[Sema] Expand ~Sendable annotation checking and diagnostics

Diagnose incorrect conformance + suppression uses, make sure
that adding `Sendable` is not suggested is type is using `~Sendable`.

include/swift/AST/DiagnosticsSema.def

@@ -8950,6 +8950,13 @@ ERROR(tilde_sendable_requires_feature_flag,none,
       "'~Sendable' requires -enable-experimental-feature TildeSendable",
       ())
 
+NOTE(sendable_conformance_is_suppressed, none,
+     "%kind0 explicitly suppresses conformance to 'Sendable' protocol",
+     (const NominalTypeDecl *))
+
+ERROR(non_sendable_type_suppressed,none,
+      "cannot both conform to and suppress conformance to 'Sendable'", ())
+
 ERROR(conformance_repression_only_on_struct_class_enum,none,
       "conformance to %0 can only be suppressed on structs, classes, and enums",
       (const ProtocolDecl *))

lib/Sema/TypeCheckConcurrency.cpp

@@ -1016,12 +1016,15 @@ static bool diagnoseSingleNonSendableType(
 
     if (type->is<FunctionType>()) {
       ctx.Diags.diagnose(loc, diag::nonsendable_function_type);
+    } else if (nominal &&
+               nominal->suppressesConformance(KnownProtocolKind::Sendable)) {
+      nominal->diagnose(diag::sendable_conformance_is_suppressed, nominal);
     } else if (nominal && nominal->getParentModule() == module) {
       // If the nominal type is in the current module, suggest adding
       // `Sendable` if it might make sense. Otherwise, just complain.
       if (isa<StructDecl>(nominal) || isa<EnumDecl>(nominal)) {
-        auto note = nominal->diagnose(
+        auto note =
-            diag::add_nominal_sendable_conformance, nominal);
+            nominal->diagnose(diag::add_nominal_sendable_conformance, nominal);
         addSendableFixIt(nominal, note, /*unchecked=*/false);
       } else {
         nominal->diagnose(diag::non_sendable_nominal, nominal);
@@ -1302,6 +1305,9 @@ inferSendableFromInstanceStorage(NominalTypeDecl *nominal,
     }
   }
 
+  if (nominal->suppressesConformance(KnownProtocolKind::Sendable))
+    return std::nullopt;
+
   class Visitor : public StorageVisitor {
   public:
     NominalTypeDecl *nominal;
@@ -1395,6 +1401,10 @@ void swift::diagnoseMissingExplicitSendable(NominalTypeDecl *nominal) {
         /*treatUsableFromInlineAsPublic=*/true).isPublic())
     return;
 
+  // If `Sendable` conformance is explicitly suppressed, do nothing.
+  if (nominal->suppressesConformance(KnownProtocolKind::Sendable))
+    return;
+
   // If the conformance is explicitly stated, do nothing.
   if (hasExplicitSendableConformance(nominal, /*applyModuleDefault=*/false))
     return;
@@ -7173,6 +7183,14 @@ static bool checkSendableInstanceStorage(
     }
   }
 
+  if (nominal->suppressesConformance(KnownProtocolKind::Sendable)) {
+    auto *conformanceDecl = dc->getAsDecl() ? dc->getAsDecl() : nominal;
+    if (!isImplicitSendableCheck(check)) {
+      conformanceDecl->diagnose(diag::non_sendable_type_suppressed);
+    }
+    return true;
+  }
+
   // Stored properties of structs and classes must have
   // Sendable-conforming types.
   class Visitor: public StorageVisitor {

test/Sema/tilde_sendable.swift

@@ -1,4 +1,4 @@
-// RUN: %target-typecheck-verify-swift -enable-experimental-feature TildeSendable
+// RUN: %target-typecheck-verify-swift -strict-concurrency=complete -swift-version 5 -enable-experimental-feature TildeSendable
 
 // REQUIRES: swift_feature_TildeSendable
 
@@ -15,6 +15,7 @@ class B: ~Sendable {}
 enum C: ~Sendable {}
 
 struct E1: Sendable, ~Sendable {}
+// expected-error@-1 {{cannot both conform to and suppress conformance to 'Sendable'}}
 
 enum E2: ~Sendable, ~Sendable {} // expected-warning {{already suppressed conformance to 'Sendable'}}
 
@@ -32,3 +33,22 @@ struct Generic<T: ~Sendable> { // expected-error {{conformance to 'Sendable' can
 
 var x: some BinaryInteger & ~Sendable // expected-error {{type 'Sendable' cannot be suppressed}}
 
+protocol W: Sendable {
+}
+
+// Check that inference is suppressed by the annotation.
+do {
+  struct NonSendable: ~Sendable {
+    // expected-note@-1 {{struct 'NonSendable' explicitly suppresses conformance to 'Sendable' protocol}}
+    let x: Int = 0
+  }
+
+  struct NoInference: W, ~Sendable {
+    // expected-error@-1 {{cannot both conform to and suppress conformance to 'Sendable'}}
+  }
+
+  func check<T: Sendable>(_: T) {}
+
+  check(NonSendable()) // expected-warning {{type 'NonSendable' does not conform to the 'Sendable' protocol}}
+  check(NoInference()) // Ok
+}