I don't think that is true. Testing with rust 1.92

enum Tree<T> { Lead(T), Branch(Vec<Tree<T>>), }

works just fine, no superfluous Box needed.

Similarly, the ‘indirect’ keyword can be omitted from Swift example in the blog post, for the same reason. A Swift Array stores its elements out of line, so an enum case payload can be an array type whose element contains the same enum, without any additional indirection.