[CoreInternal] Address Swift 6 warnings (1)

[ncooke3]

File: FirebaseCore/Internal/Sources/HeartbeatLogging/HeartbeatController.swift

public final class HeartbeatController {
  /// ...
  static let dateStandardizer: (Date) -> (Date) = {
    var calendar = Calendar(identifier: .iso8601)
    calendar.locale = Locale(identifier: "en_US_POSIX")
    calendar.timeZone = TimeZone(secondsFromGMT: 0)!
    return calendar.startOfDay(for:)
  }()
 /// ...
}

Gave the following error:

error: static property 'dateStandardizer' is not concurrency-safe because non-'Sendable' type '(Date) -> (Date)' may have shared mutable state
static let dateStandardizer: (Date) -> (Date) = {
^

I marked the closure @Sendable (because there should be no mutable state due to this static property), which silenced the Swift 6 build but the Swift 5 build issues a warning for this:

warning: converting non-sendable function value to '@Sendable (Date) -> (Date)' may introduce data races
return calendar.startOfDay(for:)
^

I can't find the link now but I found at least one Swift forums post about Swift 5 warning about things that are non-warnings in Swift 6. I think this explained the difference here as to why adding @Sendable worked in Swift 6 vs. not in Swift 5.

So, I moved the static property out into an enum, and more importantly, removed the closure in favor of a dedicated method:

  /// Used for standardizing dates for calendar-day comparison.
  private enum DateStandardizer {
    private static let calendar: Calendar = {
      var calendar = Calendar(identifier: .iso8601)
      calendar.locale = Locale(identifier: "en_US_POSIX")
      calendar.timeZone = TimeZone(secondsFromGMT: 0)!
      return calendar
    }()

    static func standardize(_ date: Date) -> (Date) {
      return calendar.startOfDay(for: date)
    }
  }

---

File: FirebaseCore/Internal/Sources/HeartbeatLogging/HeartbeatsPayload.swift

See #13454 (comment) for discussion on final approach.

/// Convenience instance that represents an empty payload.
  static let emptyPayload = HeartbeatsPayload()

Gave the following error:

error: static property 'emptyPayload' is not concurrency-safe because non-'Sendable' type 'HeartbeatsPayload' may have shared mutable state
static let emptyPayload = HeartbeatsPayload()
^

Public structs are not implicitly Sendable, so Sendable conformance needed to be explicitly added.

#no-changelog