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Feature or Enhancement Proposal

There are a number of ways to break compatibility with Swift 5 mode (strict concurrency off) that are difficult to notice. Here are some examples:

@MainActor
public class MainThreadOnly {}

public func takeBlock(_ block: @Sendable () - Void)
public func takeGlobalBlock(_ block: @MainActor () - Void)

In all of these cases, to preserve compatibility you must use @preconcurrency.

@preconcurrency @MainActor
public class MainThreadOnly {}

@preconcurrency 
public func takeBlock(_ block: @Sendable () - Void)

@preconcurrency 
public func takeGlobalBlock(_ block: @MainActor () - Void)

I think this could be a great new rule!