An AsyncSequence that will terminate with a `Timeout` error if iteration time exceeds the provided `timeout`.

AsyncTimeoutSequence.swift

/// An AsyncSequence that will terminate with a `Timeout` error if
/// iteration time exceeds the provided `timeout`.
struct AsyncTimeoutSequence<Base: AsyncSequence, C: Clock> {
    struct Timeout: Error {}

    let base: Base
    let timeout: C.Instant.Duration
    let clock: C

    init(_ base: Base, timeout: C.Instant.Duration, clock: C) {
        self.base = base
        self.timeout = timeout
        self.clock = clock
    }
}

extension AsyncTimeoutSequence: AsyncSequence {
    typealias Element = Base.Element

    struct AsyncIterator: AsyncIteratorProtocol {
        var base: Base.AsyncIterator
        let deadline: C.Instant
        let clock: C

        init(_ base: Base.AsyncIterator, timeout: C.Instant.Duration, clock: C) {
            self.base = base
            self.deadline = clock.now.advanced(by: timeout)
            self.clock = clock
        }

        mutating func next() async throws -> Base.Element? {
            guard clock.now < deadline else { throw Timeout() }
            return try await base.next()
        }
    }

    func makeAsyncIterator() -> AsyncIterator {
        AsyncIterator(base.makeAsyncIterator(), timeout: timeout, clock: clock)
    }
}

extension AsyncSequence {
    func timeout(
        after duration: Duration
    ) -> AsyncTimeoutSequence<Self, ContinuousClock> {
        .init(self, timeout: duration, clock: .continuous)
    }
}