Build.zig incorrectly handles --build-file argument / build_root

[matklad]

Here's a repro of a simple build script which breaks if used with --build-file argument:

λ pwd
/Users/matklad/tmp/subdir

λ zig build
const std = @import("std");

pub fn build(b: *std.Build) !void {
    const cat = b.addSystemCommand(&.{"cat"});
    cat.addFileArg(b.path("./build.zig"));
    b.default_step = &cat.step;
}

λ cd ..
λ zig build --build-file subdir/build.zig
cat: subdir/build.zig: No such file or directory
run cat failure
error: the following command exited with error code 1:
cat subdir/build.zig
Build Summary: 0/1 steps succeeded; 1 failed (disable with --summary none)
run cat failure
error: the following build command failed with exit code 1:
subdir/.zig-cache/o/cb04b4aa31661e7fda012bf07da1a9ac/build /Users/matklad/p/tb/work/zig/zig subdir subdir/.zig-cache /Users/matklad/.cache/zig --seed 0x8bc81711 -Z62e7f988d57764ec

The problem here is that cat always gets ~/tmp/subdir as its cwd (which looks correct), but it is passed either ./build.zig or ./subdir/build.zig as an argument, and, in the latter case, the file is not found!

This looks like a design bug to me! It feels like b.build_root.path should not exist, the build shouldn't be able to tell where it is running from that easily.