message: rust: Add missing test coverage
  The pointer can only be null if there is an invalid usage of the Rust API.
diff: @@ -46,9 +46,7 @@ impl Module {
  /// Create an instance of a module.
  // TODO: support imported functions
  pub fn instantiate(self) -> Result<Instance, ()> {
  - if self.0.is_null() {
  - return Err(());
  - }
  + assert!(!self.0.is_null());
  let ptr = unsafe {
  sys::fizzy_instantiate(
  self.0,
  @@ -309,6 +307,20 @@ mod tests {
  assert!(instance.is_ok());
  }
  + #[test]
  + fn instantiate_wasm_missing_import() {
  + /* wat2wasm
  + (module
  + (memory (import "mod" "m") 1)
  + )
  + */
  + let input = hex::decode("0061736d01000000020a01036d6f64016d020001").unwrap();
  + let module = parse(&input);
  + assert!(module.is_ok());
  + let instance = module.unwrap().instantiate();
  + assert!(instance.is_err());
  + }
  +
  #[test]
  fn find_exported_function_index() {
  /* wat2wasm
  