rustc suggest adding "ref" before trait in derive attribute

[Wasabi375]

Code

struct V(u32);

#[derive(Builder)]
#[builder(pattern = "owned")]
struct Foo {
    a: V,
    #[builder(default = "self.calc_b()")]
    b: V,
}

impl FooBuilder {
    fn calc_b(&self) -> V {
        V(self.a.as_ref().unwrap().0 + 5)
    }
}

Current output

error[E0382]: borrow of partially moved value: `self`
 --> src/main.rs:9:25
  |
5 | #[derive(Builder)]
  |          ------- value partially moved here
...
9 |     #[builder(default = "self.calc_b()")]
  |                         ^^^^^^^^^^^^^^^ value borrowed here after partial move
  |
  = note: partial move occurs because value has type `V`, which does not implement the `Copy` trait
help: borrow this binding in the pattern to avoid moving the value
  |
5 | #[derive(ref Builder)]
  |          +++

Desired output

error[E0382]: borrow of partially moved value: `self`
 --> src/main.rs:9:25
  |
5 | #[derive(Builder)]
  |          ------- value partially moved here
...
9 |     #[builder(default = "self.calc_b()")]
  |                         ^^^^^^^^^^^^^^^ value borrowed here after partial move
  |
  = note: partial move occurs because value has type `V`, which does not implement the `Copy` trait

Rationale and extra context

I encountered the error while using the rust-dervie-builder crate. The above code is invalid as explained in this issue here: colin-kiegel/rust-derive-builder#298
Basically the expansion of this derive macro leads to self being referenced after a partial move.
The first part of the error message is correct. The suggested fix however is just wrong.

For context, the expansion of the relevant code is the following

struct FooBuilder {
    a: ::derive_builder::export::core::option::Option<V>,
    b: ::derive_builder::export::core::option::Option<V>,
}

impl FooBuilder {
    fn build(
        self,
    ) -> ::derive_builder::export::core::result::Result<Foo, FooBuilderError> {
        Ok(Foo {
            a: match self.a {
                Some(value) => value,
                None => {
                    return ::derive_builder::export::core::result::Result::Err(
                        ::derive_builder::export::core::convert::Into::into(
                            ::derive_builder::UninitializedFieldError::from("a"),
                        ),
                    );
                }
            },
            b: match self.b {
                Some(value) => value,
                None => self.calc_b(),
            },
        })
    }
}

Other cases

No response

Rust Version

rustc 1.77.0 (aedd173a2 2024-03-17)
binary: rustc
commit-hash: aedd173a2c086e558c2b66d3743b344f977621a7
commit-date: 2024-03-17
host: x86_64-unknown-linux-gnu
release: 1.77.0
LLVM version: 17.0.6

Anything else?

No response

[jieyouxu]

Seems to be fixed in nightly rustc 1.79.0-nightly (0ad927c0c 2024-03-21):

error[E0382]: borrow of partially moved value: `self`
  --> src/main.rs:30:25
   |
26 | #[derive(Builder)]
   |          ------- value partially moved here
...
30 |     #[builder(default = "self.calc_b()")]
   |                         ^^^^^^^^^^^^^^^ value borrowed here after partial move
   |
   = note: partial move occurs because value has type `V`, which does not implement the `Copy` trait

For more information about this error, try `rustc --explain E0382`.

no longer suggests ref for me.

[Wasabi375]

I can confirm that it is also fixed for me when I switch to nightly rustc 1.79.0-nightly (85e449a32 2024-03-22) .