Packed Types

The GC proposal introduces packed storage types i8 and i16. These are not value types — they can only appear in struct fields and array element definitions.

Valid usage

Packed types are valid as storage types inside field and array definitions:

(module
  (type $pixel (struct
    (field $r i8)
    (field $g i8)
    (field $b i8)
    (field $a i8)))

  (type $buffer (array (mut i16)))
)

Invalid usage

Using i8 or i16 as value types in parameters, results, locals, or globals is an error:

(module
  ;; All of these are invalid:
  ;; (func (param i8))              ;; i8 is not a value type
  ;; (func (result i16))            ;; i16 is not a value type
  ;; (func (local $x i8))           ;; i8 is not a value type
  ;; (global $g i16 (i16.const 0))  ;; i16 is not a value type
)

The LSP reports: “i8 is a packed storage type and can only be used in struct/array field definitions”.

Accessing packed fields

Use struct.get_s / struct.get_u (signed/unsigned extension) to read packed fields, and struct.set to write them. Reading with plain struct.get on a packed field is invalid — you must specify the sign extension.