It’s a Continuation that doesn’t leak if not resumed, and if resumed multiple times, it finishes with the first value/error.

OptionalContinuation.swift

import os

final public class OptionalContinuation<T, E: Error>: Sendable {
    @usableFromInline let hasResumed = OSAllocatedUnfairLock(initialState: false)
    
    @usableFromInline let continuation: CheckedContinuation<T?, E>
    
    @inlinable public init(_ continuation: CheckedContinuation<T?, E>) {
        self.continuation = continuation
    }
    
    @inlinable public func resume(returning value: T) {
        self.hasResumed.withLock {
            guard !$0 else { return }; $0 = true
            self.continuation.resume(returning: value)
        }
    }
    
    @inlinable public func resume(throwing error: E) {
        self.hasResumed.withLock {
            guard !$0 else { return }; $0 = true
            self.continuation.resume(throwing: error)
        }
    }
    
    @inlinable deinit {
        self.hasResumed.withLock {
            guard !$0 else { return }; $0 = true
            self.continuation.resume(returning: nil)
        }
    }
}

@inlinable public func withOptionalContinuation<T>(
    isolation: isolated (any Actor)? = #isolation,
    function: String = #function, _ body: (OptionalContinuation<T, Never>) -> Void
) async -> T? {
    return await withCheckedContinuation(isolation: isolation, function: function) { continuation in
        let autoCompleter = OptionalContinuation(continuation)
        body(autoCompleter)
    }
}