librustc: Forbid private types in public APIs.

[pcwalton]

This breaks code like:

struct Foo {
    ...
}

pub fn make_foo() -> Foo {
    ...
}

Change this code to:

pub struct Foo {    // note `pub`
    ...
}

pub fn make_foo() -> Foo {
    ...
}

The visible_private_types lint has been removed, since it is now an
error to attempt to expose a private type in a public API.

Closes #16463.

RFC #48.

[breaking-change]

r? @alexcrichton

[alexcrichton]

There's a few corner cases which may not yet be covered by the lint (I forget), can you add some tests to make sure they're handled?

trait Foo {}
pub trait Bar: Foo {} // error

trait Foo {}
fn foo<T: Foo>() {} // error

We allow trait bounds on structs as well now, right?

trait Foo {}
struct Bar<T: Foo> {} // error (also enums(

In general the RFC has many tests as well, I just want to make sure we've got our bases covered before closing the issue.

The RFC also mentions that a feature gate would be added, I just want to confirm that you explicitly didn't implement it (which is fine by me!)

[pcwalton]

Updated. re-r? @alexcrichton

[sfackler]

Does the associated issue need to have its title updated? It refers to feature gating this, not forbidding it.

EDIT: nevermind, I misread the implementation

[alexcrichton]

r=me with as many things moved over to visitors as possible.