const subsystem: ?std.Target.SubSystem = blk: {
    if (@hasDecl(builtin, "explicit_subsystem")) break :blk builtin.explicit_subsystem;
    switch (builtin.os.tag) {
        .windows => {
            if (builtin.is_test) {
                break :blk std.Target.SubSystem.Console;
            }
            if (@hasDecl(root, "main") or
                @hasDecl(root, "WinMain") or
                @hasDecl(root, "wWinMain") or
                @hasDecl(root, "WinMainCRTStartup") or
                @hasDecl(root, "wWinMainCRTStartup"))
            {
                break :blk std.Target.SubSystem.Windows;
            } else {
                break :blk std.Target.SubSystem.Console;
            }
        },
        else => break :blk null,
    }
};

explicit_subsystem is missing when the subsystem is automatically detected, so Zig standard library has the subsystem detection logic here. This should generally be used rather than explicit_subsystem. On non-Windows targets, this is null.