[AST] Add bit to VarDecl for isPlaceholderVar

This allows the query to be consistent both during type-checking and
after.

Author: hamishknight

include/swift/AST/Decl.h

@@ -493,7 +493,7 @@ class alignas(1 << DeclAlignInBits) Decl : public ASTAllocated<Decl>, public Swi
     IsStatic : 1
   );
 
-  SWIFT_INLINE_BITFIELD(VarDecl, AbstractStorageDecl, 2+1+1+1+1+1+1+1,
+  SWIFT_INLINE_BITFIELD(VarDecl, AbstractStorageDecl, 2+1+1+1+1+1+1+1+1,
     /// Encodes whether this is a 'let' binding.
     Introducer : 2,
 
@@ -517,7 +517,11 @@ class alignas(1 << DeclAlignInBits) Decl : public ASTAllocated<Decl>, public Swi
     NoAttachedPropertyWrappers : 1,
 
     /// Whether this variable has no property wrapper auxiliary variables.
-    NoPropertyWrapperAuxiliaryVariables : 1
+    NoPropertyWrapperAuxiliaryVariables : 1,
+
+    /// Whether this variable is a placeholder that is introducing during
+    /// type-checking that has its type inferred from its use.
+    IsPlaceholderVar : 1
   );
 
   SWIFT_INLINE_BITFIELD(ParamDecl, VarDecl, 1+2+NumDefaultArgumentKindBits,
@@ -6757,6 +6761,15 @@ class VarDecl : public AbstractStorageDecl {
     Bits.VarDecl.IsLazyStorageProperty = IsLazyStorage;
   }
 
+  /// Whether this variable is a placeholder that is introducing during
+  /// type-checking that has its type inferred from its use.
+  bool isPlaceholderVar() const {
+    return Bits.VarDecl.IsPlaceholderVar;
+  }
+  void setIsPlaceholderVar() {
+    Bits.VarDecl.IsPlaceholderVar = true;
+  }
+
   /// Retrieve the backing storage property for a lazy property.
   VarDecl *getLazyStorageProperty() const;
 

lib/AST/Decl.cpp

@@ -8010,6 +8010,7 @@ VarDecl::VarDecl(DeclKind kind, bool isStatic, VarDecl::Introducer introducer,
   Bits.VarDecl.IsTopLevelGlobal = false;
   Bits.VarDecl.NoAttachedPropertyWrappers = false;
   Bits.VarDecl.NoPropertyWrapperAuxiliaryVariables = false;
+  Bits.VarDecl.IsPlaceholderVar = false;
 }
 
 Type VarDecl::getTypeInContext() const {

lib/Sema/BuilderTransform.cpp

@@ -124,6 +124,7 @@ class ResultBuilderTransform
                                SmallVectorImpl<ASTNode> &container,
                                Type type = Type(), Expr *initExpr = nullptr) {
     auto *var = builder.buildVar(loc);
+    var->setIsPlaceholderVar();
     Pattern *placeholder = TypedPattern::createImplicit(
         ctx, NamedPattern::createImplicit(ctx, var),
         type ? type : PlaceholderType::get(ctx, var));

lib/Sema/CSSyntacticElement.cpp

@@ -2779,20 +2779,13 @@ void ConjunctionElement::findReferencedVariables(
 
 Type constraints::isPlaceholderVar(PatternBindingDecl *PB) {
   auto *var = PB->getSingleVar();
-  if (!var)
-    return Type();
-
-  if (!var->getName().hasDollarPrefix())
+  if (!var || !var->isPlaceholderVar())
     return Type();
 
   auto *pattern = PB->getPattern(0);
   auto *typedPattern = dyn_cast<TypedPattern>(pattern);
   if (!typedPattern || !typedPattern->hasType())
     return Type();
 
-  auto type = typedPattern->getType();
-  if (!type->hasPlaceholder())
-    return Type();
-
-  return type;
+  return typedPattern->getType();
 }