[Proposal] ~Sendable Conformance for Suppressing Sendable Inference

This proposal introduces `~Sendable` conformance syntax to explicitly
suppress a conformance to Sendable, which would prevent automatic
`Sendable` inference on types, and provide an alternative way to mark
types as non-`Sendable` without inheritance impact.

Author: xedin

proposals/NNNN-tilde-sendable.md

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+# `~Sendable` Conformance for Suppressing Sendable Inference
+
+* **Proposal**: [SE-NNNN](NNNN-tilde-sendable.md)
+* **Authors**: [Pavel Yaskevich](https://github.com/xedin)
+* **Review Manager**: TBD
+* **Status**: **Pitch**
+* **Implementation**: [implementation](https://github.com/swiftlang/swift/pull/84777), [Interaction with ObjC](https://github.com/swiftlang/swift/pull/85105)
+* **Experimental Feature Flag**: `TildeSendable`
+* **Review**: [pitch](https://forums.swift.org/t/pitch-sendable-conformance-for-suppressing-sendable-inference/83288)
+
+## Introduction
+
+This proposal introduces `~Sendable` conformance syntax to explicitly suppress a conformance to `Sendable`, which would prevent automatic `Sendable` inference on types, and provide an alternative way to mark types as non-Sendable without inheritance impact.
+
+
+## Motivation
+
+When encountering a public type that doesn't explicitly conform to `Sendable`, it's difficult to determine the intent. It can be unclear whether the type should have an explicit `Sendable` conformance that hasn't been added yet, or whether it's deliberately non-`Sendable`. Making this determination requires understanding how the type's storage is structured and whether access to shared state is protected by a synchronization mechanism - implementation details which may not be accessible from outside the library.
+
+There are also situations when a class is not `Sendable` but some of its subclasses are. There is currently a way to expression that a type does not conform to a `Sendable` protocol:
+
+
+```swift
+class Base {
+   // ...
+}
+```
+
+
+
+```swift
+@available(*, unavailable)
+extension Base: Sendable {
+}
+```
+
+
+Like all other conformances, an unavailable conformance to `Sendable` is inherited by subclasses. An unavailable conformance means that the type never conforms to `Sendable`, including all subclasses. Attempting to declare a thread-safe subclass `ThreadSafe`:
+Attempting to declare a thread-safe subclass `ThreadSafe`:
+
+
+```swift
+final class ThreadSafe: Base, @unchecked Sendable {
+   // ...
+}
+```
+
+
+is not possible and results in the following compiler warning:
+
+
+```
+warning: conformance of 'ThreadSafe' to protocol 'Sendable' is already unavailable
+```
+
+
+because unavailable conformance to `Sendable` is inherited by the subclasses.
+
+This third state of a class not having a conformance to `Sendable` because subclasses may or may not conform to `Sendable` is not explicitly expressible in the language. Having an explicit spelling is important for library authors doing a comprehensive `Sendable` audit of their public API surface, and for communicating to clients that the lack of `Sendable` conformance is deliberate, while preserving the ability to add `@unchecked Sendable` conformances in subclasses.
+
+
+## Proposed Solution
+
+Introduce `~Sendable` conformance syntax that explicitly suppresses `Sendable`:
+
+```swift
+// This type will never be inferred as Sendable before though it could be inferred as such.
+struct MyType: ~Sendable {
+    let value: Int
+}
+```
+
+This syntax is only applicable to types because other declarations like generic parameters are already effectively `~Sendable` by default until they have an explicit `Sendable` requirement.
+
+
+## Detailed Design
+
+The `~Sendable` conformance uses the tilde (`~`) prefix to indicate suppression similar to `~Copyable`, `~Escapable`, and `~BitwiseCopyable`:
+
+```swift
+// Suppress Sendable inference
+struct NotSendableType: ~Sendable {
+    let data: String
+}
+
+// Can be combined with other conformances
+struct MyType: Equatable, ~Sendable {
+    let id: UUID
+}
+
+// Works with classes
+class MyClass: ~Sendable {
+    private let data = 0
+}
+```
+
+Just like with unavailable extensions, types with `~Sendable` conformances cannot satisfy `Sendable` requirements:
+
+```swift
+func processData<T: Sendable>(_ data: T) { }
+
+struct NotSendable: ~Sendable {
+    let value: Int
+}
+
+processData(NotSendable(value: 42)) // error: type 'NotSendable' does not conform to the 'Sendable' protocol
+```
+
+
+But, unlike unavailable extensions, `~Sendable` conformances do not affect subclasses:
+
+```swift
+class A: ~Sendable {
+}
+
+final class B: A, @unchecked Sendable {
+}
+
+func takesSendable<T: Sendable>(_: T) {
+}
+
+takesSendable(B()) // Ok!
+```
+
+
+Attempting to use `~Sendable` as a generic requirement results in a compile-time error:
+
+```swift
+func test<T: ~Sendable>(_: T) {} // error: conformance to 'Sendable' can only be suppressed on structs, classes, and enums
+```
+
+
+Attempting to explicitly conform (both conditionally and unconditionally) to both `Sendable` and `~Sendable` results in a compile-time error:
+
+```swift
+struct Container<T>: ~Sendable {
+    let value: T
+}
+
+extension Container: Sendable {} // error: cannot both conform to and suppress conformance to 'Sendable'
+extension Container: Sendable where T: Sendable {} // error: cannot both conform to and suppress conformance to 'Sendable'
+```
+
+The Swift compiler provides a way to audit Sendability of public types. The current way to do this is by enabling the `-require-explicit-sendable` flag to produce a warning for every public type without explicit `Sendable` conformance (or an unavailable extension). This flag now supports `~Sendable` and has been turned into a diagnostic group that is disabled by default - `ExplicitSendable`, and can be enabled by `-Wwarning ExplicitSendable`.
+
+## Source Compatibility
+
+This proposal is purely additive and maintains full source compatibility with existing code:
+
+* Existing code continues to work unchanged
+* No existing `Sendable` inference behavior is modified
+* Only adds new opt-in functionality
+
+## Effect on ABI Stability
+
+`~Sendable` conformance is a compile-time feature and has no ABI impact:
+
+* No runtime representation
+* No effect on existing compiled code
+
+## Effect on API Resilience
+
+The `~Sendable` annotation affects API contracts:
+
+* **Public API**: Adding `~Sendable` to a public type does not impact source compatibility because `Sendable` inference does not apply to public types. Changing a `Sendable` conformance to `~Sendable` is a source breaking change.
+
+## Alternatives Considered
+
+### `@nonSendable` Attribute
+
+```swift
+@nonSendable
+struct MyType {
+    let value: Int
+}
+```
+
+Protocol conformance is more ergonomic considering the inverse case, and it follows the existing convention of conformance suppression to other marker protocols.
+
+
+## Acknowledgements
+
+Thank you to [Holly Borla](https://github.com/hborla) for the discussion and editorial help.