[flang] Ensure lowering diagnostic handler does not outlive lowering

When the LoweringBridge is created, it registers an MLIR Diagnostics
handler with the MLIRContext. However, it never deregisters it once
lowering is finished.

This fixes this particular scenario. It also makes it so that the
Diagnostics handler is optional.

Author: Razvan Lupusoru

flang/include/flang/Lower/Bridge.h

@@ -76,6 +76,7 @@ class LoweringBridge {
                           loweringOptions, envDefaults, languageFeatures,
                           targetMachine, targetOptions, codeGenOptions);
   }
+  ~LoweringBridge();
 
   //===--------------------------------------------------------------------===//
   // Getters
@@ -174,6 +175,7 @@ class LoweringBridge {
   const std::vector<Fortran::lower::EnvironmentDefault> &envDefaults;
   const Fortran::common::LanguageFeatureControl &languageFeatures;
   std::set<std::string> tempNames;
+  std::optional<mlir::DiagnosticEngine::HandlerID> diagHandlerID;
 };
 
 } // namespace lower

flang/include/flang/Lower/LoweringOptions.def

@@ -74,5 +74,10 @@ ENUM_LOWERINGOPT(SkipExternalRttiDefinition, unsigned, 1, 0)
 /// If false, lower to the complex dialect of MLIR.
 /// On by default.
 ENUM_LOWERINGOPT(ComplexDivisionToRuntime, unsigned, 1, 1)
+
+/// When true, it registers MLIRDiagnosticsHandler for the duration
+/// of the lowering pipeline.
+ENUM_LOWERINGOPT(RegisterMLIRDiagnosticsHandler, unsigned, 1, 1)
+
 #undef LOWERINGOPT
 #undef ENUM_LOWERINGOPT

flang/lib/Lower/Bridge.cpp

@@ -6707,27 +6707,30 @@ Fortran::lower::LoweringBridge::LoweringBridge(
       loweringOptions{loweringOptions}, envDefaults{envDefaults},
       languageFeatures{languageFeatures} {
   // Register the diagnostic handler.
-  context.getDiagEngine().registerHandler([](mlir::Diagnostic &diag) {
-    llvm::raw_ostream &os = llvm::errs();
-    switch (diag.getSeverity()) {
-    case mlir::DiagnosticSeverity::Error:
-      os << "error: ";
-      break;
-    case mlir::DiagnosticSeverity::Remark:
-      os << "info: ";
-      break;
-    case mlir::DiagnosticSeverity::Warning:
-      os << "warning: ";
-      break;
-    default:
-      break;
-    }
-    if (!mlir::isa<mlir::UnknownLoc>(diag.getLocation()))
-      os << diag.getLocation() << ": ";
-    os << diag << '\n';
-    os.flush();
-    return mlir::success();
-  });
+  if (loweringOptions.getRegisterMLIRDiagnosticsHandler()) {
+    diagHandlerID =
+        context.getDiagEngine().registerHandler([](mlir::Diagnostic &diag) {
+          llvm::raw_ostream &os = llvm::errs();
+          switch (diag.getSeverity()) {
+          case mlir::DiagnosticSeverity::Error:
+            os << "error: ";
+            break;
+          case mlir::DiagnosticSeverity::Remark:
+            os << "info: ";
+            break;
+          case mlir::DiagnosticSeverity::Warning:
+            os << "warning: ";
+            break;
+          default:
+            break;
+          }
+          if (!mlir::isa<mlir::UnknownLoc>(diag.getLocation()))
+            os << diag.getLocation() << ": ";
+          os << diag << '\n';
+          os.flush();
+          return mlir::success();
+        });
+  }
 
   auto getPathLocation = [&semanticsContext, &context]() -> mlir::Location {
     std::optional<std::string> path;
@@ -6769,6 +6772,12 @@ Fortran::lower::LoweringBridge::LoweringBridge(
     fir::setCommandline(*module, *cgOpts.RecordCommandLine);
 }
 
+Fortran::lower::LoweringBridge::~LoweringBridge() {
+  if (diagHandlerID) {
+    context.getDiagEngine().eraseHandler(*diagHandlerID);
+  }
+}
+
 void Fortran::lower::genCleanUpInRegionIfAny(
     mlir::Location loc, fir::FirOpBuilder &builder, mlir::Region &region,
     Fortran::lower::StatementContext &context) {