[Serialization] Debug print decls being read

These prints can be enabled in an assert compiler with `-Xllvm
-debug-only=Serialization`.

Author: xymus

lib/Serialization/Deserialization.cpp

@@ -4750,6 +4750,14 @@ ModuleFile::getDeclChecked(
       IDC->setDeclID(DID);
     }
   }
+
+  LLVM_DEBUG(
+    if (auto *VD = dyn_cast_or_null<ValueDecl>(declOrOffset.get())) {
+      llvm::dbgs() << "Deserialized: '";
+      llvm::dbgs() << VD->getName();
+      llvm::dbgs() << "'\n";
+    });
+
   return declOrOffset;
 }
 

test/Serialization/Safety/skip-internal-details.swift

@@ -0,0 +1,70 @@
+// RUN: %empty-directory(%t)
+// RUN: split-file %s %t
+
+// REQUIRES: asserts
+
+// RUN: %target-swift-frontend -emit-module %t/HiddenLib.swift \
+// RUN:   -enable-library-evolution \
+// RUN:   -emit-module-path %t/HiddenLib.swiftmodule
+
+// RUN: %target-swift-frontend -emit-module %t/Lib.swift -I %t \
+// RUN:   -enable-library-evolution \
+// RUN:   -emit-module-path %t/Lib.swiftmodule \
+// RUN:   -emit-module-interface-path %t/Lib.swiftinterface
+
+// RUN: %target-swift-frontend -typecheck %t/Client.swift -I %t \
+// RUN:   -verify -Xllvm -debug-only=Serialization 2>&1 \
+// RUN:   | %FileCheck --check-prefixes=NEEDED,UNSAFE %s
+
+// RUN: rm %t/Lib.swiftmodule
+
+// RUN: %target-swift-frontend -typecheck %t/Client.swift -I %t \
+// RUN:   -verify -Xllvm -debug-only=Serialization 2>&1 \
+// RUN:   | %FileCheck --check-prefixes=NEEDED,CLEAN %s
+
+/// Decls part of the API needed by the client.
+// NEEDED-NOT: Deserialized: 'refToIOI()'
+// NEEDED: Deserialized: 'PublicStruct'
+// NEEDED: Deserialized: 'publicFunc()'
+
+/// Internal details dangerous to load.
+// UNSAFE: Deserialized: 'internalFunc()'
+// UNSAFE: Deserialized: 'privateFunc()'
+// UNSAFE: Deserialized: 'fileprivateFunc()'
+
+/// Decls removed by rebuilding from the swiftinterface.
+// CLEAN-NOT: Deserialized: 'internalFunc()'
+// CLEAN-NOT: Deserialized: 'privateFunc()'
+// CLEAN-NOT: Deserialized: 'fileprivateFunc()'
+
+//--- HiddenLib.swift
+
+public struct HiddenStruct {
+    public init() {}
+}
+
+//--- Lib.swift
+
+@_implementationOnly import HiddenLib
+
+public class PublicStruct {
+    public init() {}
+
+    public func publicFunc() {}
+    internal func internalFunc() {}
+    private func privateFunc() {}
+    fileprivate func fileprivateFunc() {}
+
+    internal func refToIOI() -> HiddenStruct {
+        return HiddenStruct();
+    }
+}
+
+//--- Client.swift
+
+import Lib
+
+var x = PublicStruct()
+
+// Trigger a typo correction that reads all members.
+x.notAMember() // expected-error {{value of type 'PublicStruct' has no member 'notAMember'}}