[Constraint system] Sink Expr::getDepthMap() into its one client.

NFC. It'll be easier to refactor this when it's not an API on Expr.

Author: DougGregor

include/swift/AST/Expr.h

@@ -530,11 +530,6 @@ class alignas(8) Expr {
   /// the parent map.
   llvm::DenseMap<Expr *, Expr *> getParentMap();
 
-  /// Produce a mapping from each subexpression to its depth and parent,
-  /// in the root expression. The root expression has depth 0, its children have
-  /// depth 1, etc.
-  llvm::DenseMap<Expr *, std::pair<unsigned, Expr *>> getDepthMap();
-
   /// Produce a mapping from each expression to its index according to a
   /// preorder traversal of the expressions. The parent has index 0, its first
   /// child has index 1, its second child has index 2 if the first child is a

lib/AST/Expr.cpp

@@ -721,34 +721,6 @@ llvm::DenseMap<Expr *, Expr *> Expr::getParentMap() {
   return parentMap;
 }
 
-llvm::DenseMap<Expr *, std::pair<unsigned, Expr *>> Expr::getDepthMap() {
-  class RecordingTraversal : public ASTWalker {
-  public:
-    llvm::DenseMap<Expr *, std::pair<unsigned, Expr *>> &DepthMap;
-    unsigned Depth = 0;
-
-    explicit RecordingTraversal(
-        llvm::DenseMap<Expr *, std::pair<unsigned, Expr *>> &depthMap)
-        : DepthMap(depthMap) {}
-
-    std::pair<bool, Expr *> walkToExprPre(Expr *E) override {
-      DepthMap[E] = {Depth, Parent.getAsExpr()};
-      Depth++;
-      return { true, E };
-    }
-
-    Expr *walkToExprPost(Expr *E) override {
-      Depth--;
-      return E;
-    }
-  };
-
-  llvm::DenseMap<Expr *, std::pair<unsigned, Expr *>> depthMap;
-  RecordingTraversal traversal(depthMap);
-  walk(traversal);
-  return depthMap;
-}
-
 llvm::DenseMap<Expr *, unsigned> Expr::getPreorderIndexMap() {
   class RecordingTraversal : public ASTWalker {
   public:

lib/Sema/ConstraintSystem.cpp

@@ -73,15 +73,46 @@ ExpressionTimer::~ExpressionTimer() {
       .highlight(E->getSourceRange());
 }
 
+/// Extend the given depth map by adding depths for all of the subexpressions
+/// of the given expression.
+static void extendDepthMap(
+   Expr *expr,
+   llvm::DenseMap<Expr *, std::pair<unsigned, Expr *>> &depthMap) {
+  class RecordingTraversal : public ASTWalker {
+  public:
+    llvm::DenseMap<Expr *, std::pair<unsigned, Expr *>> &DepthMap;
+    unsigned Depth = 0;
+
+    explicit RecordingTraversal(
+        llvm::DenseMap<Expr *, std::pair<unsigned, Expr *>> &depthMap)
+        : DepthMap(depthMap) {}
+
+    std::pair<bool, Expr *> walkToExprPre(Expr *E) override {
+      DepthMap[E] = {Depth, Parent.getAsExpr()};
+      Depth++;
+      return { true, E };
+    }
+
+    Expr *walkToExprPost(Expr *E) override {
+      Depth--;
+      return E;
+    }
+  };
+
+  RecordingTraversal traversal(depthMap);
+  expr->walk(traversal);
+}
+
 ConstraintSystem::ConstraintSystem(DeclContext *dc,
                                    ConstraintSystemOptions options,
                                    Expr *expr)
   : Context(dc->getASTContext()), DC(dc), Options(options),
     Arena(dc->getASTContext(), Allocator),
     CG(*new ConstraintGraph(*this))
 {
-  if (expr)
-    ExprWeights = expr->getDepthMap();
+  if (expr) {
+    extendDepthMap(expr, ExprWeights);
+  }
 
   assert(DC && "context required");
 }