Needless Mutable

TIP

This can be configured. Read more about configuration.

This checks for globals, WasmGC struct fields and arrays that are declared mutable but are never mutated.

Specifically:

• global.set can mutate a global
• struct.set can mutate a WasmGC struct field
• array.set, array.fill, array.init_data and array.init_elem can mutate a WasmGC array

For those global types and field types that are defined with a mut keyword but are never mutated by the instructions above, "mutable" is needless.

Globals

(module
  (func
    global.get 0
    drop)
  (global $global (mut i32)
    i32.const 0))

It won't report exported globals, since they may be mutated by the host.

(module
  (export "globalValue" (global 0))
  (global (mut i32)
    i32.const 0))

Struct Fields

(module
  (type (struct (field (mut i32))))
  (func (param (ref 0)) (result i32)
    local.get 0
    struct.get 0 0))

Arrays

(module
  (type (array (mut i32)))
  (func (param (ref 0)) (result i32)
    local.get 0
    i32.const 0
    array.get 0))