`std.Build` conflates libc configuration for host and cross targets

[tranzystorekk]

Zig Version

0.13.0

Steps to Reproduce and Observed Behavior

As observed in ghostty-org/ghostty#3218 zig projects that build e.g. a helper executable meant to be run on host target, attempt to link it to cross target libc, which leads to build failures.

Expected Behavior

Build artifacts mentioned above should build correctly for cross target configurations.

[ifreund]

To add a bit of detail on what is happening here, the problem arises when setting the libc for the target with --libc foo.txt. The zig build system does not provide a way to configure the host and target libc separately and all the code I see seems to assume that they are the same, which is obviously not correct for cross builds.

This is a problem when the build.zig of a project such as ghostty needs to build and run helper tools on the host as part of the build process.

[BratishkaErik]

As a workaround, would separating themnto zig build run-unigen + zig build --libc also work (if cache system does not track libc file change), or does it break too?

[BratishkaErik]

To add a bit of detail on what is happening here, the problem arises when setting the libc for the target with --libc foo.txt. The zig build system does not provide a way to configure the host and target libc separately and all the code I see seems to assume that they are the same, which is obviously not correct for cross builds.

I have been making parseZonFile at #22552 to help in the future with #20327, but considering new circumstances maybe I can adapt it sooner. What if we had this format as libc.zon:

.{
    // All omitted or null paths are skipped
    .host = .{
        .include_dir = "/whatever/for/x86_64-build-host/",
        // ..
    },
    .target = .{
        .include_dir = "/whatever/for/aarch64-target",
    },
}

?

[tranzystorekk]

I remembered that this issue exists conveniently now that 0.14 is almost released, afaik zon file parsing made it in, so this might be a good time to revive discussion about the libc config