# Pitch: Back-deployment for fixed-layout value types - #3571

**URL:** <https://forums.swift.org/t/pitch-back-deployment-for-fixed-layout-value-types-3571/90446>\
**Category:** Pitches\
**Created:** [November 4, 2026, 8:05am UTC](https://forums.swift.org/t/pitch-back-deployment-for-fixed-layout-value-types-3571/90446 "2026-11-04T08:05:38Z")\
**Posts on this page:** 1\
**Page:** 1

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**Author:** ![Douglas\_Gregor](https://sea2.discourse-cdn.com/swift/user_avatar/forums.swift.org/douglas_gregor/32/1362_2.png) [@Douglas\_Gregor](https://forums.swift.org/u/Douglas_Gregor)\
**Post date:** [November 4, 2026, 8:05am UTC](https://forums.swift.org/t/pitch-back-deployment-for-fixed-layout-value-types-3571/90446/1 "2026-11-04T08:05:38Z")

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Hi all,

@tshortli and I found a way to do back-deployment for a limited class of types, specifically, `@frozen` `struct` and `enum` types with a fixed layout. The [proposal here](https://github.com/swiftlang/swift-evolution/pull/3571) is to extend the existing `@backDeployed(before:)` attribute to cover these types. Then, we can adopt it for a number of core types in the standard library:

- Span family:

- Ref family

- Cell family (still under review)

- `UInt128` and `Int128`

- Unique (non-copyable) types

- Synchronization

- Concurrency

There is one limitation on retroactive protocol conformances to resilient protocols, which is outlined in the proposal itself.

Doug