From df3c9e2735f02a7fe8cd80db4db00fec8e13d25f Mon Sep 17 00:00:00 2001
From: Alex Eagle <eagle@post.harvard.edu>
Date: Wed, 18 Aug 2021 08:23:43 -0700
Subject: [PATCH] =?UTF-8?q?diff=5Ftest:=20add=20ability=20for=20caller=20t?=
 =?UTF-8?q?o=20specify=20a=20message=20printed=20on=20fai=E2=80=A6=20(#307?=
 =?UTF-8?q?)?=
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Useful for the use case of a pair of .test and .update targets for checked-in golden files
---
 rules/diff_test.bzl | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/rules/diff_test.bzl b/rules/diff_test.bzl
index 71faf40..acde2ea 100644
--- a/rules/diff_test.bzl
+++ b/rules/diff_test.bzl
@@ -58,7 +58,7 @@ if "!RF2!" equ "" (
 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
+    echo>&2 FAIL: files "{file1}" and "{file2}" differ. {fail_msg}
     exit /b 1
   ) else (
     fc.exe /B "!RF1!" "!RF2!"
@@ -66,6 +66,7 @@ if %ERRORLEVEL% neq 0 (
   )
 )
 """.format(
+                fail_msg = ctx.attr.failure_message,
                 file1 = _runfiles_path(ctx.file.file1),
                 file2 = _runfiles_path(ctx.file.file2),
             ),
@@ -95,10 +96,11 @@ else
   exit 1
 fi
 if ! diff "$RF1" "$RF2"; then
-  echo >&2 "FAIL: files \"{file1}\" and \"{file2}\" differ"
+  echo >&2 "FAIL: files \"{file1}\" and \"{file2}\" differ. {fail_msg}"
   exit 1
 fi
 """.format(
+                fail_msg = ctx.attr.failure_message,
                 file1 = _runfiles_path(ctx.file.file1),
                 file2 = _runfiles_path(ctx.file.file2),
             ),
@@ -112,6 +114,7 @@ fi
 
 _diff_test = rule(
     attrs = {
+        "failure_message": attr.string(),
         "file1": attr.label(
             allow_single_file = True,
             mandatory = True,