This commit hooks rustdoc into the stability index infrastructure in two
ways:
1. It looks up stability levels via the index, rather than by manual
attributes.
2. It adds stability level information throughout rustdoc output, rather
than just at the top header. In particular, a stability color (with
mouseover text) appears next to essentially every item that appears
in rustdoc's HTML output.
Along the way, the stability index code has been lightly refactored.
Both of these items are surfaced as a DefTy, so some extra logic was needed in
the decoder module to figure out whether one is actually an enum or whether it's
a typedef.
Closes#14757
Previously, documentation for an inlined trait (i.e. a trait imported
and reexported from another crate) didn't display the trait's
supertraits.
Closes#14636
There is currently no way to query all impls for a type from an external crate,
and with primitive types in play this is also quite difficult. Instead of
filtering, just suck in all impls from upstream crates into the local AST, and
have them get stripped later.
This will allow population of all implementations of traits for primitive types,
as well as filling in some corner cases with inlining documentation in other
cases.
