support for default value for optional argument without custom setting

[sehz]

Reference: #231.

I have many use cases where to provide default value for optional argument. Reason for optional argument is that I want to provide reasonable default value but to disable if need to be. For example,

#[derive(Builder)]
pub struct MyGenerator {    
    #[builder(into, required, default = Some("default_world"))]
    default_export_world_name: Option<String>
}

[Veetaha]

Hi! I may be misinterpreting the issue, but here is how I understand this. You want to have different behavior for when the setter is not called vs when you explicitly set None for it.

So for example, this should return a struct with default_export_world_name: Some("default_world")

MyGenerator::builder().build()

vs this should havedefault_export_world_name: None:

MyGenerator::builder().default_export_world_name(None).build()

The provided code example already generates the builder API like the above. However, it also creates a second method maybe_default_export_world_name(Option<Option<String>>) just so that you could dynamically pass either None to get the default value, or Some(None) to explicitly disable the default value and assign None.

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The issue title mentions "without custom setting". I'm not sure how such behavior could be requested without an explicit attribute. Do you have any ideas of the desired syntax?