Exclude self-by-value trait methods implemented on Box<T> from boxed_local

[dtolnay]

Simplest case:

trait Trait {
    fn f(self);
}

impl<T> Trait for Box<T> {
    fn f(self) {}
}

warning: local variable doesn't need to be boxed here, #[warn(boxed_local)] on by default
 --> src/main.rs:9:10
  |
9 |     fn f(self) {}
  |          ^^^^

More realistic case that we hit in dtolnay/syn#85:

trait LiftOnce<T, U> {
    type Output;
    fn lift<F>(self, f: F) -> Self::Output where F: FnOnce(T) -> U;
}

impl<T, U> LiftOnce<T, U> for Box<T> {
    type Output = Box<U>;
    fn lift<F>(self, f: F) -> Box<U> where F: FnOnce(T) -> U {
        Box::new(f(*self))
    }
}

I would consider this a false positive.

[sinkuu]

There is also false-positive with FnBox::call_box. Maybe all self: Box<_> should be excluded from boxed_local lint.

[mcarton]

@oli-obk E-easy? Do we have a method to check if a type implements Foo<…> for any … now? (here Box<T> for any T).

[oli-obk]

I'd have just excluded all impl trait for Box methods.

[phansch]

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