# XO0530: binding the payload of a field shares every heap field of a large value

Reported by `xo vet`, not `xo check`. `match s.conn { Some(c) => ... }` (or `if let`) on a field of a value that lives on binds a copy of the payload: every reference counted field of it (Str, Bytes, List, Map, handles) is shared, and released again when the copy or the field is dropped. For a payload with at least 8 such fields that is a fixed walk on every match, which profiles of the HTTP/2 server found at about 4,000 instructions a request. Test the field with `.is_none()` or a pattern without bindings first when the payload is not needed, read the one part needed (`s.conn.map(...)`), or take the value out of its owner before matching. See decision 0263.

## Example

```xo
struct Conn {
  a: Str, b: Str, c: Str, d: Str
  e: Str, f: Str, g: Str, h: Str
}

struct Srv {
  conn: Conn?
}

fn label(s: Srv) -> Str {
  match s.conn {
    Some(c) => c.a
    None => "none"
  }
}
```

## Fix

```xo
struct Conn {
  a: Str, b: Str, c: Str, d: Str
  e: Str, f: Str, g: Str, h: Str
}

struct Srv {
  conn: Conn?
}

fn label(s: Srv) -> Str {
  if s.conn.is_none() {
    return "none"
  }
  s.conn.map(fn(c) { c.a }) ?? "none"
}
```

Run `xo explain XO0530` for this text in a terminal, or see
[section 10.6 of the specification](../../spec/core/#106-diagnostic-codes).