AST: More consistent definition of a 'local capture'

This commit adds a new ValueDecl::isLocalCapture() predicate and
uses it in the right places. The predicate is true if the
declaration is in local context, *or* if its at the top level of
the main source file and follows a 'guard' statement.

Fixes <rdar://problem/23051362> / <https://bugs.swift.org/browse/SR-3528>.

Author: slavapestov

include/swift/AST/Decl.h

@@ -2713,6 +2713,10 @@ class ValueDecl : public Decl {
   AccessSemantics getAccessSemanticsFromContext(const DeclContext *DC,
                                                 bool isAccessOnSelf) const;
 
+  /// Determines if a reference to this declaration from a nested function
+  /// should be treated like a capture of a local value.
+  bool isLocalCapture() const;
+
   /// Print a reference to the given declaration.
   std::string printRef() const;
 

lib/AST/CaptureInfo.cpp

@@ -57,7 +57,7 @@ CaptureInfo CaptureInfo::empty() {
 
 bool CaptureInfo::hasLocalCaptures() const {
   for (auto capture : getCaptures())
-    if (capture.getDecl()->getDeclContext()->isLocalContext())
+    if (capture.getDecl()->isLocalCapture())
       return true;
   return false;
 }
@@ -71,7 +71,7 @@ getLocalCaptures(SmallVectorImpl<CapturedValue> &Result) const {
 
   // Filter out global variables.
   for (auto capture : getCaptures()) {
-    if (!capture.getDecl()->getDeclContext()->isLocalContext())
+    if (!capture.getDecl()->isLocalCapture())
       continue;
 
     Result.push_back(capture);

lib/AST/Decl.cpp

@@ -2866,6 +2866,16 @@ bool ValueDecl::isImplicitlyUnwrappedOptional() const {
     false);
 }
 
+bool ValueDecl::isLocalCapture() const {
+  auto *dc = getDeclContext();
+
+  if (auto *fd = dyn_cast<FuncDecl>(this))
+    if (isa<SourceFile>(dc))
+      return fd->hasTopLevelLocalContextCaptures();
+
+  return dc->isLocalContext();
+}
+
 ArrayRef<ValueDecl *>
 ValueDecl::getSatisfiedProtocolRequirements(bool Sorted) const {
   // Dig out the nominal type.

lib/Sema/TypeCheckCaptures.cpp

@@ -312,7 +312,7 @@ class FindCapturedVars : public ASTWalker {
 
     // Only capture var decls at global scope.  Other things can be captured
     // if they are local.
-    if (!isa<VarDecl>(D) && !DC->isLocalContext())
+    if (!isa<VarDecl>(D) && !D->isLocalCapture())
       return { false, DRE };
 
     // We're going to capture this, compute flags for the capture.

test/SILGen/top_level_captures.swift

@@ -0,0 +1,14 @@
+// RUN: %target-swift-frontend -emit-silgen %s | %FileCheck %s
+
+guard let x: Int = nil else { while true { } }
+
+// CHECK-LABEL: sil hidden [ossa] @$s18top_level_captures0C1XyyF : $@convention(thin) (Int) -> () {
+func capturesX() {
+  _ = x
+}
+
+// CHECK-LABEL: sil hidden [ossa] @$s18top_level_captures17transitiveCaptureyyF : $@convention(thin) (Int) -> () {
+// CHECK: [[FUNC:%.*]] = function_ref @$s18top_level_captures0C1XyyF : $@convention(thin) (Int) -> ()
+func transitiveCapture() {
+  capturesX()
+}