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I'm trying to audit some of my code to ensure it continues to work with future editions of Swift, namely when Swift Concurrency checking is tightened up. The code I have uses C APIs that are internally synchronized but have not been audited for Swift Concurrency, and I suspect will never be. Here's a very simple example for macOS:

import Darwin

Task {
	print(errno)
}

When using a Swift 5.7 toolchain (from Xcode 14) with -Xfrontend -warn-concurrency, I get a warning about this code:

<stdin>:4:8: warning: reference to var 'errno' is not concurrency-safe because it involves shared mutable state
        print(errno)
              ^
Darwin.errno:1:12: note: var declared here
public var errno: Int32 { get set }
           ^

This…makes sense, but I know errno is safe to access concurrently. But how should I tell Swift about this? I tried @preconurrency import Darwin, but doesn't seem to do anything. Actually, it just adds another warning:

<stdin>:4:8: warning: reference to var 'errno' is not concurrency-safe because it involves shared mutable state
        print(errno)
              ^
Darwin.errno:1:12: note: var declared here
public var errno: Int32 { get set }
           ^
<stdin>:1:17: remark: '@preconcurrency' attribute on module 'Darwin' is unused
@preconcurrency import Darwin
~~~~~~~~~~~~~~~~^

Is this behavior correct? I feel like this is the kind of thing @preconurrency was designed for but it seems to not work here. Am I holding it wrong?

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