std.debug.lockStderrWriter: also return ttyconf

`std.Io.tty.Config.detect` may be an expensive check (e.g. involving
syscalls), and doing it every time we need to print isn't really
necessary; under normal usage, we can compute the value once and cache
it for the whole program's execution. Since anyone outputting to stderr
may reasonably want this information (in fact they are very likely to),
it makes sense to cache it and return it from `lockStderrWriter`. Call
sites who do not need it will experience no significant overhead, and
can just ignore the TTY config with a `const w, _` destructure.

1 files changed, 4 insertions, 4 deletions

diff --git a/lib/std/testing.zig b/lib/std/testing.zig
index 7cef6f9c58..b99542e7e5 100644
--- a/lib/std/testing.zig
+++ b/lib/std/testing.zig
@@ -355,7 +355,7 @@ test expectApproxEqRel {
 /// This function is intended to be used only in tests. When the two slices are not
 /// equal, prints diagnostics to stderr to show exactly how they are not equal (with
 /// the differences highlighted in red), then returns a test failure error.
-/// The colorized output is optional and controlled by the return of `std.Io.tty.detectConfig()`.
+/// The colorized output is optional and controlled by the return of `std.Io.tty.Config.detect`.
 /// If your inputs are UTF-8 encoded strings, consider calling `expectEqualStrings` instead.
 pub fn expectEqualSlices(comptime T: type, expected: []const T, actual: []const T) !void {
     const diff_index: usize = diff_index: {
@@ -367,9 +367,9 @@ pub fn expectEqualSlices(comptime T: type, expected: []const T, actual: []const
         break :diff_index if (expected.len == actual.len) return else shortest;
     };
     if (!backend_can_print) return error.TestExpectedEqual;
-    const stderr_w = std.debug.lockStderrWriter(&.{});
+    const stderr_w, const ttyconf = std.debug.lockStderrWriter(&.{});
     defer std.debug.unlockStderrWriter();
-    failEqualSlices(T, expected, actual, diff_index, stderr_w) catch {};
+    failEqualSlices(T, expected, actual, diff_index, stderr_w, ttyconf) catch {};
     return error.TestExpectedEqual;
 }
 
@@ -379,6 +379,7 @@ fn failEqualSlices(
     actual: []const T,
     diff_index: usize,
     w: *std.Io.Writer,
+    ttyconf: std.Io.tty.Config,
 ) !void {
     try w.print("slices differ. first difference occurs at index {d} (0x{X})\n", .{ diff_index, diff_index });
 
@@ -398,7 +399,6 @@ fn failEqualSlices(
     const actual_window = actual[window_start..@min(actual.len, window_start + max_window_size)];
     const actual_truncated = window_start + actual_window.len < actual.len;
 
-    const ttyconf = std.Io.tty.detectConfig(.stderr());
     var differ = if (T == u8) BytesDiffer{
         .expected = expected_window,
         .actual = actual_window,