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diff --git a/clang/lib/CodeGen/TrapReasonBuilder.h b/clang/lib/CodeGen/TrapReasonBuilder.h
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+++ b/clang/lib/CodeGen/TrapReasonBuilder.h
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+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+///
+/// \file
+/// This file contains the declaration of TrapReasonBuilder and related classes.
+///
+//===----------------------------------------------------------------------===//
+#ifndef LLVM_CLANG_CODEGEN_TRAP_REASON_BUILDER_H
+#define LLVM_CLANG_CODEGEN_TRAP_REASON_BUILDER_H
+#include "clang/Basic/Diagnostic.h"
+
+namespace clang {
+namespace CodeGen {
+
+/// Helper class for \class TrapReasonBuilder. \class TrapReason stores the
+/// "trap reason" built by \class TrapReasonBuilder. This consists of
+/// a trap message and trap category.
+///
+/// It is intended that this object be allocated on the stack.
+class TrapReason {
+public:
+  TrapReason() = default;
+  /// \return The trap message. Note the lifetime of the underlying storage for
+  /// the returned StringRef lives in this class which means the returned
+  /// StringRef should not be used after this class is destroyed.
+  StringRef getMessage() const { return Message; }
+
+  /// \return the trap category (e.g. "Undefined Behavior Sanitizer")
+  StringRef getCategory() const { return Category; }
+
+  bool isEmpty() const {
+    // Note both Message and Category are checked because it is legitimate for
+    // the Message to be empty but for the Category to be non-empty when the
+    // trap category is known but the specific reason is not available during
+    // codegen.
+    return Message.size() == 0 && Category.size() == 0;
+  }
+
+private:
+  llvm::SmallString<64> Message;
+  // The Category doesn't need its own storage because the StringRef points
+  // to a global constant string.
+  StringRef Category;
+
+  // Only this class can set the private fields.
+  friend class TrapReasonBuilder;
+};
+
+/// Class to make it convenient to initialize TrapReason objects which can be
+/// used to attach the "trap reason" to trap instructions.
+///
+/// Although this class inherits from \class DiagnosticBuilder it has slightly
+/// different semantics.
+///
+/// * This class should only be used with trap diagnostics (declared in
+/// `DiagnosticTrapKinds.td`).
+/// * The `TrapReasonBuilder` does not emit diagnostics to the normal
+///   diagnostics consumers on destruction like normal Diagnostic builders.
+///   Instead on destruction it assigns to the TrapReason object passed into
+///   the constructor.
+///
+/// Given that this class inherits from `DiagnosticBuilder` it inherits all of
+/// its abilities to format diagnostic messages and consume various types in
+/// class (e.g. Type, Exprs, etc.). This makes it particularly suited to
+/// printing types and expressions from the AST while codegen-ing runtime
+/// checks.
+///
+///
+/// Example use via the `CodeGenModule::BuildTrapReason` helper.
+///
+/// \code
+/// {
+///   TrapReason TR;
+///   CGM.BuildTrapReason(diag::trap_diagnostic, TR) << 0 << SomeExpr;
+///   consume(&TR);
+/// }
+/// \endcode
+///
+///
+class TrapReasonBuilder : public DiagnosticBuilder {
+public:
+  TrapReasonBuilder(DiagnosticsEngine *DiagObj, unsigned DiagID,
+                    TrapReason &TR);
+  ~TrapReasonBuilder();
+
+  // Prevent accidentally copying or assigning
+  TrapReasonBuilder &operator=(const TrapReasonBuilder &) = delete;
+  TrapReasonBuilder &operator=(const TrapReasonBuilder &&) = delete;
+  TrapReasonBuilder(const TrapReasonBuilder &) = delete;
+  TrapReasonBuilder(const TrapReasonBuilder &&) = delete;
+
+private:
+  /// \return Format the trap message into `Storage`.
+  void getMessage(SmallVectorImpl<char> &Storage);
+
+  /// \return Return the trap category. These are the `CategoryName` property
+  /// of `trap` diagnostics declared in `DiagnosticTrapKinds.td`.
+  StringRef getCategory();
+
+private:
+  TrapReason &TR;
+};
+
+} // namespace CodeGen
+} // namespace clang
+
+#endif