Sema: Add TVO_CanBindToNoEscape

We have a systemic class of issues where noescape types end up bound to
type variables in places that should not. The existing diagnostic for
this is ad-hoc and duplicated in several places but it doesn't actually
address the root cause of the problem.

For now, I've changed all call sites of createTypeVariable() to set the
new flag. I plan on removing enough occurrences of the flag to replicate
the old diagnostics. Then we can continue to refine this over time.

Author: slavapestov

include/swift/AST/DiagnosticsSema.def

@@ -3044,6 +3044,9 @@ ERROR(general_noescape_to_escaping,none,
 ERROR(converting_noescape_to_type,none,
       "converting non-escaping value to %0 may allow it to escape",
       (Type))
+ERROR(unknown_escaping_use_of_noescape,none,
+      "using non-escaping value in a context where it may escape",
+      ())
 
 ERROR(capture_across_type_decl,none,
       "%0 declaration cannot close over value %1 defined in outer scope",

include/swift/AST/TypeAlignments.h

@@ -58,6 +58,7 @@ namespace swift {
   constexpr size_t TypeAlignInBits = 3;
   constexpr size_t PatternAlignInBits = 3;
   constexpr size_t SILFunctionAlignInBits = 2;
+  constexpr size_t TypeVariableAlignInBits = 4;
 }
 
 namespace llvm {
@@ -95,7 +96,7 @@ LLVM_DECLARE_TYPE_ALIGNMENT(swift::ExtensionDecl, swift::DeclAlignInBits)
 
 LLVM_DECLARE_TYPE_ALIGNMENT(swift::TypeBase, swift::TypeAlignInBits)
 LLVM_DECLARE_TYPE_ALIGNMENT(swift::ArchetypeType, swift::TypeAlignInBits)
-LLVM_DECLARE_TYPE_ALIGNMENT(swift::TypeVariableType, swift::TypeAlignInBits)
+LLVM_DECLARE_TYPE_ALIGNMENT(swift::TypeVariableType, swift::TypeVariableAlignInBits)
 
 LLVM_DECLARE_TYPE_ALIGNMENT(swift::Stmt, swift::StmtAlignInBits)
 LLVM_DECLARE_TYPE_ALIGNMENT(swift::BraceStmt, swift::StmtAlignInBits)

include/swift/AST/Types.h

@@ -329,11 +329,11 @@ class alignas(1 << TypeAlignInBits) TypeBase {
     ID : 32 - NumTypeBaseBits,
 
     /// Type variable options.
-    Options : 3,
+    Options : 4,
 
     ///  Index into the list of type variables, as used by the
     ///  constraint graph.
-    GraphIndex : 29
+    GraphIndex : 28
   );
 
   SWIFT_INLINE_BITFIELD(SILFunctionType, TypeBase, NumSILExtInfoBits+3+1+2,
@@ -5060,13 +5060,15 @@ END_CAN_TYPE_WRAPPER(Name##StorageType, ReferenceStorageType)
 #undef REF_STORAGE_HELPER
 
 /// A type variable used during type checking.
-class TypeVariableType : public TypeBase {
+class alignas(1 << TypeVariableAlignInBits)
+TypeVariableType : public TypeBase {
   // Note: We can't use llvm::TrailingObjects here because the trailing object
   // type is opaque.
 
   TypeVariableType(const ASTContext &C, unsigned ID)
     : TypeBase(TypeKind::TypeVariable, &C,
                RecursiveTypeProperties::HasTypeVariable) {
+    // Note: the ID may overflow, but its only used for printing.
     Bits.TypeVariableType.ID = ID;
   }
 
@@ -5103,6 +5105,8 @@ class TypeVariableType : public TypeBase {
     return reinterpret_cast<Implementation *>(this + 1);
   }
 
+  /// Type variable IDs are not globally unique and are only meant as a visual
+  /// aid when dumping AST.
   unsigned getID() const { return Bits.TypeVariableType.ID; }
 
   // Implement isa/cast/dyncast/etc.

lib/Sema/CSDiag.cpp

@@ -4008,7 +4008,8 @@ namespace {
       auto resultLocator =
         cs.getConstraintLocator(expr, ConstraintLocator::FunctionResult);
       auto resultType = cs.createTypeVariable(resultLocator,
-                                              TVO_CanBindToLValue);
+                                              TVO_CanBindToLValue |
+                                              TVO_CanBindToNoEscape);
 
       auto locator = cs.getConstraintLocator(expr);
       cs.addConstraint(ConstraintKind::FunctionResult,
@@ -4464,7 +4465,8 @@ bool FailureDiagnosis::diagnoseTrailingClosureErrors(ApplyExpr *callExpr) {
       } else if (auto *typeVar = resultType->getAs<TypeVariableType>()) {
         auto tv = cs.createTypeVariable(cs.getConstraintLocator(expr),
                                         TVO_CanBindToLValue |
-                                        TVO_PrefersSubtypeBinding);
+                                        TVO_PrefersSubtypeBinding |
+                                        TVO_CanBindToNoEscape);
 
         auto extInfo = FunctionType::ExtInfo().withThrows();
 
@@ -6182,7 +6184,7 @@ bool FailureDiagnosis::visitKeyPathExpr(KeyPathExpr *KPE) {
 
       bool builtConstraints(ConstraintSystem &cs, Expr *expr) override {
         auto *locator = cs.getConstraintLocator(expr);
-        auto valueType = cs.createTypeVariable(locator);
+        auto valueType = cs.createTypeVariable(locator, TVO_CanBindToNoEscape);
 
         auto keyPathType =
             BoundGenericClassType::get(Decl, ParentType, {RootType, valueType});

lib/Sema/CSDiagnostics.cpp

@@ -401,17 +401,21 @@ bool NoEscapeFuncToTypeConversionFailure::diagnoseAsError() {
     return true;
   }
 
-  auto path = getLocator()->getPath();
-  if (path.empty())
-    return false;
+  GenericTypeParamType *paramTy = nullptr;
 
-  auto &last = path.back();
-  if (last.getKind() != ConstraintLocator::GenericParameter)
-    return false;
+  auto path = getLocator()->getPath();
+  if (!path.empty()) {
+    auto &last = path.back();
+    if (last.getKind() == ConstraintLocator::GenericParameter)
+      paramTy = last.getGenericParameter();
+  }
 
-  auto *paramTy = last.getGenericParameter();
-  emitDiagnostic(anchor->getLoc(), diag::converting_noescape_to_type,
-                 paramTy);
+  if (paramTy) {
+    emitDiagnostic(anchor->getLoc(), diag::converting_noescape_to_type,
+                  paramTy);
+  } else {
+    emitDiagnostic(anchor->getLoc(), diag::unknown_escaping_use_of_noescape);
+  }
   return true;
 }