Do not rewrite ADTs mentioned in extern blocks

[aneksteind]

Fixes #948. Fixes an issue where we would rewrite structs and their fields even though they appear in extern blocks.

[kkysen]

Also, I changed "addresses" to "fixes" in the PR description as I don't think "addresses" is one of the keywords for closing an issue from a PR.

[spernsteiner]

Looks like a good approach overall

[aneksteind]

@spernsteiner I’m not clear on what further changes you’d like to see. Could you let me know if I’ve missed any concerns of yours?

[kkysen]

Just one small nit left: walk_adts instead of walk_adt, as the latter sounds like it's walking over only one. Otherwise, it all LGTM now!

[kkysen]

Why don't we need to substitute generics here? Then generic fields will just be TyKind::Param and we won't recognize that it may be an ADT or other type we need to recurse into.

[kkysen]

Oh wait, that should be covered by .walk instead, right?

[spernsteiner]

We'll indeed see TyKind::Param when visiting fields of generic structs, but walk will separately visit the type that would have been substituted in.

struct Wrapper<T> { x: T }
struct Inner { y: u32 }
extern "C" {
    fn foo(ptr: *mut Wrapper<Inner>);
}

walk(*mut Wrapper<Inner>) will visit Wrapper<Inner> and Inner, so it's okay that the traversal of the x field sees only TyKind::Param instead of Inner.