Introduce checking for ConcurrentValue conformance across actors.

When referring to an actor-isolated declaration from outside of the
actor, ensure that the types involved conform to the `ConcurrentValue`
protocol. Otherwise, produce a diagnostic stating that it is unsafe to
pass such types across actors.

Apply the same rule to local captures within concurrent code.

Author: DougGregor

include/swift/AST/DiagnosticsSema.def

@@ -4305,6 +4305,17 @@ ERROR(global_actor_isolated_requirement_witness_conflict,none,
       "requirement from protocol %3 isolated to global actor %4",
       (DescriptiveDeclKind, DeclName, Type, Identifier, Type))
 
+WARNING(non_concurrent_param_type,none,
+        "cannot pass argument of non-concurrent-value type %0 across actors",
+        (Type))
+WARNING(non_concurrent_result_type,none,
+        "cannot call function returning non-concurrent-value type %0 across "
+        "actors", (Type))
+WARNING(non_concurrent_property_type,none,
+        "cannot use %0 %1 with a non-concurrent-value type %2 "
+        "%select{across actors|from concurrently-executed code}3",
+        (DescriptiveDeclKind, DeclName, Type, bool))
+
 ERROR(actorindependent_let,none,
       "'@actorIndependent' is meaningless on 'let' declarations because "
       "they are immutable",

include/swift/Basic/LangOptions.h

@@ -244,6 +244,9 @@ namespace swift {
     /// Enable experimental concurrency model.
     bool EnableExperimentalConcurrency = false;
 
+    /// Enable experimental ConcurrentValue checking.
+    bool EnableExperimentalConcurrentValueChecking = false;
+
     /// Disable the implicit import of the _Concurrency module.
     bool DisableImplicitConcurrencyModuleImport = false;
 

include/swift/Option/FrontendOptions.td

@@ -221,7 +221,12 @@ let Flags = [FrontendOption, NoDriverOption, HelpHidden, ModuleInterfaceOption]
 def enable_experimental_concurrency :
   Flag<["-"], "enable-experimental-concurrency">,
   HelpText<"Enable experimental concurrency model">;
-  def enable_resilience : Flag<["-"], "enable-resilience">,
+
+def enable_experimental_concurrent_value_checking :
+  Flag<["-"], "enable-experimental-concurrent-value-checking">,
+  HelpText<"Enable ConcurrentValue checking">;
+
+def enable_resilience : Flag<["-"], "enable-resilience">,
      HelpText<"Deprecated, use -enable-library-evolution instead">;
 }
 

lib/Frontend/CompilerInvocation.cpp

@@ -385,6 +385,8 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
 
   Opts.EnableExperimentalConcurrency |=
     Args.hasArg(OPT_enable_experimental_concurrency);
+  Opts.EnableExperimentalConcurrentValueChecking |=
+    Args.hasArg(OPT_enable_experimental_concurrent_value_checking);
 
   Opts.DisableImplicitConcurrencyModuleImport |=
     Args.hasArg(OPT_disable_implicit_concurrency_module_import);