type Db<'a> = { Save : 'a -> unit } // <=!!!
let mock = { Db.Save = fun (x:string) -> printfn $"{x} is saved" } 
mock.Save "ok"

vaa>type Db<'a> = { Save : 'a -> unit } // <=!!!
vaa>let mock = { Db.Save = fun (x:string) -> printfn $"{x} is saved" } 
vaa>mock.Save "ok"
vaa>

trait Db {
  fn save(&self, x : &str);
}

fn use_trait<T : Db>(val : &T)
{
  val.save("value");
}

struct Mock {
}

impl Db for Mock {
  fn save(&self, val  :&str) {
    println!("{val} is saved");
  }
}

fn main() {
  let mock = Mock {};
  use_trait(&mock);
}

trait Save {
    fn save(&self);
}

struct Db1<T: Save> {
    save: T,
}

struct Db2 {
    save: Box<dyn Save>,
}

struct SomeSaveImpl {}

impl Save for SomeSaveImpl {
    fn save(&self) {
        println!("save");
    }
}

fn main() {
    let db1 = Db1 {
        save: SomeSaveImpl {},
    };
    db1.save.save();

    let db2 = Db2 {
        save: Box::new(SomeSaveImpl {}),
    };
    db2.save.save();
}