assert_that!(Some(5)).must_be(Some(5));
assert_that!(Some(5)).must_be_some_and(|val| {
  assert_that!(val).must_be(5)
}).with_msg("lazy evaluation");
Some(5).must_be_some_and(|val| {
  assert_that!(val).must_be(5)
});

// with customization
let parser: fn(&str) -> Result<Lit, ParseError> = parse_lit;
parser.must_parse("false").as_ok(Lit::Bool(false));
assert_that!(parser)
  .with_msg("")
  .must_parse("true")
  .as_ok(Lit::Bool(true))
  .with_remainder(""); // rustfmt