"Implement missing members" doesn't handle default generic type parameters

[Vlad-Shcherbina]

To reproduce, invoke "Implement missing members" assist on the following program:

struct S;
impl PartialEq for S {}

Actual result:

impl PartialEq for S {
    fn eq(&self, other: &Rhs) -> std::primitive::bool { unimplemented!() }
}

This does not compile: "cannot find type `Rhs` in this scope".

Expected result:

impl PartialEq for S {
    fn eq(&self, other: &Self) -> std::primitive::bool { unimplemented!() }
}

Because PartialEq<Rhs=Self>.

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rust-analyzer version: 2020-04-06 (c859a64)

[matklad]

Relevant bits of code:

• https://github.com/rust-analyzer/rust-analyzer/blob/b7e5d94bda362ffc21174a79aa0be113c3288e1e/crates/ra_assists/src/ast_transform.rs#L34-L38
• https://github.com/rust-analyzer/rust-analyzer/blob/b7e5d94bda362ffc21174a79aa0be113c3288e1e/crates/ra_assists/src/handlers/add_missing_impl_members.rs