Incorrect suspicious_op_assign_impl

[kjeremy]

The following triggers this lint and probably should not
https://github.com/rust-analyzer/rust-analyzer/blob/39a9e42bf8d363e83d92bedf0ad07e95a42b1602/crates/rust-analyzer/src/semantic_tokens.rs#L68

clippy 0.0.212 (69f99e7 2019-12-14)

[kjeremy]

See rust-lang/rust-analyzer#3409 (comment)

[jpospychala]

The issue here is that AssignOp kind is ignored, but expression's top level operator is checked when triggering the lint.

Basic solution is to ignore all operators in expression if it's is part of AssignOp.

But it's also noticeable that AssignOp kind is not looked at while it should so lint doesn't detect following mistake:

impl AddAssign for Foo {
    fn add_assign(&mut self, other: Foo) {
        *self -= other;
    }
}

but it detects this one:

impl AddAssign for Foo {
    fn add_assign(&mut self, other: Foo) {
        *self = *self - other ;
    }
}