[Type checker] Limit the number of type variables introduced for placeholders.

The type checker introduces fresh type variables for editor
placeholders that are encountered in the source. If there are many
such editor placeholders, we'll create a large number of type
variables with very little context, which can cause poor scaling in
the type checker. Since we get very little information out of these
type variables, artificially limit the number of type variables we
create (to 2) and rotate through them.

Another piece of rdar://problem/29389887.

(cherry picked from commit 207fffc)

Author: DougGregor

lib/Sema/CSGen.cpp

@@ -1022,6 +1022,16 @@ namespace {
     DeclContext *CurDC;
     SmallVector<DeclContext*, 4> DCStack;
 
+    static const unsigned numEditorPlaceholderVariables = 2;
+
+    /// A buffer of type variables used for editor placeholders. We only
+    /// use a small number of these (rotating through), to prevent expressions
+    /// with a large number of editor placeholders from flooding the constraint
+    /// system with type variables.
+    TypeVariableType *editorPlaceholderVariables[numEditorPlaceholderVariables]
+      = { nullptr, nullptr };
+    unsigned currentEditorPlaceholderVariable = 0;
+
     /// \brief Add constraints for a reference to a named member of the given
     /// base type, and return the type of such a reference.
     Type addMemberRefConstraints(Expr *expr, Expr *base, DeclName name,
@@ -2760,14 +2770,28 @@ namespace {
     Type visitEditorPlaceholderExpr(EditorPlaceholderExpr *E) {
       if (E->getTypeLoc().isNull()) {
         auto locator = CS.getConstraintLocator(E);
-        auto placeholderTy = CS.createTypeVariable(locator, /*options*/0);
+
         // A placeholder may have any type, but default to Void type if
         // otherwise unconstrained.
-        CS.addConstraint(ConstraintKind::Defaultable,
-                         placeholderTy, TupleType::getEmpty(CS.getASTContext()),
-                         locator);
+        auto &placeholderTy
+          = editorPlaceholderVariables[currentEditorPlaceholderVariable];
+        if (!placeholderTy) {
+          placeholderTy = CS.createTypeVariable(locator, /*options*/0);
+
+          CS.addConstraint(ConstraintKind::Defaultable,
+                           placeholderTy,
+                           TupleType::getEmpty(CS.getASTContext()),
+                           locator);
+        }
+
+        // Move to the next placeholder variable.
+        currentEditorPlaceholderVariable
+          = (currentEditorPlaceholderVariable + 1) %
+              numEditorPlaceholderVariables;
+
         return placeholderTy;
       }
+
       // NOTE: The type loc may be there but have failed to validate, in which
       // case we return the null type.
       return E->getType();