I've never worked with Zig in my life, and I don't have an opinion on it either. I've heard it's neat. Despite that, this post was intuitively clear to me.

Zig's *std.Io.Reader and Writer seem to be abstractions over many types, like Java's interface class, Go's interfaces, C++'s concepts or Rust's traits. However, it seems that abstraction itself is leaky - in that the length of the buffer is an implicit dependency which cannot be known from the type alone.

Admittedly, we don't know if there's a good explanation for the behavior - but then again, HN comments are as good a place to learn as any. Maybe someone will come up and explain.