Immovable types and self-referencing structs

[mrborrowck]

@NoamB

ExplicitMove trait Sometimes you might want to move an immovable struct after it has been constructed.

Functionally like CPP’s move constructor. It is implemented for every struct whose members implement ExplicitMove. Unlike CPP, never called implicitly.

A little dangerous to add, since people might add this bound everywhere. If rust could do without immovable structs, it can do without immovable structs that can be moved after construction.

I've had this idea which is similar to the ExplicitMove trait but is more restrictive (The RefMove trait must be implemented when the struct contains field with ref-lifetimes) and is called implicitly. The idea is to apply default move semantics first, but fields which have a ref-lifetime are in an uninitialized state. They must be initialized by the method specified by the RefMove trait.

https://internals.rust-lang.org/t/improving-self-referential-structs/4808/65?u=mrborrowck