Existential Types in Rust

a year ago

Rust has been working on adding explicit existential types since 2017, which are types that exist but cannot be directly manipulated outside their scope.
Existential types invert the control of generic types, letting the function decide the concrete type while the caller treats it as unknown.
Rust currently has a limited form of existential types called `impl Trait`, which allows functions to return types implementing a trait without specifying the concrete type.
`impl Trait` is useful for abstracting API details without runtime indirection, especially in asynchronous functions and closures.
Rust 1.75 (December 2023) allowed `impl Trait` as return values from trait methods, but they still have limitations compared to full existential types.
Future improvements aim to allow existential types in type aliases and structures, enabling patterns like guaranteed same-type returns across functions.
Existential types would improve ergonomics in asynchronous programming, a focus for Rust 2024, by allowing `impl Future` values to be stored in structures.

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