add option to pass args to diff in diff_test

lib/private/BUILD.bazel

@@ -104,6 +104,7 @@ bzl_library(
     deps = [
         ":directory_path",
         "//lib:utils",
+        "@bazel_skylib//lib:shell",
     ],
 )
 

lib/private/diff_test.bzl

@@ -21,6 +21,7 @@ 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).
 """
 
+load("@bazel_skylib//lib:shell.bzl", "shell")
 load(":directory_path.bzl", "DirectoryPathInfo")
 
 def _runfiles_path(f):
@@ -71,6 +72,10 @@ def _diff_test_impl(ctx):
             "{file1}": file1_path,
             "{file2}": file2_path,
             "{build_file_path}": ctx.build_file_path,
+            "{diff_args}": " ".join([
+                shell.quote(arg)
+                for arg in ctx.attr.diff_args
+            ]),
         },
         is_executable = True,
     )
@@ -92,6 +97,7 @@ _diff_test = rule(
             allow_files = True,
             mandatory = True,
         ),
+        "diff_args": attr.string_list(),
         "_windows_constraint": attr.label(default = "@platforms//os:windows"),
         "_diff_test_tmpl_sh": attr.label(
             default = ":diff_test_tmpl.sh",
@@ -106,7 +112,7 @@ _diff_test = rule(
     implementation = _diff_test_impl,
 )
 
-def diff_test(name, file1, file2, size = "small", **kwargs):
+def diff_test(name, file1, file2, diff_args = [], size = "small", **kwargs):
     """A test that compares two files.
 
     The test succeeds if the files' contents match.
@@ -115,6 +121,7 @@ def diff_test(name, file1, file2, size = "small", **kwargs):
       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>.
+      diff_args: Arguments to pass to the `diff` command. (Ignored on Windows)
       size: standard attribute for tests
       **kwargs: The <a href="https://docs.bazel.build/versions/main/be/common-definitions.html#common-attributes-tests">common attributes for tests</a>.
     """
@@ -123,5 +130,6 @@ def diff_test(name, file1, file2, size = "small", **kwargs):
         file1 = file1,
         file2 = file2,
         size = size,
+        diff_args = diff_args,
         **kwargs
     )

lib/private/diff_test_tmpl.sh

@@ -72,11 +72,11 @@ if [[ ! "$DF1" ]] && [[ "$DF2" ]]; then
   exit 1
 fi
 if [[ "$DF1" ]] || [[ "$DF2" ]]; then
-  if ! diff -r "$RF1" "$RF2"; then
+  if ! diff {diff_args} -r "$RF1" "$RF2"; then
     fail "directories \"{file1}\" and \"{file2}\" differ. {fail_msg}"
   fi
 else
-  if ! diff "$RF1" "$RF2"; then
+  if ! diff {diff_args} "$RF1" "$RF2"; then
     fail "files \"{file1}\" and \"{file2}\" differ. {fail_msg}"
   fi
 fi

lib/private/write_source_file.bzl

@@ -22,6 +22,7 @@ def write_source_file(
         diff_test = True,
         diff_test_failure_message = "{{DEFAULT_MESSAGE}}",
         file_missing_failure_message = "{{DEFAULT_MESSAGE}}",
+        diff_args = [],
         check_that_out_file_exists = True,
         **kwargs):
     """Write a file or directory to the source tree.
@@ -66,6 +67,8 @@ def write_source_file(
         file_missing_failure_message: Text to print when the output file is missing. Subject to the same
              substitutions as diff_test_failure_message.
 
+        diff_args: Arguments to pass to the `diff` command. (Ignored on Windows)
+
         check_that_out_file_exists: Test that the output file exists and print a helpful error message if it doesn't.
 
             If `True`, the output file or directory must be in the same containing Bazel package as the target since the underlying mechanism
@@ -177,6 +180,7 @@ To update *only* this file, run:
             file1 = in_file,
             file2 = out_file,
             failure_message = message,
+            diff_args = diff_args,
             **kwargs
         )