box-default wrongly suggest on dyn trait

[flisky]

Summary

I have a function signature like fn with_bar(self, bar: Box<dyn Bar>), and box-default will suggest Foo::new().with_bar(Box::default()) over Foo::new().with_bar(Box::new(BarImpl::default)), which clearly cannot be compiled.

Lint Name

box-default

Reproducer

No response

Version

rustc 1.66.0-nightly (8796e7a9c 2022-10-08)
binary: rustc
commit-hash: 8796e7a9cfd4c5c4f1de15ec1c53994ddf288665
commit-date: 2022-10-08
host: aarch64-apple-darwin
release: 1.66.0-nightly
LLVM version: 15.0.2

Additional Labels

No response

[llogiq]

@rustbot claim

I think it should still work with Box::<BarImpl>::default(). Can you check that it does?

[Vlad-Shcherbina]

Box::<T>::default() works, but Clippy suggests Box::default(), not Box::<T>::default(). The suggestion is incorrect.

Consider this artifical example:

trait MyTrait {}

#[derive(Default)]
struct MyStruct;

impl MyTrait for MyStruct {}

fn take_box_dyn(_: Box<dyn MyTrait>) {}

fn main() {
    let _: Box<dyn MyTrait> = Box::new(MyStruct::default());
    take_box_dyn(Box::new(MyStruct::default()));
}

Clippy says this:

warning: `Box::new(_)` of default value
  --> src\main.rs:11:31
   |
11 |     let _: Box<dyn MyTrait> = Box::new(MyStruct::default());
   |                               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `Box::<MyStruct>::default()`
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#box_default
   = note: `#[warn(clippy::box_default)]` on by default

warning: `Box::new(_)` of default value
  --> src\main.rs:12:18
   |
12 |     take_box_dyn(Box::new(MyStruct::default()));
   |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `Box::default()`
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#box_default

So, for a variable of type Box<dyn Trait> it correctly suggests Box::<T>::default(), but for a function expecting Box<dyn Trait> it incorrectly and inconsistently suggests Box::default().

> cargo clippy --version
clippy 0.1.68 (0468a00a 2022-12-17)

[rekby]

I have same problem: box to dyn trait and clippy suggest variant with Box::default(), which can't compiled.

[Eugeny]

Same on the latest nightly - this issue is only partially fixed.

[rwthompsonii]

I'm having an issue with a crate that provides a builder-pattern "default" function and clippy is complaining about it, even though the object does not (and cannot) implement Default. I'm silencing the warning for now.

[llogiq]

@rwthompsonii can you provide code?

[rwthompsonii]

It's closed source, I'll try and get an unencumbered example tomorrow.