Rule to check for Swift 5-mode-incompatible API

[mattmassicotte]

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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!

[mattmassicotte]

Ok I'm trying to come up with examples. You have to catch public API that uses global actors, @Sendable closures, Sendable generic constraints, and the use of sending parameters (sending returns are not a problem).

@preconcurrency can address all of these, except for sending parameters for which there is no mechanism to preserve source compatibility.

non-triggering:

Example("public struct S: Sendable {}"),
Example("public class C: Sendable {}"),
Example("public actor A {}"),
Example("public @MyActor enum E { case a }"),
Example("private @MainActtor struct S { }"),
Example("@MainActtor struct S { }"),

triggering:

Example("↓@MainActor public struct S {}"),
Example("↓@MainActor public func globalActor()"),
Example("public func sendableClosure(_ block: ↓@Sendable () -> Void)"),
Example("public func globalActorClosure(_ block: ↓@MainActor () -> Void)"),
Example(
	"public func globalActorClosure(_ block: ↓@MyActor () -> Void)",
	configuration: ["global_actors": ["MyActor"]]
),
Example("public func sendingParameter(p: sending NonSendable)"),
Example("↓@MainActor public protocol GlobalActor {}"),
Example("public struct S { public func sendableClosure(_ block: ↓@Sendable () -> Void) }"),
Example("↓@MyActor public struct S {}", configuration: ["global_actors": ["MyActor"]]),
Example("public func generic<T> where T: ↓Sendable")

[SimplyDanny]

Thank you for the idea a the examples! What would be a good name for the new rule? Probably the hardest part ... 😅

[mattmassicotte]

I started playing around with an implementation, and I used "IncompatibleConcurrencyAnnotationRule". But I rapidly realized that I don't have enough Swift syntax knowledge.

What do you think of that one?

[SimplyDanny]

With "incompatible" meaning "not back-deployable"? But that's probably too long and a too weird combination of words. I think "IncompatibleConcurrencyAnnotationRule" is good with an explanation of what it means in the description.

But I rapidly realized that I don't have enough Swift syntax knowledge.

It's a hurdle to get used to it. But with some experience it's fun to work with it.