diff --git a/clang/include/clang/Tooling/Refactoring/Stencil.h b/clang/include/clang/Tooling/Refactoring/Stencil.h
--- a/clang/include/clang/Tooling/Refactoring/Stencil.h
+++ b/clang/include/clang/Tooling/Refactoring/Stencil.h
@@ -23,6 +23,7 @@
 #include "clang/AST/ASTContext.h"
 #include "clang/AST/ASTTypeTraits.h"
 #include "clang/ASTMatchers/ASTMatchFinder.h"
+#include "clang/Tooling/Refactoring/MatchConsumer.h"
 #include "clang/Tooling/Refactoring/RangeSelector.h"
 #include "llvm/ADT/StringRef.h"
 #include "llvm/Support/Error.h"
@@ -175,6 +176,10 @@
   return ifBound(Id, text(TrueText), text(FalseText));
 }
 
+/// Wraps a MatchConsumer in a StencilPart, so that it can be used in a Stencil.
+/// This supports user-defined extensions to the Stencil language.
+StencilPart run(MatchConsumer<std::string> C);
+
 /// For debug use only; semantics are not guaranteed.
 ///
 /// \returns the string resulting from calling the node's print() method.
diff --git a/clang/lib/Tooling/Refactoring/Stencil.cpp b/clang/lib/Tooling/Refactoring/Stencil.cpp
--- a/clang/lib/Tooling/Refactoring/Stencil.cpp
+++ b/clang/lib/Tooling/Refactoring/Stencil.cpp
@@ -26,6 +26,7 @@
 using ast_matchers::MatchFinder;
 using llvm::errc;
 using llvm::Error;
+using llvm::Expected;
 using llvm::StringError;
 
 // A down_cast function to safely down cast a StencilPartInterface to a subclass
@@ -102,6 +103,9 @@
   return A.Id == B.Id && A.TruePart == B.TruePart && A.FalsePart == B.FalsePart;
 }
 
+// Equality is not defined over MatchConsumer<std::string>s. They are, by definition, opaque.
+bool isEqualData(const MatchConsumer<std::string> &A, const MatchConsumer<std::string> &B) { return false; }
+
 // The `evalData()` overloads evaluate the given stencil data to a string, given
 // the match result, and append it to `Result`. We define an overload for each
 // type of stencil data.
@@ -159,6 +163,15 @@
       .eval(Match, Result);
 }
 
+Error evalData(const MatchConsumer<std::string> &Fn, const MatchFinder::MatchResult &Match,
+               std::string *Result) {
+  Expected<std::string> Value = Fn(Match);
+  if (!Value)
+    return Value.takeError();
+  *Result += *Value;
+  return Error::success();
+}
+
 template <typename T>
 class StencilPartImpl : public StencilPartInterface {
   T Data;
@@ -233,3 +246,9 @@
   return StencilPart(std::make_shared<StencilPartImpl<IfBoundData>>(
       Id, std::move(TruePart), std::move(FalsePart)));
 }
+
+StencilPart stencil::run(MatchConsumer<std::string> Fn) {
+  return StencilPart(
+      std::make_shared<StencilPartImpl<MatchConsumer<std::string>>>(
+          std::move(Fn)));
+}
diff --git a/clang/unittests/Tooling/StencilTest.cpp b/clang/unittests/Tooling/StencilTest.cpp
--- a/clang/unittests/Tooling/StencilTest.cpp
+++ b/clang/unittests/Tooling/StencilTest.cpp
@@ -29,6 +29,7 @@
 using stencil::cat;
 using stencil::dPrint;
 using stencil::ifBound;
+using stencil::run;
 using stencil::text;
 
 // In tests, we can't directly match on llvm::Expected since its accessors
@@ -300,6 +301,15 @@
   EXPECT_THAT_EXPECTED(Stencil.eval(StmtMatch->Result), HasValue("field"));
 }
 
+TEST_F(StencilTest, RunOp) {
+  StringRef Id = "id";
+  auto SimpleFn = [Id](const MatchResult &R) {
+    return std::string(R.Nodes.getNodeAs<Stmt>(Id) != nullptr ? "Bound"
+                                                              : "Unbound");
+  };
+  testExpr(Id, "3;", cat(run(SimpleFn)), "Bound");
+}
+
 TEST(StencilEqualityTest, Equality) {
   auto Lhs = cat("foo", dPrint("dprint_id"));
   auto Rhs = cat("foo", dPrint("dprint_id"));
@@ -324,4 +334,12 @@
   auto S2 = cat(node("node"));
   EXPECT_NE(S1, S2);
 }
+
+// `run` is opaque.
+TEST(StencilEqualityTest, InEqualityRun) {
+  auto F = [](const MatchResult &R) { return "foo"; };
+  auto S1 = cat(run(F));
+  auto S2 = cat(run(F));
+  EXPECT_NE(S1, S2);
+}
 } // namespace