[clang] [clang][transformer] Make `describe()` terser for `NamedDecl`s. (PR #108215)

[Clement Courbet via cfe-commits]

https://github.com/legrosbuffle created https://github.com/llvm/llvm-project/pull/108215

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

>From 42f1e67d2186fd90b8fc08206b33e077912039a1 Mon Sep 17 00:00:00 2001
From: Clement Courbet <courbet at google.com>
Date: Wed, 11 Sep 2024 12:35:14 +0000
Subject: [PATCH] [clang][transformer] Make `describe()` terser for
 `NamedDecl`s.

Right now `describe()`ing a `FunctionDecl` dups the whole code of the
function. Dump only its name.
---
 clang/lib/Tooling/Transformer/Stencil.cpp |  8 +++++++-
 clang/unittests/Tooling/StencilTest.cpp   | 22 ++++++++++++++++++++++
 2 files changed, 29 insertions(+), 1 deletion(-)

diff --git a/clang/lib/Tooling/Transformer/Stencil.cpp b/clang/lib/Tooling/Transformer/Stencil.cpp
index bc4fa6e36057c1..223fb5a7689751 100644
--- a/clang/lib/Tooling/Transformer/Stencil.cpp
+++ b/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();
 }
diff --git a/clang/unittests/Tooling/StencilTest.cpp b/clang/unittests/Tooling/StencilTest.cpp
index 26257cf2ca3a5f..445912a53e8b62 100644
--- a/clang/unittests/Tooling/StencilTest.cpp
+++ b/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) {