Add -require-explicit-sendable to warn about non-Sendable public types

include/swift/AST/DiagnosticsSema.def

@@ -1949,6 +1949,15 @@ NOTE(non_sendable_nominal,none,
 NOTE(add_nominal_sendable_conformance,none,
      "consider making %0 %1 conform to the 'Sendable' protocol",
      (DescriptiveDeclKind, DeclName))
+WARNING(public_decl_needs_sendable,none,
+        "public %0 %1 does not specify whether it is 'Sendable' or not",
+        (DescriptiveDeclKind, DeclName))
+NOTE(explicit_unchecked_sendable,none,
+     "add '@unchecked Sendable' conformance to %0 %1 if this type manually implements concurrency safety",
+     (DescriptiveDeclKind, DeclName))
+NOTE(explicit_disable_sendable,none,
+     "make %0 %1 explicitly non-Sendable to suppress this warning",
+     (DescriptiveDeclKind, DeclName))
 
 NOTE(required_by_opaque_return,none,
     "required by opaque return type of %0 %1", (DescriptiveDeclKind, DeclName))

include/swift/Basic/LangOptions.h

@@ -176,6 +176,9 @@ namespace swift {
     // Availability macros definitions to be expanded at parsing.
     SmallVector<std::string, 4> AvailabilityMacros;
 
+    /// Require public declarations to declare that they are Sendable (or not).
+    bool RequireExplicitSendable = false;
+
     /// If false, '#file' evaluates to the full path rather than a
     /// human-readable string.
     bool EnableConcisePoundFile = false;

include/swift/Option/Options.td

@@ -413,6 +413,10 @@ def require_explicit_availability_target : Separate<["-"], "require-explicit-ava
   HelpText<"Suggest fix-its adding @available(<target>, *) to public declarations without availability">,
   MetaVarName<"<target>">;
 
+def require_explicit_sendable : Flag<["-"], "require-explicit-sendable">,
+  Flags<[FrontendOption, NoInteractiveOption]>,
+  HelpText<"Require explicit Sendable annotations on public declarations">;
+
 def define_availability : Separate<["-"], "define-availability">,
   Flags<[FrontendOption, NoInteractiveOption]>,
   HelpText<"Define an availability macro in the format 'macroName : iOS 13.0, macOS 10.15'">,

lib/Driver/ToolChains.cpp

@@ -233,6 +233,7 @@ void ToolChain::addCommonFrontendArgs(const OutputInfo &OI,
   inputArgs.AddLastArg(arguments, options::OPT_enable_library_evolution);
   inputArgs.AddLastArg(arguments, options::OPT_require_explicit_availability);
   inputArgs.AddLastArg(arguments, options::OPT_require_explicit_availability_target);
+  inputArgs.AddLastArg(arguments, options::OPT_require_explicit_sendable);
   inputArgs.AddLastArg(arguments, options::OPT_enable_testing);
   inputArgs.AddLastArg(arguments, options::OPT_enable_private_imports);
   inputArgs.AddLastArg(arguments, options::OPT_g_Group);

lib/Frontend/CompilerInvocation.cpp

@@ -573,6 +573,7 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
     }
   }
 
+  Opts.RequireExplicitSendable |= Args.hasArg(OPT_require_explicit_sendable);
   for (const Arg *A : Args.filtered(OPT_define_availability)) {
     Opts.AvailabilityMacros.push_back(A->getValue());
   }

lib/Sema/TypeCheckAvailability.cpp

@@ -2446,6 +2446,7 @@ bool swift::diagnoseExplicitUnavailability(SourceLoc loc,
   auto proto = rootConf->getProtocol()->getDeclaredInterfaceType();
 
   StringRef platform;
+  auto behavior = DiagnosticBehavior::Unspecified;
   switch (attr->getPlatformAgnosticAvailability()) {
   case PlatformAgnosticAvailabilityKind::Deprecated:
     llvm_unreachable("shouldn't see deprecations in explicit unavailability");
@@ -2456,6 +2457,12 @@ bool swift::diagnoseExplicitUnavailability(SourceLoc loc,
       // This was platform-specific; indicate the platform.
       platform = attr->prettyPlatformString();
       break;
+    } else if (rootConf->getProtocol()->isSpecificProtocol(
+                   KnownProtocolKind::Sendable) &&
+               !ctx.LangOpts.isSwiftVersionAtLeast(6)) {
+      // Downgrade unavailable Sendable conformances to warnings prior to
+      // Swift 6.
+      behavior = DiagnosticBehavior::Warning;
     }
     LLVM_FALLTHROUGH;
 
@@ -2474,7 +2481,8 @@ bool swift::diagnoseExplicitUnavailability(SourceLoc loc,
   EncodedDiagnosticMessage EncodedMessage(attr->Message);
   diags.diagnose(loc, diag::conformance_availability_unavailable,
                  type, proto,
-                 platform.empty(), platform, EncodedMessage.Message);
+                 platform.empty(), platform, EncodedMessage.Message)
+      .limitBehavior(behavior);
 
   switch (attr->getVersionAvailability(ctx)) {
   case AvailableVersionComparison::Available: