[AST] Mark Sendable as a repressible protocol

This allows to spell `~Sendable` to suppress the conformance
inference on types when `TildeSendable` flag is enabled.

Author: xedin

include/swift/AST/DiagnosticsSema.def

@@ -8943,6 +8943,13 @@ ERROR(attr_inline_always_on_accessor,none,
       "'@inline(always)' on class variable accessors whose variable declaration"
       " is not final are not allowed", ())
 
+//===----------------------------------------------------------------------===//
+// MARK: `~Sendable`
+//===----------------------------------------------------------------------===//
+ERROR(tilde_sendable_requires_feature_flag,none,
+      "'~Sendable' requires -enable-experimental-feature TildeSendable",
+      ())
+
 //===----------------------------------------------------------------------===//
 // MARK: Swift Performance hints
 //===----------------------------------------------------------------------===//

include/swift/AST/KnownProtocols.def

@@ -107,7 +107,7 @@ PROTOCOL(CodingKey)
 PROTOCOL(Encodable)
 PROTOCOL(Decodable)
 
-PROTOCOL(Sendable)
+REPRESSIBLE_PROTOCOL(Sendable)
 PROTOCOL(SendableMetatype)
 PROTOCOL(UnsafeSendable)
 

lib/Sema/TypeCheckConcurrency.cpp

@@ -7499,6 +7499,10 @@ ProtocolConformance *swift::deriveImplicitSendableConformance(
   if (isa<ProtocolDecl>(nominal))
     return nullptr;
 
+  // An explicit use `~Sendable` suppresses the conformance.
+  if (nominal->suppressesConformance(KnownProtocolKind::Sendable))
+    return nullptr;
+
   // Actor types are always Sendable; they don't get it via this path.
   auto classDecl = dyn_cast<ClassDecl>(nominal);
   if (classDecl && classDecl->isActor())

lib/Sema/TypeCheckDeclPrimary.cpp

@@ -123,6 +123,15 @@ class CheckRepressions {
                       ctx.getProtocol(*kp));
       return Type();
     }
+
+    if (rpk == RepressibleProtocolKind::Sendable) {
+      if (!ctx.LangOpts.hasFeature(Feature::TildeSendable)) {
+        diagnoseInvalid(repr, repr.getLoc(),
+                        diag::tilde_sendable_requires_feature_flag);
+        return Type();
+      }
+    }
+
     if (auto *extension = dyn_cast<const ExtensionDecl *>(decl)) {
       diagnoseInvalid(repr, extension,
                       diag::suppress_inferrable_protocol_extension,