Is there a way to declare builder only fields?

[inflation]

I happen to encounter this case, some attributes are used in the building function, but not needed in the final struct. It also needs to be a reference and I'd like not to include lifetime in the final struct.

[Veetaha]

Hi, if you'd like to have custom fields only during the building process, then consider using a function-based builder instead of derive(Builder):

struct Example {
     x: u32,
     y: u32,
}

#[bon::bon]
impl Example {
    #[builder]
    fn new(
        x: u32,
        y: u32,
        custom_builder_only_field: &u32,
    ) -> Self {
        // ... use the `custom_builder_only_field` here to build the struct
        
        // Return the final struct
        Self { x, y }
    }
}

Example::builder()
    .x(1)
    .y(2)
    .custom_builder_only_field(&3)
    .build();

Or I may have misunderstood your use case. Could you share some specific example of code to better demonstrate the problem?

[Veetaha]

If you want a #[builder(field)] of a reference type, then you should instead use a #[builder(start_fn)] and initialize that reference when constructing the builder. See the section about native fields for start_fn members here