rustdoc: don't crash when an external trait's docs needs to import another trait

[QuietMisdreavus]

Fixes#48414

When resolving intra-paths for an item, rustdoc needs to have information about their items on hand, for proper bookkeeping. When loading a path for an external item, it needs to load these items from their host crate, since their information isn't otherwise available. This includes resolving paths for those docs. which can cause this process to recurse. Rustdoc keeps a map of external traits in a RefCell<HashMap<DefId, Trait>>, and it keeps a borrow of this active when importing an external trait. In the linked crash, this led to a RefCell borrow error, panic, and ICE.

This PR manually releases the borrow while importing the trait, and also keeps a list of traits being imported at the given moment. The latter keeps rustdoc from infinitely recursing as it tries to import the same trait repeatedly.

[rust-highfive]

r? @GuillaumeGomez

(rust_highfive has picked a reviewer for you, use r? to override)

[Manishearth]

r=me but needs a test

[Manishearth]

The test I had was

pubtraitFoo{/// [x](::Bar)fnfoo();}pubtraitBar{/// [x](::Foo)fnfoo();}

and then

pubuse aux::Foo;pubuse aux::Bar;

[Manishearth]

I have a test at https://github.com/manishearth/rust/tree/rustdoc-trait-ice , but haven't run it yet

[QuietMisdreavus]

The test i wrote up was:

/// Woah, this trait links to [OtherTrait](OtherTrait)!pubtraitSomeTrait{}/// Woah, this trait links to [SomeTrait](SomeTrait)!pubtraitOtherTrait{}

#[doc(inline)]pubuse issue_48414::{SomeTrait,OtherTrait};

...which still stack-overflows. >_> Need to investigate.

[Manishearth]

wooooooooooo

[QuietMisdreavus]

It turns out, i wrote a && when i needed to have written a ||. 😓

[Manishearth]

@bors r+

[bors]

📌 Commit 8872e7b has been approved by Manishearth