rustdoc should maybe only consider trait definitions and not trait impls for the search

[sdroege]

See for example https://doc.rust-lang.org/std/?search=fmt . This shows the Debug::fmt impl of each of these types, which can easily get quite noisy and make it hard to spot that actually it's the Debug::fmt somewhere at the top that is the origin of all the results.

In addition, rustdoc only lists the Debug::fmt impls of each of the types but not the Display::fmt impl, which is also quite inconsistent.