False positive for `boxed_local` with trait default methods

[dmarcuse]

When defining a trait that has a method accepting a self: Box<Self> which has a default implementation on the trait itself, the boxed_local lint is triggered. This was fixed for impls in #3321, but still triggers for traits.

Clippy version: clippy 0.0.212 (b2601be 2018-11-27)

Example Code

trait ExampleTrait {
    fn method(self: Box<Self>) { // boxed_local lint triggered here
        unimplemented!()
    }       
}

struct SomeObject;

impl ExampleTrait for SomeObject {}

#[allow(unused_variables)]
fn main() {
    let trait_object: Box<dyn ExampleTrait> = Box::new(SomeObject {});
    trait_object.method();
}

clippy output

warning: local variable doesn't need to be boxed here
 --> src/main.rs:2:15
  |
2 |     fn method(self: Box<Self>) {
  |               ^^^^
  |
  = note: #[warn(clippy::boxed_local)] on by default
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#boxed_local

method(self: Box<Self>) can't be replaced with method(self) because the Self isn't sized, but couldn't you change it to method(&self) or method(&mut self) instead?

[dmarcuse]

Not for this particular case. The method spawns a thread which takes ownership of self, so using a reference wouldn't work as the thread's reference would outlive the object.

The lint is supposed to check if the box is moved inside the body and not warn in that case. In the example below method2 will get a warning but method1 won't.

#![warn(clippy::boxed_local)]

use std::mem;

trait ExampleTrait {
    fn method1(self: Box<Self>) { // boxed_local lint is _not_ triggered here
        mem::drop(self)
    }       
    fn method2(self: Box<Self>) { // boxed_local lint triggered here
        unimplemented!();
    }       
}

fn main() {}

Playground

Can you post more of your code?

[dmarcuse]

It seems to not detect the value being moved when it's implicitly moved into a closure (e.g. to spawn a thread)

Playground

[dmarcuse]

Thank you!