Using default generic parameter types for skippable fields

[srithon]

In a project I'm working on, I have an immutable buildable struct that contains an optional generic field. When this field is not set, the compiler throws an error because it cannot infer its type.

However, since the struct is immutable, this field would stay None forever, meaning that its corresponding type would not be relevant. I attempted to fix this by giving it a Default Generic Type Parameter with a throwaway type, but it did not work as I expected.

This is a minimal example of the issue.

I set the placeholder type to Vec<String> because in my project the type of B has bounds which prevent it from being ().

use derive_builder::Builder;

#[derive(Builder)]
struct GenericStruct<A, B = Vec<String>> {
    required_field: A,

    #[builder(setter(strip_option), default)]
    optional_field: Option<B>
}

fn main() {
    let a = GenericStructBuilder::default()
        .optional_field(30)
        .required_field("hello")
        .build();

    // **Error**
    let b = GenericStructBuilder::default()
        .required_field("hello")
        .build();
}

error[E0282]: type annotations needed for `std::result::Result<GenericStruct<&str, B>, String>`
  --> src/main.rs:18:13
   |
18 |     let b = GenericStructBuilder::default()
   |         -   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ cannot infer type for type parameter `B` declared on the struct `GenericStructBuilder`
   |         |
   |         consider giving `b` the explicit type `std::result::Result<GenericStruct<&str, B>, String>`, where the type parameter `B` is specified

Potential User Workarounds

1. Remove strip_option and default, and make users explicitly pass in Option::<()>::None to the setter
2. Something with Box?

Potential Crate Solutions

1. Internally use workaround Add MIT/Apache2 License, Readme #1 before build() to set the type

Other Notes

• I tried plugging in the default_type_parameter_fallback feature gate, but could not get it to work
• I also tried specifying default = "Option::<Vec<String>>::None", but that didn't work

Resources

• Manual Builder Solution (without derive_builder): stackoverflow post asking about a similar situation
• 0213-defaulted-type-params RFC
• Type inference fails in light of generic type aliases with default parameter. #50822: GitHub issue for Rust
• Default generic type inference: reddit post

[TedDriggs]

I'm not sure there's anything this crate can do, I'm afraid.