Run doctest does not work in impl trait blocks

[fehrnah]

On VScode, the Run doctest does not work if the doctest is in a trait impl block.

Cargo test is called on Struct::function instead of Struct::Trait::function

[matklad]

cc @bnjjj

[bnjjj]

Ok will fix it this weekend. @fehrnah can you provide a minimal example to help me reproduce ?

[Woyten]

In fact, any documented function that is not a module-level function can't work. The fix seems straight-forward but I believe that there are some tricky cases like generics and impls of the same trait for different structs in the same module.