Add StrictConcurrency as an always-enabled experimental feature

include/swift/Basic/Features.def

@@ -210,6 +210,9 @@ EXPERIMENTAL_FEATURE(ReferenceBindings, false)
 /// Enable the explicit 'import Builtin' and allow Builtin usage.
 EXPERIMENTAL_FEATURE(BuiltinModule, true)
 
+// Enable strict concurrency.
+EXPERIMENTAL_FEATURE(StrictConcurrency, true)
+
 #undef EXPERIMENTAL_FEATURE_EXCLUDED_FROM_MODULE_INTERFACE
 #undef EXPERIMENTAL_FEATURE
 #undef UPCOMING_FEATURE

lib/AST/ASTPrinter.cpp

@@ -3173,6 +3173,10 @@ static bool usesFeatureExistentialAny(Decl *decl) {
   return false;
 }
 
+static bool usesFeatureStrictConcurrency(Decl *decl) {
+  return false;
+}
+
 static bool usesFeatureImportObjcForwardDeclarations(Decl *decl) {
   ClangNode clangNode = decl->getClangNode();
   if (!clangNode)

lib/Frontend/CompilerInvocation.cpp

@@ -509,6 +509,15 @@ static void diagnoseCxxInteropCompatMode(Arg *verArg, ArgList &Args,
   diags.diagnose(SourceLoc(), diag::valid_cxx_interop_modes, versStr);
 }
 
+static llvm::Optional<StrictConcurrency>
+parseStrictConcurrency(StringRef value) {
+  return llvm::StringSwitch<llvm::Optional<StrictConcurrency>>(value)
+      .Case("minimal", StrictConcurrency::Minimal)
+      .Case("targeted", StrictConcurrency::Targeted)
+      .Case("complete", StrictConcurrency::Complete)
+      .Default(llvm::None);
+}
+
 static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
                           DiagnosticEngine &Diags,
                           const FrontendOptions &FrontendOpts) {
@@ -763,9 +772,33 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
     addFutureFeatureIfNotImplied(Feature::BareSlashRegexLiterals);
 
   for (const Arg *A : Args.filtered(OPT_enable_experimental_feature)) {
+    // Allow StrictConcurrency to have a value that corresponds to the
+    // -strict-concurrency=<blah> settings.
+    StringRef value = A->getValue();
+    if (value.startswith("StrictConcurrency")) {
+      auto decomposed = value.split("=");
+      if (decomposed.first == "StrictConcurrency") {
+        bool handled;
+        if (decomposed.second == "") {
+          Opts.StrictConcurrencyLevel = StrictConcurrency::Complete;
+          handled = true;
+        } else if (auto level = parseStrictConcurrency(decomposed.second)) {
+          Opts.StrictConcurrencyLevel = *level;
+          handled = true;
+        } else {
+          handled = false;
+        }
+
+        if (handled) {
+          Opts.Features.insert(Feature::StrictConcurrency);
+          continue;
+        }
+      }
+    }
+
     // If this is a known experimental feature, allow it in +Asserts
     // (non-release) builds for testing purposes.
-    if (auto feature = getExperimentalFeature(A->getValue())) {
+    if (auto feature = getExperimentalFeature(value)) {
 #ifdef NDEBUG
       if (!isFeatureAvailableInProduction(*feature)) {
         Diags.diagnose(SourceLoc(),
@@ -928,6 +961,8 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
 
   } else if (Args.hasArg(OPT_warn_concurrency)) {
     Opts.StrictConcurrencyLevel = StrictConcurrency::Complete;
+  } else if (Opts.hasFeature(Feature::StrictConcurrency)) {
+    // Already set above.
   } else {
     // Default to minimal checking in Swift 5.x.
     Opts.StrictConcurrencyLevel = StrictConcurrency::Minimal;

test/Concurrency/experimental_feature_strictconcurrency.swift

@@ -0,0 +1,22 @@
+// RUN: %target-typecheck-verify-swift -enable-experimental-feature StrictConcurrency
+// RUN: %target-typecheck-verify-swift -enable-experimental-feature StrictConcurrency=complete
+// REQUIRES: concurrency
+
+class C1 { } // expected-note{{class 'C1' does not conform to the 'Sendable' protocol}}
+class C2 { }
+
+@available(*, unavailable)
+extension C2: Sendable {} // expected-note{{conformance of 'C2' to 'Sendable' has been explicitly marked unavailable here}}
+
+protocol TestProtocol {
+  associatedtype Value: Sendable
+}
+
+struct Test1: TestProtocol { // expected-warning{{type 'Test1.Value' (aka 'C1') does not conform to the 'Sendable' protocol}}
+  typealias Value = C1
+}
+
+struct Test2: TestProtocol { // expected-warning{{conformance of 'C2' to 'Sendable' is unavailable}}
+  // expected-note@-1{{in associated type 'Self.Value' (inferred as 'C2')}}
+  typealias Value = C2
+}

test/Concurrency/experimental_feature_strictconcurrency_targeted.swift

@@ -0,0 +1,18 @@
+// RUN: %target-typecheck-verify-swift -enable-experimental-feature StrictConcurrency=targeted
+// REQUIRES: concurrency
+
+class C { // expected-note{{class 'C' does not conform to the 'Sendable' protocol}}
+  var counter = 0
+}
+
+func acceptsSendable<T: Sendable>(_: T) { }
+
+func testNoConcurrency(c: C) {
+  acceptsSendable(c)
+}
+
+@available(SwiftStdlib 5.1, *)
+func testConcurrency(c: C) async {
+  acceptsSendable(c) // expected-warning{{type 'C' does not conform to the 'Sendable' protocol}}
+}
+