[Concurrency] Fix preconcurrency downgrade behavior for Sendable closures and generic requirements.

[hborla]

Sendable violations inside @preconcurrency @Sendable closures should be suppressed in minimal checking, and diagnosed as warnings under complete checking, including the Swift 6 language mode. This change fixes an issue where warnings were still produced in minimal checking, and errors were still produced under -swift-version 6, which means that libraries cannot stage in new Sendable annotations if their clients have already migrated to Swift 6.

The preconcurrency downgrade infrastructure is currently pretty difficult to follow, because the information is passed through SendableCheckContext throughout the diagnoseNonSendableTypes APIs, and there are different code paths for @preconcurrency on nominal declarations versus import statements. This PR does not attempt to clean any of this code up, because I'd like to cherry pick the bug fix to release/6.0. I'll look at clarifying this code in a follow up change.

This change also downgrades Sendable requirement failures when the requirement is on a @preconcurrency declaration. However, the change in this PR is not sufficient for requirement inference, e.g.

// module A
@preconcurrency
struct RequireSendable<T: Sendable> {}

// module B

class NotSendable {}

func f<T, U: Sendable>(_: T, _: U) -> RequireSendable<T>? { nil }

func call(ns: NotSendable) {
  f(ns, ns)
}

A case like the above requires more careful handling. @preconcurrency can't simply be inferred on f, because that would break ABI by stripping U: Sendable from the generic signature. @preconcurrency may need to apply per requirement in order to maintain source compatibility without breaking ABI.

Resolves: rdar://138535438, rdar://139234188, #76652

[hborla]

@swift-ci please smoke test

[hborla]

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[hborla]

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[hborla]

@swift-ci please test source compatibility

[DougGregor]

This all looks safe, and will downgrade a number of errors to warnings based on @preconcurrency. Thank you!

[DougGregor]

This is why isolatedByPreconcurrency is a closure that's passed in, right?

[hborla]

Yes, and it looks like ClosureIsolatedByPreconcurrency (used by constraint system calls) does check the constraint system state, but I ran into a case while debugging where this line of code returned an unexpected result mid constraint solving. It didn't seem to be causing any issues for the preconcurrency downgrade behavior, so I left this comment for myself to figure out what happened there after I land this.

[Lukasa]

@swift-ci build toolchain

[Lukasa]

@swift-ci build toolchain macOS platform

[Lukasa]

Hmm, this toolchain did not appear to work. Building the current main of Hummingbird using the current main of swift-nio (by putting swift-nio in edit mode) produces the same errors using the downloaded Ubuntu 20.04 toolchain as we see with the current 6.0 toolchain:

/hummingbird/Sources/HummingbirdTLS/TLSChannel.swift:40:44: error: conformance of 'NIOSSLHandler' to 'Sendable' is unavailable
38 |     @inlinable
39 |     public func setup(channel: Channel, logger: Logger) -> EventLoopFuture<Value> {
40 |         return channel.pipeline.addHandler(NIOSSLServerHandler(context: self.sslContext)).flatMap {
   |                                            `- error: conformance of 'NIOSSLHandler' to 'Sendable' is unavailable
41 |             self.baseChannel.setup(channel: channel, logger: logger)
42 |         }

/hummingbird/.build/checkouts/swift-nio-ssl/Sources/NIOSSL/NIOSSLHandler.swift:791:1: note: conformance of 'NIOSSLHandler' to 'Sendable' has been explicitly marked unavailable here
 789 | 
 790 | @available(*, unavailable)
 791 | extension NIOSSLHandler: Sendable {}
     | `- note: conformance of 'NIOSSLHandler' to 'Sendable' has been explicitly marked unavailable here
 792 | 
 793 | extension NIOSSLHandler {
/hummingbird/Sources/HummingbirdTLS/TLSChannel.swift:40:44: error: conformance of 'NIOSSLHandler' to 'Sendable' is unavailable
38 |     @inlinable
39 |     public func setup(channel: Channel, logger: Logger) -> EventLoopFuture<Value> {
40 |         return channel.pipeline.addHandler(NIOSSLServerHandler(context: self.sslContext)).flatMap {
   |                                            `- error: conformance of 'NIOSSLHandler' to 'Sendable' is unavailable
41 |             self.baseChannel.setup(channel: channel, logger: logger)
42 |         }

/hummingbird/.build/checkouts/swift-nio-ssl/Sources/NIOSSL/NIOSSLHandler.swift:791:1: note: conformance of 'NIOSSLHandler' to 'Sendable' has been explicitly marked unavailable here
 789 | 
 790 | @available(*, unavailable)
 791 | extension NIOSSLHandler: Sendable {}
     | `- note: conformance of 'NIOSSLHandler' to 'Sendable' has been explicitly marked unavailable here
 792 | 
 793 | extension NIOSSLHandler {

This is the addHandler method in question, with the @preconcurrency annotation on it.

[hborla]

@swift-ci build toolchain

[hborla]

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[hborla]

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[hborla]

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