Unnecessary nonisolated(unsafe) produces no warning

[mattmassicotte]

Motivation

During the process of migrating a system towards supporting concurrency, you might need to use nonisolated(unsafe) from SE-0412.

public final class ActuallySendableClass {
	public init() {
	}
}

struct NonIsolatedContainer {
	nonisolated(unsafe) static let a = ActuallySendableClass()
}

But, later on, the class might be updated to conform to Sendable. Today, no diagnostics are produced.

public final class ActuallySendableClass: Sendable {
	public init() {
	}
}

struct NonIsolatedContainer {
	// now unnecessary
	nonisolated(unsafe) static let a = ActuallySendableClass()
}

Proposed solution

I think a warning would be appropriate here now that nonisolated(unsafe) is no longer required.

Alternatives considered

No response

Additional information

No response

[hborla]

I think this is a good idea. There are cases where nonisolated is still necessary (e.g. for a public actor property), but in those cases we can still warn about removing the (unsafe) part.

[hborla]

@simanerush This is a good concurrency starter bug! Interested in taking a look?