diff --git a/lib/std/mem.zig b/lib/std/mem.zig
index 5ba984ecdc..a40334e587 100644
--- a/lib/std/mem.zig
+++ b/lib/std/mem.zig
@@ -514,6 +514,25 @@ pub fn eql(comptime T: type, a: []const T, b: []const T) bool {
     return true;
 }
 
+/// Compares two slices and returns the index of the first inequality.
+/// Returns null if the slices are equal.
+pub fn indexOfDiff(comptime T: type, a: []const T, b: []const T) ?usize {
+    const shortest = math.min(a.len, b.len);
+    if (a.ptr == b.ptr)
+        return if (a.len == b.len) null else shortest;
+    var index: usize = 0;
+    while (index < shortest) : (index += 1) if (a[index] != b[index]) return index;
+    return if (a.len == b.len) null else shortest;
+}
+
+test "indexOfDiff" {
+    testing.expectEqual(indexOfDiff(u8, "one", "one"), null);
+    testing.expectEqual(indexOfDiff(u8, "one two", "one"), 3);
+    testing.expectEqual(indexOfDiff(u8, "one", "one two"), 3);
+    testing.expectEqual(indexOfDiff(u8, "one twx", "one two"), 6);
+    testing.expectEqual(indexOfDiff(u8, "xne", "one"), 0);
+}
+
 pub const toSliceConst = @compileError("deprecated; use std.mem.spanZ");
 pub const toSlice = @compileError("deprecated; use std.mem.spanZ");
 
diff --git a/lib/std/testing.zig b/lib/std/testing.zig
index 4b629156f6..fed2b15bf5 100644
--- a/lib/std/testing.zig
+++ b/lib/std/testing.zig
@@ -1,4 +1,5 @@
 const std = @import("std.zig");
+const warn = std.debug.warn;
 
 pub const LeakCountAllocator = @import("testing/leak_count_allocator.zig").LeakCountAllocator;
 pub const FailingAllocator = @import("testing/failing_allocator.zig").FailingAllocator;
@@ -212,3 +213,65 @@ test "expectEqual vector" {
 
     expectEqual(a, b);
 }
+
+pub fn expectEqualStrings(expected: []const u8, actual: []const u8) void {
+    if (std.mem.indexOfDiff(u8, actual, expected)) |diff_index| {
+        warn("\n====== expected this output: =========\n", .{});
+        printWithVisibleNewlines(expected);
+        warn("\n======== instead found this: =========\n", .{});
+        printWithVisibleNewlines(actual);
+        warn("\n======================================\n", .{});
+
+        var diff_line_number: usize = 1;
+        for (expected[0..diff_index]) |value| {
+            if (value == '\n') diff_line_number += 1;
+        }
+        warn("First difference occurs on line {}:\n", .{diff_line_number});
+
+        warn("expected:\n", .{});
+        printIndicatorLine(expected, diff_index);
+
+        warn("found:\n", .{});
+        printIndicatorLine(actual, diff_index);
+
+        @panic("test failure");
+    }
+}
+
+fn printIndicatorLine(source: []const u8, indicator_index: usize) void {
+    const line_begin_index = if (std.mem.lastIndexOfScalar(u8, source[0..indicator_index], '\n')) |line_begin|
+        line_begin + 1
+    else
+        0;
+    const line_end_index = if (std.mem.indexOfScalar(u8, source[indicator_index..], '\n')) |line_end|
+        (indicator_index + line_end)
+    else
+        source.len;
+
+    printLine(source[line_begin_index..line_end_index]);
+    {
+        var i: usize = line_begin_index;
+        while (i < indicator_index) : (i += 1)
+            warn(" ", .{});
+    }
+    warn("^\n", .{});
+}
+
+fn printWithVisibleNewlines(source: []const u8) void {
+    var i: usize = 0;
+    while (std.mem.indexOf(u8, source[i..], "\n")) |nl| : (i += nl + 1) {
+        printLine(source[i .. i + nl]);
+    }
+    warn("{}␃\n", .{source[i..]}); // End of Text symbol (ETX)
+}
+
+fn printLine(line: []const u8) void {
+    switch (line[line.len - 1]) {
+        ' ', '\t' => warn("{}⏎\n", .{line}), // Carriage return symbol,
+        else => warn("{}\n", .{line}),
+    }
+}
+
+test "" {
+    expectEqualStrings("foo", "foo");
+}
diff --git a/lib/std/zig/parser_test.zig b/lib/std/zig/parser_test.zig
index a6975929d9..28d3fa0be2 100644
--- a/lib/std/zig/parser_test.zig
+++ b/lib/std/zig/parser_test.zig
@@ -2971,14 +2971,7 @@ fn testTransform(source: []const u8, expected_source: []const u8) !void {
         var failing_allocator = std.testing.FailingAllocator.init(&fixed_allocator.allocator, maxInt(usize));
         var anything_changed: bool = undefined;
         const result_source = try testParse(source, &failing_allocator.allocator, &anything_changed);
-        if (!mem.eql(u8, result_source, expected_source)) {
-            warn("\n====== expected this output: =========\n", .{});
-            warn("{}", .{expected_source});
-            warn("\n======== instead found this: =========\n", .{});
-            warn("{}", .{result_source});
-            warn("\n======================================\n", .{});
-            return error.TestFailed;
-        }
+        std.testing.expectEqualStrings(expected_source, result_source);
         const changes_expected = source.ptr != expected_source.ptr;
         if (anything_changed != changes_expected) {
             warn("std.zig.render returned {} instead of {}\n", .{ anything_changed, changes_expected });