ActiveSupport's delegate as a Rust macro: https://play.rust-lang.org/?gist=432651935f3bb53ee5b62b5cec242fc7&version=stable&backtrace=0

playground.rs

#![allow(dead_code)]

/// Create method to access field of sub-resource
///
/// Similar to ActiveSupport's [`delegate`].
///
/// [`delegate`]: http://api.rubyonrails.org/classes/Module.html#method-i-delegate
///
/// # Limitations
///
/// - You need to specify the return type
/// - No prefixing, because `concat_idents` is [useless]
///
/// [useless]: https://github.com/rust-lang/rust/issues/29599
macro_rules! delegate {
    ($field:ident ($ty:ty) to $subresource:ident) => {
        fn $field(&self) -> &$ty {
            &self.$subresource.$field
        }
    }
}

enum Availability { Available, NotAvailable }

struct Variant {
    color: String,
    availability: Availability,
}

struct SpecificProduct {
    title: String,
    variant: Variant,
}

impl SpecificProduct {
    fn title(&self) -> &str { &self.title }
    
    delegate!(color (String) to variant);
    delegate!(availability (Availability) to variant);
}

#[test]
fn success() {
    let prod = SpecificProduct {
        title: "Thingy".into(),
        variant: Variant {
            color: "Blue".into(),
            availability: Availability::NotAvailable,
        },
    };
    
    assert_eq!(prod.color(), &"Blue".to_string());
}