[Frontend] SE-0466: Add -default-isolation frontend that accepts MainActor and nonisolated

Author: xedin

include/swift/Basic/LangOptions.h

@@ -96,6 +96,11 @@ namespace swift {
     TaskToThread,
   };
 
+  enum class DefaultIsolation : uint8_t {
+    MainActor,
+    Nonisolated
+  };
+
   /// Describes the code size optimization behavior for code associated with
   /// declarations that are marked unavailable.
   enum class UnavailableDeclOptimization : uint8_t {
@@ -632,6 +637,9 @@ namespace swift {
     /// Disables `DynamicActorIsolation` feature.
     bool DisableDynamicActorIsolation = false;
 
+    /// Defines the default actor isolation.
+    DefaultIsolation DefaultIsolationBehavior = DefaultIsolation::Nonisolated;
+
     /// Whether or not to allow experimental features that are only available
     /// in "production".
 #ifdef NDEBUG

include/swift/Option/Options.td

@@ -985,6 +985,15 @@ def strict_concurrency : Joined<["-"], "strict-concurrency=">,
            "concurrency model, or 'complete' ('Sendable' and other checking is "
            "enabled for all code in the module)">;
 
+def default_isolation : Separate<["-"], "default-isolation">,
+  Flags<[FrontendOption]>,
+  HelpText<"Specify the default actor isolation: MainActor or nonisolated. "
+  "Defaults to nonisolated.">,
+  MetaVarName<"MainActor|nonisolated">;
+def default_isolation_EQ :  Joined<["-"], "default-isolation=">,
+  Flags<[FrontendOption]>,
+  Alias<default_isolation>;
+
 def enable_experimental_feature :
   Separate<["-"], "enable-experimental-feature">,
   Flags<[FrontendOption, ModuleInterfaceOption]>,

lib/Frontend/CompilerInvocation.cpp

@@ -1817,6 +1817,24 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
   Opts.DisableDynamicActorIsolation |=
       Args.hasArg(OPT_disable_dynamic_actor_isolation);
 
+  if (const Arg *A = Args.getLastArg(options::OPT_default_isolation)) {
+    auto behavior =
+        llvm::StringSwitch<std::optional<DefaultIsolation>>(A->getValue())
+            .Case("MainActor", DefaultIsolation::MainActor)
+            .Case("nonisolated", DefaultIsolation::Nonisolated)
+            .Default(std::nullopt);
+
+    if (behavior) {
+      Opts.DefaultIsolationBehavior = *behavior;
+    } else {
+      Diags.diagnose(SourceLoc(), diag::error_invalid_arg_value,
+                     A->getAsString(Args), A->getValue());
+      HadError = true;
+    }
+  } else {
+    Opts.DefaultIsolationBehavior = DefaultIsolation::Nonisolated;
+  }
+
 #if !defined(NDEBUG) && SWIFT_ENABLE_EXPERIMENTAL_PARSER_VALIDATION
   /// Enable round trip parsing via the new swift parser unless it is disabled
   /// explicitly. The new Swift parser can have mismatches with C++ parser -