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

[hokein]

Summary

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

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

[sammccall]

Ah, nice work!
Agree it's a hack (and how many more instances are lurking) but I don't know how better to fix it and it's been reported 3 times...

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

529

I'm not sure we need to mention clangd here - I'd expect standalone clang-tidy to be able to hit the same condition?
At least logically I think the bug exists either way.

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

275

Does the bug repro with clang-tidy? If so it'd be nice to make this a clang-tidy test.

[hokein]

refine the fix.

[hokein]

In D78715#1999236, @sammccall wrote:

Ah, nice work!
Agree it's a hack (and how many more instances are lurking) but I don't know how better to fix it and it's been reported 3 times...

this is the only place in clang-tidy calling getTypeInfo.

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

529

yeah logically, but it doesn't trigger the assertion somehow (haven't dig into it). removed it.

I realized that the code below may change Descriptor.ElemType, refined the fix (we store the fixits, and emit the diagnostic at the end of the function).

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

275

unfortunately, not for standalone clang-tidy :(

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

 
 /// Computes the changes needed to convert a given for loop, and
 /// applies them.
 void LoopConvertCheck::doConversion(
     ASTContext *Context, const VarDecl *IndexVar,
     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();
 
     // Use the type of the alias if it's not the same
     QualType AliasVarType = AliasVar->getType();
     assert(!AliasVarType.isNull() && "Type in VarDecl is null");
     if (AliasVarType->isReferenceType()) {
if (AliasUseRequired) {
       // in a for loop's init clause. Need to put this ';' back while removing
       // the declaration of the alias variable. This is probably a bug.
       ReplacementText = ";";
     } else {
       // Avoid leaving empty lines or trailing whitespaces.
       getAliasRange(Context->getSourceManager(), ReplaceRange);
     }
 
-    Diag << FixItHint::CreateReplacement(
-        CharSourceRange::getTokenRange(ReplaceRange), ReplacementText);
+    FixIts.push_back(FixItHint::CreateReplacement(
+        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 {
     VariableNamer Namer(&TUInfo->getGeneratedDecls(),
                         &TUInfo->getParentFinder().getStmtToParentStmtMap(),
                         Loop, IndexVar, MaybeContainer, Context, NamingStyle);
     VarName = Namer.createIndexName();
     // First, replace all usages of the array subscript expression with our new
for (const auto &Usage : Usages) {
       } else {
         // The Usage expression is only null in case of lambda captures (which
         // are VarDecl). If the index is captured by value, add '&' to capture
         // by reference instead.
         ReplaceText =
             Usage.Kind == Usage::UK_CaptureByCopy ? "&" + VarName : VarName;
       }
       TUInfo->getReplacedVars().insert(std::make_pair(Loop, IndexVar));
-      Diag << FixItHint::CreateReplacement(
-          CharSourceRange::getTokenRange(Range), ReplaceText);
+      FixIts.push_back(FixItHint::CreateReplacement(
+          CharSourceRange::getTokenRange(Range), ReplaceText));
     }
   }
 
   // Now, we need to construct the new range expression.
   SourceRange ParenRange(Loop->getLParenLoc(), Loop->getRParenLoc());
 
   QualType Type = Context->getAutoDeductType();
   if (!Descriptor.ElemType.isNull() && Descriptor.ElemType->isFundamentalType())
if (!UseCopy) {
     }
   }
 
   StringRef MaybeDereference = Descriptor.ContainerNeedsDereference ? "*" : "";
   std::string TypeString = Type.getAsString(getLangOpts());
   std::string Range = ("(" + TypeString + " " + VarName + " : " +
                        MaybeDereference + Descriptor.ContainerString + ")")
                           .str();
-  Diag << FixItHint::CreateReplacement(
-      CharSourceRange::getTokenRange(ParenRange), Range);
+  FixIts.push_back(FixItHint::CreateReplacement(
+      CharSourceRange::getTokenRange(ParenRange), Range));
+  diag(Loop->getForLoc(), "use range-based for loop instead") << FixIts;
   TUInfo->getGeneratedDecls().insert(make_pair(Loop, VarName));
 }
 
 /// Returns a string which refers to the container iterated over.
 StringRef LoopConvertCheck::getContainerString(ASTContext *Context,
                                                const ForStmt *Loop,
                                                const Expr *ContainerExpr) {
   StringRef ContainerString;

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

EXPECT_THAT(
                    "use a trailing return type for this function"),
               DiagSource(Diag::ClangTidy),
               DiagName("modernize-use-trailing-return-type"),
               // Verify that we don't have "[check-name]" suffix in the message.
               WithFix(FixMessage(
                   "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() {
       int i = 3;
       double d = 8 / i;  // NOLINT
       // NOLINTNEXTLINE
       double e = 8 / i;
       #define BAD 8 / i