Anonymous/inferred field types

language design
5

My use-case requires custom trait implementations (currently achieved via a derive macro and custom attributes). I’m not sure your structural records accomplish that. Currently I use a macro to generate code like this:

{
    #[my_attr(...)]
    #[derive(Clone, Debug, MyTrait)]
    struct AnonStruct<A: MyTrait> {
        fixed_field: u8,
        gen_field: A,
    }

    impl<A: MyTrait> AnonStruct {
        // custom impls passed into macro
    }

    AnonStruct {
        fixed_field: 0,
        gen_field: field_val,  // passed into macro by user
    }
}

Locally the struct has a name, but effectively the macro just returns an anonymous type with some trait bounds.

This approach works, except that within the impls the type of gen_field is merely A: MyTrait, which makes it more difficult to use than necessary.

(Actually, I already have support for explicit typing by the user, but as mentioned that’s not always possible due to use of closures and often not simple.)