow from other definitions. Multiply for every type with an explicit or implicit where clause (T: Foo, where <A as B>::C: D, associated types, etc.).