False positive on boxed_local for Box<Self> parameters

[leoyvens]

Some traits such as FnBox define methods that take Box<Self>. Implementing such a method will incorrectly trigger the boxed_local lint.

[llogiq]

See also #916

[0ndorio]

I think I hit this old issue with the following playground example [1]. I know that self: Box<Self> as function parameter is a bit odd, but the trait implementation can't change much about the trait definition.

[1] https://play.rust-lang.org/?gist=5720b0cda84efeb89752c7491acd40ab&version=stable&mode=debug

[phansch]

Fixed via #3321 (feel free to re-open if it's still a problem)