Added experimental feature to allow weak let and immutable weak captures

Author: nickolas-pohilets

include/swift/Basic/Features.def

@@ -517,6 +517,9 @@ EXPERIMENTAL_FEATURE(CDecl, false)
 /// Allow use of `@extensible` on public enums
 SUPPRESSIBLE_EXPERIMENTAL_FEATURE(ExtensibleAttribute, false)
 
+/// Allow `weak let` and make weak captures immutable
+EXPERIMENTAL_FEATURE(WeakLet, true)
+
 #undef EXPERIMENTAL_FEATURE_EXCLUDED_FROM_MODULE_INTERFACE
 #undef EXPERIMENTAL_FEATURE
 #undef UPCOMING_FEATURE

lib/AST/Expr.cpp

@@ -1366,9 +1366,8 @@ CaptureListEntry CaptureListEntry::createParsed(
     SourceRange ownershipRange, Identifier name, SourceLoc nameLoc,
     SourceLoc equalLoc, Expr *initializer, DeclContext *DC) {
 
-  auto introducer =
-      (ownershipKind != ReferenceOwnership::Weak ? VarDecl::Introducer::Let
-                                                 : VarDecl::Introducer::Var);
+  bool forceVar = ownershipKind == ReferenceOwnership::Weak && !Ctx.LangOpts.hasFeature(Feature::WeakLet);
+  auto introducer = forceVar ? VarDecl::Introducer::Var : VarDecl::Introducer::Let;
   auto *VD =
       new (Ctx) VarDecl(/*isStatic==*/false, introducer, nameLoc, name, DC);
 

lib/AST/FeatureSet.cpp

@@ -628,6 +628,15 @@ static bool usesFeatureExtensibleAttribute(Decl *decl) {
   return decl->getAttrs().hasAttribute<ExtensibleAttr>();
 }
 
+static bool usesFeatureWeakLet(Decl *decl) {
+  if (auto *VD = dyn_cast<VarDecl>(decl)) {
+    if (auto *refAttr = VD->getAttrs().getAttribute<ReferenceOwnershipAttr>()) {
+      return VD->isLet() && refAttr->get() == ReferenceOwnership::Weak;
+    }
+  }
+  return false;
+}
+
 // ----------------------------------------------------------------------------
 // MARK: - FeatureSet
 // ----------------------------------------------------------------------------

lib/Sema/TypeCheckAttr.cpp

@@ -5260,7 +5260,7 @@ Type TypeChecker::checkReferenceOwnershipAttr(VarDecl *var, Type type,
   case ReferenceOwnershipOptionality::Allowed:
     break;
   case ReferenceOwnershipOptionality::Required:
-    if (var->isLet()) {
+    if (var->isLet() && !ctx.LangOpts.hasFeature(Feature::WeakLet)) {
       var->diagnose(diag::invalid_ownership_is_let, ownershipKind);
       attr->setInvalid();
     }