Conditionally enable behaviors by a frontend flag.

Since the feature is incomplete and yet to be accepted or implemented as proposed, hide it behind an -enable-experimental-property-behaviors frontend flag.

include/swift/Basic/LangOptions.h

@@ -137,6 +137,9 @@ namespace swift {
     /// \brief Enable experimental "switch" pattern-matching features.
     bool EnableExperimentalPatterns = false;
 
+    /// \brief Enable experimental property behavior feature.
+    bool EnableExperimentalPropertyBehaviors = false;
+
     /// Should we check the target OSs of serialized modules to see that they're
     /// new enough?
     bool EnableTargetOSChecking = true;

include/swift/Option/FrontendOptions.td

@@ -216,6 +216,10 @@ def emit_verbose_sil : Flag<["-"], "emit-verbose-sil">,
 def enable_experimental_patterns : Flag<["-"], "enable-experimental-patterns">,
   HelpText<"Enable experimental 'switch' pattern matching features">;
 
+def enable_experimental_property_behaviors :
+  Flag<["-"], "enable-experimental-property-behaviors">,
+  HelpText<"Enable experimental property behaviors">;
+
 def disable_availability_checking : Flag<["-"],
   "disable-availability-checking">,
   HelpText<"Disable checking for potentially unavailable APIs">;

lib/Frontend/CompilerInvocation.cpp

@@ -726,6 +726,9 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
   Opts.EnableExperimentalPatterns |=
     Args.hasArg(OPT_enable_experimental_patterns);
 
+  Opts.EnableExperimentalPropertyBehaviors |=
+    Args.hasArg(OPT_enable_experimental_property_behaviors);
+
   Opts.DisableAvailabilityChecking |=
       Args.hasArg(OPT_disable_availability_checking);
   

lib/Parse/ParseDecl.cpp

@@ -3830,7 +3830,8 @@ ParserStatus Parser::parseDeclVar(ParseDeclOptions Flags,
   };
   
   // Check for a behavior declaration.
-  if (Tok.is(tok::l_square)) {
+  if (Context.LangOpts.EnableExperimentalPropertyBehaviors
+      && Tok.is(tok::l_square)) {
     BehaviorLBracket = consumeToken(tok::l_square);
     // TODO: parse visibility (public/private/internal)
     auto type = parseType(diag::expected_behavior_name,

test/Interpreter/property_behavior.swift

@@ -1,4 +1,7 @@
-// RUN: %target-run-simple-swift
+// RUN: rm -rf %t
+// RUN: mkdir -p %t
+// RUN: %target-build-swift -Xfrontend -enable-experimental-property-behaviors %s -o %t/a.out
+// RUN: %target-run %t/a.out
 // REQUIRES: executable_test
 
 import StdlibUnittest

test/SILGen/property_behavior.swift

@@ -1,4 +1,4 @@
-// RUN: %target-swift-frontend -emit-silgen %s | FileCheck %s
+// RUN: %target-swift-frontend -emit-silgen -enable-experimental-property-behaviors %s | FileCheck %s
 protocol behavior {
   associatedtype Value
 }

test/decl/var/behaviors.swift

@@ -1,4 +1,4 @@
-// RUN: %target-parse-verify-swift -module-name Main
+// RUN: %target-parse-verify-swift -enable-experimental-property-behaviors -module-name Main
 
 protocol behavior {
   associatedtype Value