Pattern 12: Type Safety with GATs

Context

You provide fixtures or builders that return references tied to the fixture lifetime.

Problem

Without explicit lifetimes, consumers can hold onto references after cleanup, causing dangling pointers or logic bugs.

Solution

Use Generic Associated Types (GATs) to bind returned data to the fixture lifetime. In AsyncFixtureProvider, Fixture<'a> ensures the borrow cannot outlive the provider. Pair GATs with sealed traits to prevent downstream crates from violating invariants.

Forces

• Safety vs. ergonomics: GATs constrain lifetimes but keep APIs pleasant
• Extensibility vs. soundness: sealing the trait permits internal evolution while preserving invariants
• Async vs. sync: async fixtures require lifetimes that sync code cannot express without GATs

Examples

#![allow(unused)]
fn main() {
pub trait AsyncFixtureProvider: private::Sealed {
    type Fixture<'a>: Send where Self: 'a;
    type Error: std::error::Error + Send + Sync + 'static;

    fn create_fixture<'a>(&'a self) -> Pin<Box<dyn Future<Output = Result<Self::Fixture<'a>, Self::Error>> + Send + 'a>>;
}
}

Related Patterns

• Pattern 13: Sealed Traits for API Safety
• Pattern 16: Fixture Lifecycle Management
• Pattern 17: Builder-Driven Test Data