Missing imports warn 'this is an error in Swift 6'

[drewster99]

Short description of the issue:

Release builds of RxSwift (and other targets) emit many warnings that various references to Foundation types, such as Foundation.Date and Dispatch.DispatchTimeInterval, cannot be used because their definition was never imported, and that this warning will be a hard error in Swift 6.

Example from Timeout.swift:

'RxTimeInterval' aliases 'Dispatch.DispatchTimeInterval' and cannot be used here because 'Dispatch' was not imported by this file; this is an error in Swift 6

Example in screenshot:

Expected outcome:

Warnings against using types which have not been imported by the current file should not appear during Xcode archive build.

What actually happens:

Such warnings do appear.

Self contained code example that reproduces the issue:

The easiest way to reproduce is to:

1. Clone the repo
2. Open Xcode
3. Open Rx.xcworkspace
4. Select the RxSwift scheme
5. Select an iOS simulator
6. Choose Product -> Build For -> Profiling -- or hit Command-Shift-I (that's "eye" not "el")

You'll see a bunch of warnings like the ones described above.

RxSwift/RxCocoa/RxBlocking/RxTest version/commit

Confirmed with 6.6.0 tag as well as the current tip of main branch, which is 214f7ff.

Platform/Environment

• [ X] iOS
• macOS
• tvOS
• watchOS
• playgrounds

How easy is to reproduce? (chances of successful reproduce after running the self contained code)

• [ X] easy, 100% repro
• sometimes, 10%-100%
• hard, 2% - 10%
• extremely hard, %0 - 2%

Xcode version:

15.0.1

Installation method:

• CocoaPods
• Carthage
• [X ] Git submodules

I have multiple versions of Xcode installed:
(so we can know if this is a potential cause of your issue)

• yes (which ones)
• [ X] no

Level of RxSwift knowledge:
(this is so we can understand your level of knowledge
and formulate the response in an appropriate manner)

• just starting
• I have a small code base
• [X ] I have a significant code base

I'm building under macOS Sonoma 14.1.1 on a 2021 M1 Max macbook pro, with Xcode 15.0.1.

These warnings can be resolved by adding "import Foundation" (or similar) to most of the files in question.