[clangd] Fix type printing in the presence of qualifiers

When printing QualType with qualifiers like "const", or pointing to an
elaborated type, we would print garbage like:
  std::const std::vector<int>&
with the initial std:: being calculated correctly, but inserted in the
wrong place and the second std:: not removed (due to elaborated type).

This affected, among others, ExtractFunction and ExpandAuto tweaks.

This change introduces a new callback to PrintingPolicy, which allows us
to influence the printing of namespace qualifiers. In the future, the
same callback can be used to improve handling of "using namespace"
directives as well.

Fixes:
  https://github.com/clangd/clangd/issues/640 (ExtractFunction)
  https://github.com/clangd/clangd/issues/264 (ExpandAuto)
  First point of https://github.com/clangd/clangd/issues/524

Differential Revision: https://reviews.llvm.org/D94259

clang-tools-extra/clangd/AST.cpp

@@ -299,19 +299,27 @@ SymbolID getSymbolID(const llvm::StringRef MacroName, const MacroInfo *MI,
   return SymbolID(USR);
 }
 
-// FIXME: This should be handled while printing underlying decls instead.
 std::string printType(const QualType QT, const DeclContext &CurContext) {
   std::string Result;
   llvm::raw_string_ostream OS(Result);
-  auto Decls =
-      explicitReferenceTargets(DynTypedNode::create(QT), DeclRelation::Alias);
-  if (!Decls.empty())
-    OS << getQualification(CurContext.getParentASTContext(), &CurContext,
-                           Decls.front(),
-                           /*VisibleNamespaces=*/llvm::ArrayRef<std::string>{});
   PrintingPolicy PP(CurContext.getParentASTContext().getPrintingPolicy());
-  PP.SuppressScope = true;
   PP.SuppressTagKeyword = true;
+  PP.SuppressUnwrittenScope = true;
+
+  class PrintCB : public PrintingCallbacks {
+  public:
+    PrintCB(const DeclContext *CurContext) : CurContext(CurContext) {}
+    virtual ~PrintCB() {}
+    virtual bool isScopeVisible(const DeclContext *DC) const {
+      return DC->Encloses(CurContext);
+    }
+
+  private:
+    const DeclContext *CurContext;
+  };
+  PrintCB PCB(&CurContext);
+  PP.Callbacks = &PCB;
+
   QT.print(OS, PP);
   return OS.str();
 }

clang-tools-extra/clangd/unittests/tweaks/ExpandAutoTypeTests.cpp

@@ -65,6 +65,11 @@ TEST_F(ExpandAutoTypeTest, Test) {
   EXPECT_EQ(apply(R"cpp(au^to x = "test";)cpp"),
             R"cpp(const char * x = "test";)cpp");
 
+  EXPECT_EQ(apply("ns::Class * foo() { au^to c = foo(); }"),
+            "ns::Class * foo() { ns::Class * c = foo(); }");
+  EXPECT_EQ(apply("void ns::Func() { au^to x = new ns::Class::Nested{}; }"),
+            "void ns::Func() { Class::Nested * x = new ns::Class::Nested{}; }");
+
   EXPECT_UNAVAILABLE("dec^ltype(au^to) x = 10;");
   // expanding types in structured bindings is syntactically invalid.
   EXPECT_UNAVAILABLE("const ^auto &[x,y] = (int[]){1,2};");

clang-tools-extra/clangd/unittests/tweaks/ExtractFunctionTests.cpp

@@ -97,6 +97,22 @@ void f(const int c) {
 })cpp";
   EXPECT_EQ(apply(ConstCheckInput), ConstCheckOutput);
 
+  // Check const qualifier with namespace
+  std::string ConstNamespaceCheckInput = R"cpp(
+namespace X { struct Y { int z; }; }
+int f(const X::Y &y) {
+  [[return y.z + y.z;]]
+})cpp";
+  std::string ConstNamespaceCheckOutput = R"cpp(
+namespace X { struct Y { int z; }; }
+int extracted(const X::Y &y) {
+return y.z + y.z;
+}
+int f(const X::Y &y) {
+  return extracted(y);
+})cpp";
+  EXPECT_EQ(apply(ConstNamespaceCheckInput), ConstNamespaceCheckOutput);
+
   // Don't extract when we need to make a function as a parameter.
   EXPECT_THAT(apply("void f() { [[int a; f();]] }"), StartsWith("fail"));
 

clang/include/clang/AST/PrettyPrinter.h

@@ -18,6 +18,7 @@
 
 namespace clang {
 
+class DeclContext;
 class LangOptions;
 class SourceManager;
 class Stmt;
@@ -39,6 +40,15 @@ public:
   virtual std::string remapPath(StringRef Path) const {
     return std::string(Path);
   }
+
+  /// When printing type to be inserted into code in specific context, this
+  /// callback can be used to avoid printing the redundant part of the
+  /// qualifier. For example, when inserting code inside namespace foo, we
+  /// should print bar::SomeType instead of foo::bar::SomeType.
+  /// To do this, shouldPrintScope should return true on "foo" NamespaceDecl.
+  /// The printing stops at the first isScopeVisible() == true, so there will
+  /// be no calls with outer scopes.
+  virtual bool isScopeVisible(const DeclContext *DC) const { return false; }
 };
 
 /// Describes how types, statements, expressions, and declarations should be

clang/lib/AST/TypePrinter.cpp

@@ -1225,6 +1225,9 @@ void TypePrinter::AppendScope(DeclContext *DC, raw_ostream &OS,
   if (DC->isFunctionOrMethod())
     return;
 
+  if (Policy.Callbacks && Policy.Callbacks->isScopeVisible(DC))
+    return;
+
   if (const auto *NS = dyn_cast<NamespaceDecl>(DC)) {
     if (Policy.SuppressUnwrittenScope && NS->isAnonymousNamespace())
       return AppendScope(DC->getParent(), OS, NameInScope);