False positive for boxed local in default Trait method implementation

[MentalAllergen]

Output of cargo clippy -V: clippy 0.0.212 (3aea8603 2019-09-03)

Given the following code:

trait Foo {
    fn bar(self: Box<Self>) -> u32 {
        5
    }
}

Clippy lint boxed local is triggered, advising us to remove the Box and have plain self in the argument list, like this:

trait Foo {
    fn bar(self) -> u32 {
        5
    }
}

However, doing this makes the code fail compilation with the following error:
the size for values of type "Self" cannot be known at compilation time,
which is always the case for traits.

I've encountered this while implementing the state design pattern with dynamic dispatch. I wouldn't usually opt for dynamic dispatch, however the problem domain requires it.

Ideally this lint should check weather the implementation is a default trait method implementation before triggering.

[MentalAllergen]

Looking at the implementation, I should be able to make a fix for this. Will give it a try and report here. How would I test the changes locally?

[flip1995]

Hey @MentalAllergen, sorry for the late reply. I think every Clippy maintainer is currently pretty busy.

How the testing in Clippy works is described here: https://github.com/rust-lang/rust-clippy/blob/master/doc/adding_lints.md#testing

TL;DR: Add a test case for this in the corresponding test file in tests/ui/ and then test it with TESTNAME=boxed_local cargo +master uitest (I'm not sure if the corresponding test file is really called boxed_local.rs). You can install the master toolchain with the setup-toolchain.sh script in the project root.

[MentalAllergen]

Thanks for the explanation @flip1995!
PR is up.

I still couldn't test locally as cargo +master build failed for the master branch of clippy. It seems to be a problem with a change from rust-lang itself so until that is resolved, testing will not be possible.

[benmkw]

I think this is another/ the same case where this lint may not be accurate https://github.com/probe-rs/probe-rs/pull/437/files#file-probe-rs-src-probe-mod-rs-L459. I tried to apply the suggestion at some point but I'm pretty convinced that its not possible.