Printing failure in test

matklad · May 14, 2026, 5:44pm

What’s the sanctioned way to communicate details about failure in tests?

std.testing is using this function:

https://codeberg.org/ziglang/zig/src/commit/bdfbf432dd626b495cee0a8aa53e77b4ef718ff2/lib/std/testing.zig#L52-L58

But that’s private. I can use std.debug.print directly, but that feels like perhaps the wrong abstraction layer here? Ideally, tests shouldn’t assume that stderr is anything in particular, I would think?

Southporter · May 14, 2026, 6:08pm

Can you not use std.log for that? I’ve used this in my testing to output more information. You may have to set the std.testing.log_level if you want more detaild logs (default is warn).

matklad · May 14, 2026, 6:14pm

I can(and do), but am I supposed to, given than stdlib doesn’t?

Southporter · May 14, 2026, 6:16pm

github.com/ziglang/zig

Integrate std.log with std.testing

opened 06:38AM - 28 Jun 20 UTC

andrewrk

enhancement contributor friendly standard library

Inspired by https://github.com/ziglang/zig/pull/5734#discussion_r446585745 Th…e goal of the `std.log` API is to allow developers to leave log statements in finished code, without causing any performance issues or unwanted stderr/stdout for API consumers. This harmonizes with testing! Here's an example: ```zig const std = @import("std"); const testing = std.testing; const log = std.log; test "make sure an uncommon branch is hit" { testing.expect(myCoolCode(3, 4) == 7); testing.expect(myCoolCode(999, 100) == 100); testing.expectLog("a special case occurred: 999\n"); } fn myCoolCode(a: i32, b: i32) i32 { if (a > 100) { log.info(.scope, "a special case occurred: {}\n", .{a}); return b; } return a + b; } ``` Much like the [test runner](https://github.com/ziglang/zig/blob/master/lib/std/special/test_runner.zig) clears the testing allocator and checks for leaks between tests, it can also specify a custom log handler, which buffers the log statements during tests, and provides the API to make sure log messages did or did not occur. It could provide an API for exact matches, substring matches, or checking for the absence of a log message. An open question would be how the "scope" parameter fits into this. It could be part of the API for checking, or maybe there would be a `testing` scope, which by default would never get printed and only would be activated for tests.

Looks like it is expected behavior. It seems like they want users to be able to do the std.log stuff they normally would and have it work with the default test runner. It’s still an open issue, so it’s not fully implemented, but at least confirms the direction.