"42 implementations" CodeLens shouldn't count derived traits as implementations

[Vlad-Shcherbina]

#[derive(Debug)]
// lens says “2 implementations”
struct MyStruct { ... }

...

impl MyStruct { ... } // the only "interesting" implementation

Why it's an issue:

• The derive line is literally adjacent, so the counting and navigation aid is pointless
• When all impls are derived, the lens still takes space and distracts, while providing no value at all

A possible solution would be to exclude derived traits from the cound and from preview/navigation,
or at least show them separately: "2 implemenations | 40 derived".

[ChayimFriedman2]

I disagree. Derive is a trait implementation, and derive may not implement a trait (or implement more than one).

With #19130 it will become even more important.