Zig simple runner through function pointer

fnproto_runner.zig

const std = @import("std");
const builtin = @import("builtin");
const FnProto = switch (builtin.zig_backend) {
    // workaround until we replace stage1 with stage2
    .stage1 => ?fn (*Runner) anyerror!void,
    else => ?*fn (*Runner) anyerror!void,
};

const Runner = struct {
    file: std.fs.File,
    pub fn init(file: std.fs.File) Runner {
        return .{
            .file = file,
        };
    }
    pub fn run(runner: *Runner, func: FnProto) anyerror!void {
        if (func != null)
            try func.?(runner);
    }
};

fn dumpPrint(runner: *Runner) anyerror!void {
    try runner.file.writer().writeAll("ok\n");
}

test "fnproto" {
    const stdout = std.io.getStdOut();
    var runner = Runner.init(stdout);
    try runner.run(dumpPrint);
    // doesnt work:
    //var dump_print = dumpPrint(&runner);
    //try runner.run(dump_print);
    // stage2
    //try runner.run(&dumpPrint);
}