Two structures inherit one trait, but with extra parameters

Question

How do I make two structures inherit one trait, but with extra parameters?

And I heard that doing so in Rust is not recommended. Why?

struct Foo {name: String,}
struct FooPlus {name: String, lvl: i128,}

trait Trait {
    fn show(&self);
    fn new(name: &str) -> Self;
    // fn new(name: &str, lvl: i128) -> Self;
}

impl Trait for Foo {
    fn new(name: &str) -> Self {}
    fn show(&self) {}
}

impl Trait for FooPlus {
    fn show(&self) {}
    fn new(name: &str, lvl: i128) -> Self {}
}

Answer 1

You can make the extra parameter be an associated type to the trait.

trait Trait {
  type ConstructParam
  fn show(&self);
  fn new(name: &str, arg: ConstructParam) -> Self;
}

impl Trait for Foo {
  type ConstructParam = ();
  fn show(&self) { ... }
  fn new(name: &str, arg: ()) -> Self { ... }
}

impl Trait for FooPlus {
  type ConstructParam = i128;
  fn show(&self) { ... }
  fn new(name: &str, arg: i128) -> Self { ... }
}

Note that you have to call the "no-argument" constructor with an extra unit argument, so Foo::new("example name", ()), but that unit argument is zero-sized and thus will almost certainly be optimized to nothing.

Though, as already mentioned in the comments, having new in a trait is odd. How often are you going to polymorphically call new on a type that's generic? If you really need to polymorphically generate instances, that sounds like an application of the factory pattern, not a good use for traits in this way. And if you're not doing it polymorphically, then just make new part of the impl for Foo and FooPlus separately.