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

[ghost]

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.

[ghost]

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!