This is now valid:

const Simple {
    name: []const u8,
};

var arena = std.heap.ArenaAllocator.init(gpa);
defer arena.deinit();

var parsed: Simple = blk: {
    const source = try gpa.alloc(u8, file_size);
    defer gpa.free(source);

    _ = try file.preadAll(source, 0);

    var yaml: Yaml = .{ .source = source };
    defer yaml.deinit(gpa);

    try yaml.load(gpa);

    break :blk try yaml.parse(arena.allocator(), Simple);
};

std.debug.print("name={s}", .{parsed.name});