rustdoc: Show methods from the whole Deref/DerefMut chain instead of only one level

[sdroege]

See for example https://gstreamer.pages.freedesktop.org/gstreamer-rs/gstreamer/message/struct.Message.html#deref-methods.

The type it derefs to also has another deref impl that would be useful to include here, see https://gstreamer.pages.freedesktop.org/gstreamer-rs/gstreamer/miniobject/struct.GstRc.html#impl-Deref .

In this case the expected result would be that the methods of both GstRc and MessageRef would be shown in the docs of Message.

CC @GuillaumeGomez @kinnison @rust-lang/rustdoc

[GuillaumeGomez]

I think that all non-direct elements should be listed with no docs and just a link to their original items' page. Just adding a title before them to say that they come from the deref type.

What do you think?

[sdroege]

Non-direct meaning that also e.g. the docs of Vec<T> wouldn't show the methods from &[T] anymore? IMHO it would make sense to show all levels. Can't become much longer than the docs of Vec<T> right now already :)

[kinnison]

One option would be to show the direct deref target as we do now, and pre-collapsed indirect deref targets all the way down.

[Nemo157]

Dup of #53038 and #26207

[jyn514]

Duplicate of #26207.