So once we have reference types we can use those types for fd but currently we can also enforce nearly the same behavior as if we had reference types by annotating functions. This would enable the host system to enforce the integrity of fd across module instances.

For example lets say we have to have 2 wasm instances (a, b) that share no memory or tables but instance a imports some exported functions from instance b. If those exported functions expected fd in its augments, those augments would be annotated in a custom section so that when a called b the host would enforce that values were valid fd.

The custom sections could looks something like the following

LANGUAGE_TYPES = {
  'i32': 0x7f,
  'i64': 0x7e,
  'f32': 0x7d,
  'f64': 0x7c,
  'fd': 0x7b,
  'anyFunc': 0x70,
  'func': 0x60,
  'block_type': 0x40,
}

The custom section has a name of "wasi.type".

The typeMap section maps the custom types to functions. And is encoded as follows

And a map is Encoded as follows and named "wasi.typeMap".

Lastly the custom type information for globals are encoded as follows. The section name is "wasi.globals"