[clang][transformer] Make describe() terser for NamedDecls. (#108215)

Right now `describe()`ing a `FunctionDecl` dups the whole code of the
function. Dump only its name.

Author: legrosbuffle

clang/lib/Tooling/Transformer/Stencil.cpp

@@ -50,7 +50,13 @@ static Error printNode(StringRef Id, const MatchFinder::MatchResult &Match,
   auto NodeOrErr = getNode(Match.Nodes, Id);
   if (auto Err = NodeOrErr.takeError())
     return Err;
-  NodeOrErr->print(Os, PrintingPolicy(Match.Context->getLangOpts()));
+  const PrintingPolicy PP(Match.Context->getLangOpts());
+  if (const auto *ND = NodeOrErr->get<NamedDecl>()) {
+    // For NamedDecls, we can do a better job than printing the whole thing.
+    ND->getNameForDiagnostic(Os, PP, false);
+  } else {
+    NodeOrErr->print(Os, PP);
+  }
   *Result += Output;
   return Error::success();
 }

clang/unittests/Tooling/StencilTest.cpp

@@ -565,6 +565,28 @@ TEST_F(StencilTest, DescribeAnonNamespaceType) {
                        HasValue(std::string(Expected)));
 }
 
+TEST_F(StencilTest, DescribeFunction) {
+  std::string Snippet = "int F(); F();";
+  std::string Expected = "F";
+  auto StmtMatch = matchStmt(Snippet, callExpr(callee(namedDecl().bind("fn"))));
+  ASSERT_TRUE(StmtMatch);
+  EXPECT_THAT_EXPECTED(describe("fn")->eval(StmtMatch->Result),
+                       HasValue(std::string(Expected)));
+}
+
+TEST_F(StencilTest, DescribeImplicitOperator) {
+  std::string Snippet = "struct Tag {}; [](Tag){};";
+  std::string Expected = "operator()";
+  auto StmtMatch = matchStmt(
+      Snippet,
+      stmt(hasDescendant(
+          cxxMethodDecl(hasParameter(0, hasType(namedDecl(hasName("Tag")))))
+              .bind("fn"))));
+  ASSERT_TRUE(StmtMatch);
+  EXPECT_THAT_EXPECTED(describe("fn")->eval(StmtMatch->Result),
+                       HasValue(std::string(Expected)));
+}
+
 TEST_F(StencilTest, RunOp) {
   StringRef Id = "id";
   auto SimpleFn = [Id](const MatchResult &R) {