You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: tests.rs
    path of file: ./repos/boa/core/parser/src/parser/expression/primary/async_function_expression
    
    the code of the file until where you have to start completion: use crate::parser::tests::check_script_parser;
use boa_ast::{
    declaration::{Declaration, LexicalDeclaration, Variable},
    expression::literal::Literal,
    function::{AsyncFunction, FormalParameterList
    
fn check_async_expression() {
    let interner = &mut Interner::default();
    let add = interner.get_or_intern_static("add", utf16!("add"));
    check_script_parser(
        "const add = async function() {
            return 1;
        };
        ",
        vec![Declaration::Lexical(LexicalDeclaration::Const(
            vec![Variable::from_identifier(
                add.into(),
                Some(
                    AsyncFunction::new(
                        Some(add.into()),
                        FormalParameterList::default(),
                        FunctionBody::new(
                            vec![StatementListItem::Statement(Statement::Return(
                                Return::new(Some(Literal::from(1).into())),
                            ))]
                            .into(),
                        ),
                        false,
                    )
                    .into(),
                ),
            )]
            .try_into()
            .unwrap(),
        ))
        .into()],
        interner,
    );
}