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diff --git a/rules/diff_test.bzl b/rules/diff_test.bzl
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+# Copyright 2019 The Bazel Authors. All rights reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#    http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""A test rule that compares two binary files.
+
+The rule uses a Bash command (diff) on Linux/macOS/non-Windows, and a cmd.exe
+command (fc.exe) on Windows (no Bash is required).
+"""
+
+def _runfiles_path(f):
+    if f.root.path:
+        return f.path[len(f.root.path) + 1:]  # generated file
+    else:
+        return f.path  # source file
+
+def _diff_test_impl(ctx):
+    if ctx.attr.is_windows:
+        test_bin = ctx.actions.declare_file(ctx.label.name + "-test.bat")
+        ctx.actions.write(
+            output = test_bin,
+            content = """@rem Generated by diff_test.bzl, do not edit.
+@echo off
+SETLOCAL ENABLEEXTENSIONS
+SETLOCAL ENABLEDELAYEDEXPANSION
+set MF=%RUNFILES_MANIFEST_FILE:/=\\%
+set PATH=%SYSTEMROOT%\\system32
+set F1={file1}
+set F2={file2}
+if "!F1:~0,9!" equ "external/" (set F1=!F1:~9!) else (set F1=!TEST_WORKSPACE!/!F1!)
+if "!F2:~0,9!" equ "external/" (set F2=!F2:~9!) else (set F2=!TEST_WORKSPACE!/!F2!)
+for /F "tokens=2* usebackq" %%i in (`findstr.exe /l /c:"!F1! " "%MF%"`) do (
+  set RF1=%%i
+  set RF1=!RF1:/=\\!
+)
+if "!RF1!" equ "" (
+  echo>&2 ERROR: !F1! not found
+  exit /b 1
+)
+for /F "tokens=2* usebackq" %%i in (`findstr.exe /l /c:"!F2! " "%MF%"`) do (
+  set RF2=%%i
+  set RF2=!RF2:/=\\!
+)
+if "!RF2!" equ "" (
+  echo>&2 ERROR: !F2! not found
+  exit /b 1
+)
+fc.exe 2>NUL 1>NUL /B "!RF1!" "!RF2!"
+if %ERRORLEVEL% neq 0 (
+  if %ERRORLEVEL% equ 1 (
+    echo>&2 FAIL: files "{file1}" and "{file2}" differ
+    exit /b 1
+  ) else (
+    fc.exe /B "!RF1!" "!RF2!"
+    exit /b %errorlevel%
+  )
+)
+""".format(
+                file1 = _runfiles_path(ctx.file.file1),
+                file2 = _runfiles_path(ctx.file.file2),
+            ),
+            is_executable = True,
+        )
+    else:
+        test_bin = ctx.actions.declare_file(ctx.label.name + "-test.sh")
+        ctx.actions.write(
+            output = test_bin,
+            content = r"""#!/bin/bash
+set -euo pipefail
+F1="{file1}"
+F2="{file2}"
+[[ "$F1" =~ ^external/* ]] && F1="${{F1#external/}}" || F1="$TEST_WORKSPACE/$F1"
+[[ "$F2" =~ ^external/* ]] && F2="${{F2#external/}}" || F2="$TEST_WORKSPACE/$F2"
+if [[ -d "${{RUNFILES_DIR:-/dev/null}}" && "${{RUNFILES_MANIFEST_ONLY:-}}" != 1 ]]; then
+  RF1="$RUNFILES_DIR/$F1"
+  RF2="$RUNFILES_DIR/$F2"
+elif [[ -f "${{RUNFILES_MANIFEST_FILE:-/dev/null}}" ]]; then
+  RF1="$(grep -F -m1 "$F1 " "$RUNFILES_MANIFEST_FILE" | sed 's/^[^ ]* //')"
+  RF2="$(grep -F -m1 "$F2 " "$RUNFILES_MANIFEST_FILE" | sed 's/^[^ ]* //')"
+elif [[ -f "$TEST_SRCDIR/$F1" && -f "$TEST_SRCDIR/$F2" ]]; then
+  RF1="$TEST_SRCDIR/$F1"
+  RF2="$TEST_SRCDIR/$F2"
+else
+  echo >&2 "ERROR: could not find \"{file1}\" and \"{file2}\""
+  exit 1
+fi
+if ! diff "$RF1" "$RF2"; then
+  echo >&2 "FAIL: files \"{file1}\" and \"{file2}\" differ"
+  exit 1
+fi
+""".format(
+                file1 = _runfiles_path(ctx.file.file1),
+                file2 = _runfiles_path(ctx.file.file2),
+            ),
+            is_executable = True,
+        )
+    return DefaultInfo(
+        executable = test_bin,
+        files = depset(direct = [test_bin]),
+        runfiles = ctx.runfiles(files = [test_bin, ctx.file.file1, ctx.file.file2]),
+    )
+
+_diff_test = rule(
+    attrs = {
+        "file1": attr.label(
+            allow_single_file = True,
+            mandatory = True,
+        ),
+        "file2": attr.label(
+            allow_single_file = True,
+            mandatory = True,
+        ),
+        "is_windows": attr.bool(mandatory = True),
+    },
+    test = True,
+    implementation = _diff_test_impl,
+)
+
+def diff_test(name, file1, file2, **kwargs):
+    """A test that compares two files.
+
+    The test succeeds if the files' contents match.
+
+    Args:
+      name: The name of the test rule.
+      file1: Label of the file to compare to <code>file2</code>.
+      file2: Label of the file to compare to <code>file1</code>.
+      **kwargs: The <a href="https://docs.bazel.build/versions/master/be/common-definitions.html#common-attributes-tests">common attributes for tests</a>.
+    """
+    _diff_test(
+        name = name,
+        file1 = file1,
+        file2 = file2,
+        is_windows = select({
+            "@bazel_tools//src/conditions:host_windows": True,
+            "//conditions:default": False,
+        }),
+        **kwargs
+    )