[SIL] Don't assume we always have an associated DeclContext. (#2730) (#2735)

...in code that I wrote. The integrated REPL, deprecated though it may
be, does not have an associated DeclContext because its SourceFile is
not considered complete. (The proper LLDB REPL does not suffer from
this problem because they use a new SourceFile for every block of
input.)

Elsewhere, tighten up code that may have hit similar bugs, though we
haven't seen anything hit these yet.

rdar://problem/26476281
(cherry picked from commit 740ddd8)

Author: jrose-apple

lib/SILGen/SILGenExpr.cpp

@@ -2304,9 +2304,11 @@ static bool isNullableTypeInC(SILModule &M, Type ty) {
     return true;
 
   // Other types like UnsafePointer can also be nullable.
+  const DeclContext *DC = M.getAssociatedContext();
+  if (!DC)
+    DC = M.getSwiftModule();
   ty = OptionalType::get(ty);
-  return ty->isTriviallyRepresentableIn(ForeignLanguage::C,
-                                        M.getAssociatedContext());
+  return ty->isTriviallyRepresentableIn(ForeignLanguage::C, DC);
 }
 
 /// Determine whether the given declaration returns a non-optional object that

lib/SILOptimizer/IPO/DeadFunctionElimination.cpp

@@ -217,8 +217,7 @@ class FunctionLivenessComputation {
     // If a vtable or witness table (method) is only visible in another module
     // it can be accessed inside that module and we don't see this access.
     // We hit this case e.g. if a table is imported from the stdlib.
-    if (decl->getDeclContext()->getParentModule() !=
-        Module->getAssociatedContext()->getParentModule())
+    if (decl->getDeclContext()->getParentModule() != Module->getSwiftModule())
       return true;
 
     return false;

lib/SILOptimizer/IPO/LetPropertiesOpts.cpp

@@ -338,7 +338,7 @@ static bool isAssignableExternally(VarDecl *Property, SILModule *Module) {
 // be analyzed by this pass.
 static bool mayHaveUnknownUses(VarDecl *Property, SILModule *Module) {
   if (Property->getDeclContext()->getParentModule() !=
-      Module->getAssociatedContext()->getParentModule()) {
+      Module->getSwiftModule()) {
     DEBUG(llvm::dbgs() << "Property " << *Property
                        << " is defined in a different module\n");
     // We don't see the bodies of initializers from a different module

lib/Serialization/SerializeSIL.cpp

@@ -1817,6 +1817,7 @@ void SILSerializer::writeSILBlock(const SILModule *SILMod) {
   // serialize everything.
   // FIXME: Resilience: could write out vtable for fragile classes.
   const DeclContext *assocDC = SILMod->getAssociatedContext();
+  assert(assocDC && "cannot serialize SIL without an associated DeclContext");
   for (const SILVTable &vt : SILMod->getVTables()) {
     if (ShouldSerializeAll &&
         vt.getClass()->isChildContextOf(assocDC))

test/Interpreter/SDK/Cocoa_repl.swift

@@ -18,3 +18,14 @@ extension CGRect: Q {
 
 (CGRect() as Any as! Q).foo()
 // CHECK: (0.0, 0.0, 0.0, 0.0)
+
+// Test the "mayLieAboutNonOptionalReturn" hack for both imported and
+// non-imported types.
+struct Empty {}
+let _: Optional = Empty()
+// CHECK: Optional(REPL.Empty())
+let _: Optional = CGPoint.zero
+// CHECK: Optional((0.0, 0.0))
+let _: Optional = NSString.availableStringEncodings()
+// CHECK: Optional(0x{{[0-9a-fA-F]+}})
+