In this article, I’d like to discuss a design pattern I widely use in my Rust projects — one that I feel is underrepresented in the public discourse, despite the fact that many experienced developers independently discover it as a natural solution. I call it the Configurer Trait Pattern.

This pattern is especially useful for app- and component-wide compile-time configuration, where multiple type parameters must be managed in a scalable and ergonomic way.

The problem it solves

The pattern addresses the explosion of generic parameters in public interfaces. When code is private to a crate (i.e., all usage is tightly controlled), this isn’t much of an issue. But exposing types like this:

struct SomeType<T, M, Payload: Payload, HashType, UserType>

in a public API is deeply uncomfortable to me. Every time a generic is added, removed, or changed, dependent code breaks — and migrating that code becomes a burden.

The Pattern

The idea is simple: replace the multitude of generic parameters with a single one — C: Configurer. Define the Configurer trait like this:

trait Configurer {
    type T;
    type M;
    type Payload: Payload;
    type HashType;
}

Then use this trait for a zero-sized type C, which is plugged into the public-facing type:

struct SomeType<C: Configurer> {
    t: C::T,
    m: C::M,
    payload: C::Payload,
    hash: C::HashType,
}

This drastically reduces surface complexity and decouples your public API from the intricacies of the underlying type machinery.

Downsides and Caveats

This pattern, while elegant, comes with several caveats.

1. The Need for “perfect derive”

In some cases, deriving standard traits like Debug, Clone, or Copy on types like SomeType<C> becomes non-trivial. This is where crates like derive_more or manual deriving become necessary. derive_more provides improved derive macros that understand complex trait bounds and associated types.

2. Trait Solver False-Negative Conflicts

Rust’s trait solver currently produces false-negative conflicts for patterns like the following:

struct Foo<C: Configurer>(C::Bar);

trait Configurer {
    type Bar;
}

trait Baz {}

impl<C: Configurer<Bar=i8>> Baz for Foo<C> {}

impl<C: Configurer<Bar=u8>> Baz for Foo<C> {}

This results in:

conflicting implementations of trait `Baz` for type `Foo<_>`

Yet the semantically identical version without C: Configurer compiles just fine:

struct Foo<Bar>(Bar);

trait Baz {}

impl Baz for Foo<i8> {}

impl Baz for Foo<u8> {}

A workaround is to use handle types to “break” the overlap detection:

struct Foo<C: Configurer>(C::Bar);

trait Configurer {
    type Bar;
}

trait Baz {}

struct DeriveHandle<C: Configurer<Bar=Bar>, Bar>(Foo<C>);

impl<C: Configurer<Bar=Bar>, Bar> Baz for Foo<C>
    where DeriveHandle<C, Bar>: Baz
{}

impl<C: Configurer<Bar=i8>> Baz for DeriveHandle<C, i8> {}

impl<C: Configurer<Bar=u8>> Baz for DeriveHandle<C, u8> {}

It’s clunky, but effective.

3. GAT Invariance and Lifetime Variance

Associated traits in Rust are always invariant over their parameters — which becomes problematic in certain advanced use cases.

For example, consider a library for concurrent object interning. It defines a type like this:

/// The result of internalization.
///
/// # Generic Parameters
/// * `C`: Internalization [`Configurer`].
pub struct Id<'a, C: Configurer> {
    /* Some code */
    interner_provider: C::InternerProvider<'a>,
}

/// [`Id`] configurer.
pub trait Configurer: 'static + Sized {
    /* Some code */
    type InternerProvider<'a>: InternerProvider<'a, Self>;
}

pub trait InternerProvider<'a, C: Configurer>: Copy
{
    type Interner;
    fn interner(self) -> &'a Self::Interner;
}

You may want to support two cases:

One interner per Id<C> — InternerProvider::interner returns a static reference, and Configurer::InternerProvider is a zero-sized type.
Multiple interners per Id<C> — here, Configurer::InternerProvider becomes a reference to a specific interner, and the method simply returns self.

The second case requires that Id<'a, C> be covariant over 'a. Otherwise, we will get errors on the user side, which will reduce the usability of the library to zero. For example, when the user wants to use an Id<'a, C> with an extended lifetime 'a in a context that assumes Id<'b, C>s with a shorter, but nested lifetime 'b such that 'a: 'b, the compiler will not let him do this.

This is because associated types in GAT are invariant by default, and there is currently no way to customize this.

🧠 For more information on subtyping and variance, see The Rustonomicon.

The Workaround: “Private” Generic Parameters with Derived Defaults

The solution lies in adding “private” generic parameters with default values derived from C: Configurer:

/// The result of internalization.
///
/// # Generic Parameters
/// * `'a`: [`InternerProvider`] lifetime. `Id` is covariant over it if and only
///   if the corresponding [`C::InternerProvider`](Configurer::InternerProvider)
///   is also covariant over it.
/// * `C`: Internalization [`Configurer`].
///
/// _The presence of the remaining parameters in the list is only necessary
/// to achieve covariance over them, since direct specification of GATs leads
/// to invariance. **Don't fill them by yourself, use the provided
/// defaults only.**_
///
/// * `_IP`: [`InternerProvider`] type.
pub struct Id<
    'a,
    C: Configurer,
    _IP = <C as Configurer>::InternerProvider<'a>,
> {
    /* Some code */
    interner_provider: _IP,
    /// Variance plug: it's covariant over `'a` and invariant over `C`.
    _phantom: PhantomData<(CovariantLt<'a>, Invariant<C>)>,
}

type CovariantLt<'a> = &'a ();
type Invariant<T> = fn(T) -> T;

This guarantees the variance behavior we want, without burdening end users — they never have to manually specify _IP.

Example Test

A simple test demonstrating correct covariance:

#[cfg(test)]
mod tests {
    /* Some exports */

    struct DummyConfigurer;

    impl Configurer for DummyConfigurer {
        type InternerProvider<'a> = DummyInternerProvider<'a>;
    }

    #[derive(Copy, Clone)]
    struct DummyInternerProvider;

    impl<'a, C: Configurer> InternerProvider<'a, C> for DummyInternerProvider {
        type Interner = ();

        fn interner(self) -> &'a Self::Interner {
            &()
        }
    }

    fn covariance<'short, 'long: 'short>(id: Id<'long, DummyConfigurer>) {
        let _: Id<'short, DummyConfigurer> = id;
    }
}

Conclusion

The Configurer Trait Pattern is a flexible and robust solution for managing compile-time configurations in Rust — especially in public APIs. It hides complexity, reduces breakage, and enables expressive compile-time composition. However, it does require some cleverness to work around current limitations in the Rust compiler.

If you’re building large or flexible libraries, I highly recommend considering this pattern in your design.

Configurer Trait Pattern