E4042

Compiler diagnostic name: invalid_stub_type.

Invalid stub type.

When defining an FFI function, each parameter and return type must be a valid stub type that can be represented natively by the selected backend.

For example, FixedArray[T] is not a valid stub type for the Wasm GC backend.

Erroneous example

pub fn ffi(array : FixedArray[Int]) -> Unit = "mod" "ffi"
//                 ^~~~~~~~~~~~~~~
// Error: Invalid stub type.

Suggestion

The way to pass non-native types through the FFI interface depends on both the backend of the MoonBit compiler and the runtime you are using. Use a native stub type directly, or pass a handle/index and manage the compound value on the host side.

pub fn ffi(value : Int) -> Unit = "mod" "ffi"