Clippy incorrectly complains about `borrowed_box` when trait object is boxed

[idubrov]

The following code triggers clippy::borrowed_box:

trait Whatever {}
struct Something;
impl Whatever for Something {}
fn func(flag: bool, secret: &mut Box<Whatever>) {
    *secret = Box::new(Something);
}

However, I don't think it should be complaining here since Whatever is not Sized (so I cannot overwrite it directly)

I think, this issue (or similar) was raised before: #1884 (also mentioned in #3128)

The fix for #1884 was, however, just to check for Any trait as a special case.

Playground

[flip1995]

borrowed_box doesn't trigger on the code anymore.

[MarcelCoding]

It is doing it again. I updated the example: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=f1d0ac1aa9020c047580ac51061f1af5

[flip1995]

What prevents you from just using &dyn Whatever in this example? According to the lint documentation this is exactly the case this lint was made for.

[MarcelCoding]

Whatever is a trait and at the point without a reference I have to box the trait. (not in the example, I could add it )

[xanathar]

Can you do like this_is_fine in this example?

trait Whatever {}
struct Something;
impl Whatever for Something {}

fn this_is_fine(_: &dyn Whatever) { }

fn this_is_also_fine_but_moves(_: Box<dyn Whatever>) { }

fn this_triggers_clippy(_: &Box<dyn Whatever>) { }


fn main() {
    let wt: Box<dyn Whatever> = Box::new(Something);

    this_triggers_clippy(&wt);
    this_is_fine(&*wt);
    this_is_also_fine_but_moves(wt);
}

[flip1995]

Alternatively this_is_fine(wt.as_ref()). The advantage of having it as just &dyn _ is that if you have a concrete type that you want to pass to the function, you don't have to box it. So with this_is_fine you can just call this_is_fine(&Something). But with this_triggers_clippy you have to box Something first: this_triggers_clippy(&Box::new(Something)). Playground

[MarcelCoding]

Thanks, with the help of your example I got it working. I think it would be helpful for others to include this conversation in the message provided by clippy.

[flip1995]

Hard to include all of this in the message, but we should improve the documentation a bit.

[MarcelCoding]

Ether that, I rather thought about a link to this conversation.