从另一个 static 方法调用特征 static 方法 (rust)

Question

Can you call a trait static method implemented by types from another trait static method implemented in the trait? For example:

trait SqlTable {
  fn table_name() -> String;

  fn load(id: i32) -> Something {
    ...
    Self::table_name()    // <-- this is not right
    ...
  }
}

This is now working thanks to Chris and Arjan (see comments/answers below)

fn main() {
  let kiwibank = SqlTable::get_description(15,None::<Account>);
}

trait SqlTable {
    fn table_name(_: Option<Self>) -> String;

    fn get_description(id: i32, _: Option<Self>) -> String {
        println!("Fetching from {} table", SqlTable::table_name(None::<Self>) );
        String::from_str("dummy result")
    }
}

struct Account {
    id: i32,
    name: String,
}
impl SqlTable for Account {
    fn table_name(_: Option<Account>) -> String { String::from_str("account") }
}

Answer 1

You have to change Self to SqlTable:

trait SqlTable {
  fn table_name() -> String;

  fn load(id: i32) -> Self {
    ...
    SqlTable::table_name()    // <-- this is not right
    ...
  }
}

Static methods are always called on a trait like SomeTrait::some_method(). Bug #6894 covers this issue.

Answer 2

• Yes, you can call a trait static method [implemented by types] from another trait static method [implemented in the trait].
• Static methods are always called on a trait like SomeTrait::some_method() .
• Where there is no Self or self in [a trait] function signature, it is not callable at present. The standard workaround until UFCS comes is to take an argument _: Option<Self> and pass it None::<T> .

See original question for code that (as of today) compiles.

Answer 3

This works without any workarounds with current Rust (verified with 1.65.0):

trait Trait {
    fn x() -> i32 {
        1
    }
}

struct Struct {
    y: i32,
}

impl Struct {
    fn new(y: i32) -> Self {
        Self { y }
    }

    fn f(self) -> i32 {
        Self::x() + self.y
    }
}

impl Trait for Struct {}

fn main() {
    assert_eq!(Struct::new(1).f(), 2);
}