Add impl members assist shold not copy docstrings and attrs

[matklad]

when invoking assist for impl PartialEq for Diagnostics, I get

impl PartialEq for Diagnostics {
        /// This method tests for `self` and `other` values to be equal, and is used
        /// by `==`.
        #[must_use]
        #[stable(feature = "rust1", since = "1.0.0")]
        fn eq(&self, other: &Rhs) -> bool { unimplemented!() }
        /// This method tests for `!=`.
        #[inline]
        #[must_use]
        #[stable(feature = "rust1", since = "1.0.0")]
        fn ne(&self, other: &Rhs) -> bool { !self.eq(other) }
}

There are two problems here:

• doc comments and attributes should not be copied
• default methods like ne should not be copied

Code and tests for this feature are here:

https://github.com/rust-analyzer/rust-analyzer/blob/3d7a330f320ac2ebf73df012556d64be9b120445/crates/ra_assists/src/add_missing_impl_members.rs

[DJMcNab]

We should have a new assist for adding default methods too I guess

[flodiebold]

I think adding default methods should be done via completions, i.e. if you type fn inside an impl, it should propose all missing methods including default methods, and complete the whole signature if you choose the completion.