[clangd] Fix modernize-loop-convert "multiple diag in flight" crash.

Summary:
this maybe not ideal, but it is trivial and does fix the crash.

Fixes https://github.com/clangd/clangd/issues/156.

Reviewers: sammccall

Subscribers: ilya-biryukov, MaskRay, jkorous, arphaman, kadircet, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D78715

clang-tools-extra/clang-tidy/modernize/LoopConvertCheck.cpp

@@ -525,13 +525,11 @@ void LoopConvertCheck::doConversion(
     const ValueDecl *MaybeContainer, const UsageResult &Usages,
     const DeclStmt *AliasDecl, bool AliasUseRequired, bool AliasFromForInit,
     const ForStmt *Loop, RangeDescriptor Descriptor) {
-  auto Diag = diag(Loop->getForLoc(), "use range-based for loop instead");
-
   std::string VarName;
   bool VarNameFromAlias = (Usages.size() == 1) && AliasDecl;
   bool AliasVarIsRef = false;
   bool CanCopy = true;
-
+  std::vector<FixItHint> FixIts;
   if (VarNameFromAlias) {
     const auto *AliasVar = cast<VarDecl>(AliasDecl->getSingleDecl());
     VarName = AliasVar->getName().str();
@@ -563,8 +561,8 @@ void LoopConvertCheck::doConversion(
       getAliasRange(Context->getSourceManager(), ReplaceRange);
     }
 
-    Diag << FixItHint::CreateReplacement(
+    FixIts.push_back(FixItHint::CreateReplacement(
-        CharSourceRange::getTokenRange(ReplaceRange), ReplacementText);
+        CharSourceRange::getTokenRange(ReplaceRange), ReplacementText));
     // No further replacements are made to the loop, since the iterator or index
     // was used exactly once - in the initialization of AliasVar.
   } else {
@@ -609,8 +607,8 @@ void LoopConvertCheck::doConversion(
             Usage.Kind == Usage::UK_CaptureByCopy ? "&" + VarName : VarName;
       }
       TUInfo->getReplacedVars().insert(std::make_pair(Loop, IndexVar));
-      Diag << FixItHint::CreateReplacement(
+      FixIts.push_back(FixItHint::CreateReplacement(
-          CharSourceRange::getTokenRange(Range), ReplaceText);
+          CharSourceRange::getTokenRange(Range), ReplaceText));
     }
   }
 
@@ -648,8 +646,9 @@ void LoopConvertCheck::doConversion(
   std::string Range = ("(" + TypeString + " " + VarName + " : " +
                        MaybeDereference + Descriptor.ContainerString + ")")
                           .str();
-  Diag << FixItHint::CreateReplacement(
+  FixIts.push_back(FixItHint::CreateReplacement(
-      CharSourceRange::getTokenRange(ParenRange), Range);
+      CharSourceRange::getTokenRange(ParenRange), Range));
+  diag(Loop->getForLoc(), "use range-based for loop instead") << FixIts;
   TUInfo->getGeneratedDecls().insert(make_pair(Loop, VarName));
 }
 

clang-tools-extra/clangd/unittests/DiagnosticsTests.cpp

@@ -272,6 +272,33 @@ TEST(DiagnosticsTest, ClangTidy) {
                   "use a trailing return type for this function")))));
 }
 
+TEST(DiagnosticTest, NoMultipleDiagnosticInFlight) {
+  Annotations Main(R"cpp(
+    template <typename T> struct Foo {
+      T *begin();
+      T *end();
+    };
+    struct LabelInfo {
+      int a;
+      bool b;
+    };
+
+    void f() {
+      Foo<LabelInfo> label_info_map;
+      [[for]] (auto it = label_info_map.begin(); it != label_info_map.end(); ++it) {
+        auto S = *it;
+      }
+    }
+  )cpp");
+  TestTU TU = TestTU::withCode(Main.code());
+  TU.ClangTidyChecks = "modernize-loop-convert";
+  EXPECT_THAT(
+      TU.build().getDiagnostics(),
+      UnorderedElementsAre(::testing::AllOf(
+          Diag(Main.range(), "use range-based for loop instead"),
+          DiagSource(Diag::ClangTidy), DiagName("modernize-loop-convert"))));
+}
+
 TEST(DiagnosticTest, ClangTidySuppressionComment) {
   Annotations Main(R"cpp(
     int main() {