diagnose-api-breaking-changes treats nonisolated as an API breaking change

[glbrntt]

Is it reproducible with SwiftPM command-line tools: swift build, swift test, swift package etc?

• Confirmed reproduction steps with SwiftPM CLI. The description text must include reproduction steps with either of command-line SwiftPM commands, swift build, swift test, swift package etc.

Description

Adding the nonisolated keyword to a public struct in a module with default module isolation (i.e. non-isolated) is reported as an API breaking change by swift package diagnose-api-breaking-changes.

Under default module isolation, a plain struct is already nonisolated, so adding the keyword is a no-op and shouldn't be an issue.

Expected behavior

diagnose-api-breaking-changes shouldn't report issues.

Actual behavior

diagnose-api-breaking-changes reports that the API has been broken.

Steps to reproduce

nonisolated-struct-repro.zip

1. Checkout attached Swift project
2. Run swift package diagnose-api-breaking-changes baseline

3 breaking changes detected in Repro:
  💔 API breakage: struct S is now with nonisolated
  💔 API breakage: constructor S.init() is now with nonisolated
  💔 API breakage: func S.foo() is now with nonisolated

Swift Package Manager version/commit hash

No response

Swift & OS version (output of swift --version ; uname -a)

No response