SE-0523: Hashable conformance for `UnownedTaskExecutor`

Summary of changes

This proposal adds Hashable conformance to UnownedTaskExecutor, enabling its use as a dictionary key and in sets. UnownedTaskExecutor already conforms to Equatable; this is a natural and source-compatible extension of that conformance.

Motivation

SE-0417 introduced task executor preferences in Swift 6 and extended withUnsafeCurrentTask to expose the unownedTaskExecutor of the currently running task. This allows performance-sensitive code to make scheduling decisions based on the executor a task is already running on.

A concrete example arises in connection pooling. A ConnectionPool may maintain connections across multiple executors. When a task requests a connection, the pool can minimize context switches by preferring a connection already associated with the requesting task's executor. With only Equatable conformance, this requires a linear scan over all known executors:

// Today: O(n) linear search
func pickConnection(preferring executor: UnownedTaskExecutor) -> Connection {
    for (e, connection) in executorConnectionList {
        if e == executor { return connection }
    }
    // fallback
    return executorConnectionList.first!.connection
}

With Hashable conformance, this becomes a constant-time dictionary lookup:

// With Hashable: O(1) dictionary lookup
func pickConnection(preferring executor: UnownedTaskExecutor) -> Connection {
    if let connection = connectionsByExecutor[executor] {
        return connection
    }
    // fallback
    return connectionsByExecutor.values.first!
}

This pattern is applicable to any system that indexes resources, caches, or scheduling metadata by executor identity.

Proposed solution

Add Hashable conformance to UnownedTaskExecutor.

Detailed design

UnownedTaskExecutor is a struct wrapping a Builtin.Executor value. It already provides Equatable conformance based on the identity of the underlying executor reference. The Hashable conformance hashes the same underlying identity value, maintaining consistency with the existing equality semantics.

extension UnownedTaskExecutor: Hashable {}

API Surface

The only public API change is the addition of the Hashable protocol conformance on UnownedTaskExecutor:

extension UnownedTaskExecutor: Hashable {}

This is purely additive and source-compatible with existing code.

Source compatibility

This proposal is purely additive. All existing code continues to compile without changes. Types that are Equatable can adopt Hashable without breaking source compatibility.

ABI compatibility

Adding a protocol conformance is an additive ABI change. This proposal does not modify existing ABI surface.

Implications on adoption

This feature requires a minimum deployment target matching the Swift standard library version in which it ships. It has no other adoption implications.

Alternatives considered

Do nothing

If we don't do anything, users are forced into using the implementation to get the Hashable conformance themselves:

extension UnownedTaskExecutor: Hashable {
  @inlinable
  public func hash(into hasher: inout Hasher) {
    let (ident, impl) = unsafeBitCast(self, to: (Int, Int).self)
    hasher.combine(ident)
    hasher.combine(impl)
  }
}