rustdoc search: find types containing some other type

[lolbinarycat]

when searching through crates with many complicated and nested types, it is often useful to know how you can get an instance of some type. this is partially covered by the -> SomeType syntax, but in codebases where you are expected to directly access fields instead of using getter methods, this is insufficient.

there should be a syntax to search for all types that use a given type in a field (not necessarily a struct, it could also be a union or enum).

tangentially related: #118912

[GuillaumeGomez]

We could update structfield to handle other cases as well.

[lolbinarycat]

it could probably use similar logic, but maybe we should have a different prefix, like has:

[notriddle]

Personally, I would just treat (visible) fields as functions with one parameter and one return value. It's how fields work in Hoogle, and for good reason, since the "I have a TyCtxt and want an hir::Map" is a use case that hardly cares if it goes through a method, a free function, or a struct field.

[lolbinarycat]

well, in haskell, field accessors are free functions. but yeah that's probably a good idea.

[lolbinarycat]

if we're treating fields as unary functions, we should probably also treat constants and statics as nullary functions.