Fix indexing in the test runner's log formatting

[LewisGaul]

Without this fix the following fails due to the index of the args being changed by the test runner log function.

const std = @import("std");
test {
    std.log.warn("{[0]d}, 0x{[0]X}", .{123});
}

[LewisGaul]

The failing CI logs shows all tests passing (2094 passed; 59 skipped; 0 failed.) but "Step is killed (canceled)" - I don't think this is an issue with the PR.

[marler8997]

Bleh didn't know about this potential issue until now.

Combining multiple format strings/arg tuples is something I do from time to time. Here's a solution I tried, it's a bit of a monstrosity. The direct call to multiFormat works but the logWithPrefixExample segfaults, I think Zig might have issues with tuples within tuples?

const std = @import("std");

pub fn multiFormat(
    writer: anytype,
    comptime fmts: []const []const u8,
    arg_tuples: anytype,
) !void {
    comptime std.debug.assert(fmts.len == arg_tuples.len);
    // NOTE: this should work but maybe we can do something more efficient here?
    inline for (fmts) |fmt, i| {
        try std.fmt.format(writer, fmt, arg_tuples[i]);
    }
}

pub fn logWithPrefixExample(level: []const u8, comptime fmt: []const u8, args: anytype) !void {
    const stderr = std.io.getStdErr().writer();
    std.debug.getStderrMutex().lock();
    defer std.debug.getStderrMutex().unlock();
    try multiFormat(
        stderr,
        &.{ "[{s}] ", fmt },
        .{ .{ level } , .{ args } }
    );
}

pub fn main() !void {
    try multiFormat(
        std.io.getStdOut().writer(),
        &.{ "hello '{0s}'", ", how are you doing '{0s}'?\n"},
        .{ .{ "person" }, .{ "today" }});
    // uncomment to try this, but it will segfault
    //try logWithPrefixExample("yo", "hello again '{0s}'", .{ "person" });
}

[motiejus]

I think Zig might have issues with tuples within tuples?

Yes, see #3915 . Anonymous tuples in anonymous tuples don't really work in stage1.