How to implement AsRef for a struct containing references

With fn as_ref(&self) -> &Struct the compiler has to infer the (implicit) generic lifetime in the return type and fails to do so. The compiler is expecting a Struct<'a> but the signature promises a free parameter. That's why you get

      expected fn(&Struct<'a>) -> &Struct<'a>
         found fn(&Struct<'a>) -> &Struct<'_>  // '_ is some anonymous lifetime,
                                               // which needs to come from somewhere

The solution is to modify the signature to return Struct<'a> instead of Struct. Even shorter and more clear:

impl<'a> AsRef<Struct<'a>> for Struct<'a> {
    fn as_ref(&self) -> &Self {  // Self is Struct<'a>, the type for which we impl AsRef
        self
    }
}