[Sema] Set env var to report public import of private modules as an error

The env var ENABLE_PUBLIC_IMPORT_OF_PRIVATE_AS_ERROR must be set for
public imports of private module to be reported as an error, otherwise
it's still a warning.

Author: xymus

lib/Sema/TypeCheckDeclPrimary.cpp

@@ -1715,6 +1715,10 @@ class DeclChecker : public DeclVisitor<DeclChecker> {
            inFlight.fixItInsert(ID->getStartLoc(),
                               "@_implementationOnly ");
         }
+
+        static bool treatAsError = getenv("ENABLE_PUBLIC_IMPORT_OF_PRIVATE_AS_ERROR");
+        if (!treatAsError)
+          inFlight.limitBehavior(DiagnosticBehavior::Warning);
       }
     }
   }

test/Sema/implementation-only-import-suggestion-as-error.swift

@@ -0,0 +1,28 @@
+/// Same test as implementation-only-import-suggestion upgrading warnings to
+/// errors. We can remove this test when this becomes the default behavior.
+
+// RUN: %empty-directory(%t)
+// REQUIRES: VENDOR=apple
+
+/// Prepare the SDK.
+// RUN: cp -r %S/Inputs/public-private-sdk %t/sdk
+// RUN: %target-swift-frontend -emit-module -module-name PublicSwift \
+// RUN:   %t/sdk/System/Library/Frameworks/PublicSwift.framework/Modules/PublicSwift.swiftmodule/source.swift \
+// RUN:   -o %t/sdk/System/Library/Frameworks/PublicSwift.framework/Modules/PublicSwift.swiftmodule/%target-swiftmodule-name
+// RUN: %target-swift-frontend -emit-module -module-name PrivateSwift \
+// RUN:   %t/sdk/System/Library/PrivateFrameworks/PrivateSwift.framework/Modules/PrivateSwift.swiftmodule/source.swift \
+// RUN:   -o %t/sdk/System/Library/PrivateFrameworks/PrivateSwift.framework/Modules/PrivateSwift.swiftmodule/%target-swiftmodule-name
+
+/// Expect errors when building a public client.
+// RUN: env ENABLE_PUBLIC_IMPORT_OF_PRIVATE_AS_ERROR=1 \
+// RUN:    %target-swift-frontend -typecheck -sdk %t/sdk -module-cache-path %t %s \
+// RUN:   -F %t/sdk/System/Library/PrivateFrameworks/ \
+// RUN:   -library-level api -verify
+
+import PublicSwift
+import PrivateSwift // expected-error{{private module 'PrivateSwift' is imported publicly from the public module 'main'}}
+
+import PublicClang
+import PublicClang_Private // expected-error{{private module 'PublicClang_Private' is imported publicly from the public module 'main'}}
+import FullyPrivateClang // expected-error{{private module 'FullyPrivateClang' is imported publicly from the public module 'main'}}
+import main // expected-warning{{'implementation-only-import-suggestion-as-error.swift' is part of module 'main'; ignoring import}}

test/Sema/implementation-only-import-suggestion.swift

@@ -34,11 +34,11 @@
 // RUN:   -library-level other -D PUBLIC_IMPORTS
 #if PUBLIC_IMPORTS
 import PublicSwift
-import PrivateSwift // expected-error{{private module 'PrivateSwift' is imported publicly from the public module 'main'}}
+import PrivateSwift // expected-warning{{private module 'PrivateSwift' is imported publicly from the public module 'main'}}
 
 import PublicClang
-import PublicClang_Private // expected-error{{private module 'PublicClang_Private' is imported publicly from the public module 'main'}}
-import FullyPrivateClang // expected-error{{private module 'FullyPrivateClang' is imported publicly from the public module 'main'}}
+import PublicClang_Private // expected-warning{{private module 'PublicClang_Private' is imported publicly from the public module 'main'}}
+import FullyPrivateClang // expected-warning{{private module 'FullyPrivateClang' is imported publicly from the public module 'main'}}
 import main // expected-warning{{'implementation-only-import-suggestion.swift' is part of module 'main'; ignoring import}}
 
 /// Expect no warnings with implementation-only imports.