[Serialization] Move deserialization safety behind an env var or flag

Author: xymus

include/swift/Basic/LangOptions.h

@@ -366,7 +366,8 @@ namespace swift {
 
     /// Enable early skipping deserialization of decls that are marked as
     /// unsafe to read.
-    bool EnableDeserializationSafety = false;
+    bool EnableDeserializationSafety =
+      ::getenv("SWIFT_ENABLE_DESERIALIZATION_SAFETY");
 
     /// Whether to enable the new operator decl and precedencegroup lookup
     /// behavior. This is a staging flag, and will be removed in the future.

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

@@ -23,6 +23,18 @@
 // RUN:   -enable-deserialization-safety 2>&1 \
 // RUN:   | %FileCheck --check-prefixes=NEEDED,CLEAN,SAFE %s
 
+/// Disabled by default.
+// RUN: %target-swift-frontend -typecheck %t/Client.swift -I %t \
+// RUN:   -verify -Xllvm -debug-only=Serialization \
+// RUN:   -disable-deserialization-safety 2>&1 \
+// RUN:   | %FileCheck --check-prefixes=NEEDED,UNSAFE %s
+
+/// Enable with env var.
+// RUN: env SWIFT_ENABLE_DESERIALIZATION_SAFETY=true \
+// RUN:   %target-swift-frontend -typecheck %t/Client.swift -I %t \
+// RUN:   -verify -Xllvm -debug-only=Serialization 2>&1 \
+// RUN:   | %FileCheck --check-prefixes=NEEDED,CLEAN,SAFE %s
+
 /// Build against the swiftinterface.
 // RUN: rm %t/Lib.swiftmodule
 // RUN: %target-swift-frontend -typecheck %t/Client.swift -I %t \