WASM Language Tools

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Lint ​

Each lint has these levels:

  • "allow" - don't report problems
  • "hint" - report problems as slight tips
  • "warn" - report problems as warnings
  • "deny" - report problems as errors

unused ​

default: "warn"

This lint reports unused items, such as functions, globals, types, and even function parameters and locals. For WasmGC, fields in struct type are also supported.

Here is an example of unused functions and globals with or without identifiers:

wasm
(module
  (func)
  (func $f)
  (global i32
    i32.const 0)
  (global $g i32
    i32.const 0))

Sometimes, there may be unused items but you want to ignore them intentionally, then you can add an underscore prefix to the identifier:

wasm
(module
  (func $_)
  (func $_f))

So they won't be reported as unused.

shadow ​

default: "warn"

WebAssembly allows shadowing identifiers in block labels. For example:

wasm
(module
  (func
    (block $label
      (block $label
        br $label))))

However, it's confusing that you may want to jump to the label at line 3 or line 4. This lint reports such cases when you accidentally give the same name to different block labels.

implicitModule ​

default: "allow"

WABT allows defining module fields without the outer (module) syntax:

wasm
(func)
(global i32
  i32.const 0)

However, you may hope it to be more explicit, or your compiler doesn't support this syntax. In such cases, you can set this lint to "warn" or "deny".

multiModules ​

default: "deny"

Most compilers (such as WABT) only allow one module in a file.

wasm
(module)
(module)

If this doesn't match your needs, you can set this lint to "allow".

unreachable ​

default: "hint"

This lint reports unreachable code, for example those code after br, return and unreachable instructions or infinite loops:

wasm
(module
  (func
    (loop
      br 0)
    nop)
  (func
    return
    nop))

deprecated ​

default: "warn"

This lint reports usages of deprecated items which are marked with the @deprecated annotation.

wasm
(module
  (@deprecated)
  (func $func)
  (func
    call $func))

needlessMut ​

default: "warn"

This lint reports mutable items that are never mutated:

wasm
(module
  (global (mut i32)
    i32.const 0)
  (func (result i32)
    global.get 0))

For WasmGC, fields in struct type and array type is also supported:

wasm
(module
  (type $array (array (mut i32)))
  (type $struct (struct (field (mut i32))))
  (func (param (ref $array) (ref $struct)) (result i32 i32)
    local.get 0
    i32.const 0
    array.get $array
    local.get 1
    struct.get $struct 0))

needlessTryTable ​

default: "warn"

A try_table instruction without catch or catch_all clauses behaves like a block instruction, which is unnecessary. Any exception thrown inside such try_table will be propagated.

This lint reports such cases:

wasm
(module
  (tag)
  (func
    try_table
      throw 0
    end))

uselessCatch ​

default: "warn"

This lint reports catch clauses that will never be matched.

When there're multiple handlers for the same tag, only the first one will be matched:

wasm
(module
  (tag $e)
  (func
    block
      try_table (catch 0 0) (catch $e 1)
      end
    end))

Any catch clauses after a catch_all clause will never be matched:

wasm
(module
  (tag $e)
  (func
    block
      try_table (catch_all 0) (catch $e 1)
      end
    end))