Index: test/Misc/find-diagnostic-id.c
===================================================================
--- /dev/null
+++ test/Misc/find-diagnostic-id.c
@@ -0,0 +1,5 @@
+// RUN: diagtool find-diagnostic-id warn_unused_variable | FileCheck %s
+// RUN: not diagtool find-diagnostic-id warn_unused_vars 2>&1 | FileCheck --check-prefix=ERROR %s
+
+// CHECK: {{^[0-9]+$}}
+// ERROR: error: invalid diagnostic 'warn_unused_vars'
Index: tools/diagtool/CMakeLists.txt
===================================================================
--- tools/diagtool/CMakeLists.txt
+++ tools/diagtool/CMakeLists.txt
@@ -6,6 +6,7 @@
   diagtool_main.cpp
   DiagTool.cpp
   DiagnosticNames.cpp
+  FindDiagnosticID.cpp
   ListWarnings.cpp
   ShowEnabledWarnings.cpp
   TreeView.cpp
Index: tools/diagtool/FindDiagnosticID.cpp
===================================================================
--- /dev/null
+++ tools/diagtool/FindDiagnosticID.cpp
@@ -0,0 +1,58 @@
+//===- FindDiagnosticID.cpp - diagtool tool for finding diagnostic id -----===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#include "DiagTool.h"
+#include "DiagnosticNames.h"
+#include "clang/Basic/AllDiagnostics.h"
+#include "llvm/Support/CommandLine.h"
+
+DEF_DIAGTOOL("find-diagnostic-id", "Print the id of the given diagnostic",
+             FindDiagnosticID)
+
+using namespace clang;
+using namespace diagtool;
+
+static Optional<DiagnosticRecord>
+findDiagnostic(ArrayRef<DiagnosticRecord> Diagnostics, StringRef Name) {
+  for (const auto &Diag : Diagnostics) {
+    StringRef DiagName = Diag.getName();
+    if (DiagName == Name)
+      return Diag;
+  }
+  return None;
+}
+
+int FindDiagnosticID::run(unsigned int argc, char **argv,
+                          llvm::raw_ostream &OS) {
+  static llvm::cl::OptionCategory FindDiagnosticIDOptions(
+      "diagtool find-diagnostic-id options");
+
+  static llvm::cl::opt<std::string> DiagnosticName(
+      llvm::cl::Positional, llvm::cl::desc("<diagnostic-name>"),
+      llvm::cl::Required, llvm::cl::cat(FindDiagnosticIDOptions));
+
+  std::vector<const char *> Args;
+  Args.push_back("find-diagnostic-id");
+  for (const char *A : llvm::makeArrayRef(argv, argc))
+    Args.push_back(A);
+
+  llvm::cl::HideUnrelatedOptions(FindDiagnosticIDOptions);
+  llvm::cl::ParseCommandLineOptions((int)Args.size(), Args.data(),
+                                    "Diagnostic ID mapping utility");
+
+  ArrayRef<DiagnosticRecord> AllDiagnostics = getBuiltinDiagnosticsByName();
+  Optional<DiagnosticRecord> Diag =
+      findDiagnostic(AllDiagnostics, DiagnosticName);
+  if (!Diag) {
+    llvm::errs() << "error: invalid diagnostic '" << DiagnosticName << "'\n";
+    return 1;
+  }
+  OS << Diag->DiagID << "\n";
+  return 0;
+}