[6.2] AST: Add a warning group for @_implementationOnly deprecation diagnostics

include/swift/AST/DiagnosticGroups.def

@@ -48,6 +48,7 @@ GROUP(DynamicCallable, "dynamic-callable-requirements")
 GROUP(ErrorInFutureSwiftVersion, "error-in-future-swift-version")
 GROUP(ExistentialAny, "existential-any")
 GROUP(ExistentialMemberAccess, "existential-member-access-limitations")
+GROUP(ImplementationOnlyDeprecated, "implementation-only-deprecated")
 GROUP(IsolatedConformances, "isolated-conformances")
 GROUP(MemberImportVisibility, "member-import-visibility")
 GROUP(MissingModuleOnKnownPaths, "missing-module-on-known-paths")

include/swift/AST/DiagnosticsSema.def

@@ -1220,11 +1220,13 @@ ERROR(module_not_compiled_with_library_evolution,none,
       "module %0 was not compiled with library evolution support; "
       "using it means binary compatibility for %1 can't be guaranteed",
       (Identifier, Identifier))
-WARNING(implementation_only_requires_library_evolution,none,
+GROUPED_WARNING(implementation_only_requires_library_evolution,
+        ImplementationOnlyDeprecated,none,
         "using '@_implementationOnly' without enabling library evolution "
         "for %0 may lead to instability during execution",
         (Identifier))
-WARNING(implementation_only_deprecated,none,
+GROUPED_WARNING(implementation_only_deprecated,
+        ImplementationOnlyDeprecated,none,
         "'@_implementationOnly' is deprecated, use 'internal import' instead",
         ())
 

test/Sema/implementation-only-deprecated.swift

@@ -11,6 +11,12 @@
 // RUN:   -enable-library-evolution -swift-version 5 \
 // RUN:   -enable-upcoming-feature InternalImportsByDefault \
 // RUN:   -verify
+// RUN: %target-swift-frontend -typecheck %t/Client.swift -I %t \
+// RUN:   -enable-library-evolution -swift-version 5 \
+// RUN:   -enable-upcoming-feature InternalImportsByDefault \
+// RUN:   -warnings-as-errors -Wwarning ImplementationOnlyDeprecated \
+// RUN:   -verify
+
 
 // REQUIRES: swift_feature_InternalImportsByDefault
 

test/Sema/implementation-only-import-from-non-resilient.swift

@@ -9,6 +9,8 @@
 
 /// Build a client with and without library-evolution.
 // RUN: %target-swift-frontend -typecheck %t/client-non-resilient.swift -I %t -verify
+// RUN: %target-swift-frontend -typecheck %t/client-non-resilient.swift -I %t -verify \
+// RUN:   -warnings-as-errors -Wwarning ImplementationOnlyDeprecated
 // RUN: %target-swift-frontend -typecheck %t/client-resilient.swift -I %t -verify \
 // RUN:   -enable-library-evolution -swift-version 5
 

userdocs/diagnostics/implementation-only-deprecated.md

@@ -0,0 +1,23 @@
+# Deprecated implementation-only imports (ImplementationOnlyDeprecated)
+
+Warnings that identify `import` declarations with the `@_implementationOnly` attribute.
+
+## Overview
+
+When applied to `import` declarations, the compiler-internal attribute `@_implementationOnly` attempts prevents declarations from the imported module from being exposed in the ABI or public interface of the dependent module. This attribute became deprecated when support for access levels on `import` declarations was introduced with [SE-0409].
+
+One reason `@_implementationOnly import` is deprecated is that it is unsafe when used in modules that are built _without_ [library evolution] enabled. For example, suppose the following code were part of a library named `Foo`:
+
+```swift
+// Library `Foo`
+@_implementationOnly import ImplementationDetail
+
+public struct Bar {
+  internal var x: Baz // defined in ImplementationDetail
+}
+```
+
+If `Foo` is not compiled with library evolution, then the memory layout of values of `Bar` must be known at compile time in clients of `Foo`. However, the `@_implementationOnly import` of `ImplementationDetail` prevents clients from being able to look up `Baz` which is a type that contributes to the layout of `Foo`. As a result, the layout of `Foo` will be miscalculated resulting in undefined behavior.
+
+[SE-0409]: https://github.com/swiftlang/swift-evolution/blob/main/proposals/0409-access-level-on-imports.md
+[library evolution]: https://www.swift.org/blog/library-evolution/