Stack overflow with recursive types

[peterjoel]

Any recursive type will cause a stack overflow.

e.g.

#[derive(TypeHash)]
struct Recursive {
    foo: Option<Box<Recursive>>,
}

Produces:

thread 'recursive_generic_type_terminates' has overflowed its stack
fatal runtime error: stack overflow

That simple example could be detected in the macro, but it can't in the general case. e.g. mutually recursive types:

#[derive(TypeHash)]
struct A {
    foo: Option<Box<B>>,
}

#[derive(TypeHash)]
struct B {
    foo: Option<Box<A>>,
}

[peterjoel]

I don't particularly want to add runtime overhead (in particular heap allocation) in order to detect cycles, especially since most types are not recursive.

A couple of possible solutions:

1. Support an optional recursive attribute, which turns on recursion tracking just for that one type.
2. Add a new method, type_hash_recursive(bread_crumb_trail), so you can opt in at the call-site.

The best solution might be to do both, and have the generated implementation automatically call type_hash_recursive() when the attribute is present.