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rusttraits

What does the trait after the trait name mean?

I came across this trait definition while reading about Rust:

trait Enchanter: std::fmt::Debug {
    ...
}

From this I understand that the name of the trait is Enchanter, but I do not understand what the std::Format:Debug part implies since it is also a trait (I think).

Solution

  • This is declaring a supertrait. It is equivalent to:

    trait Enchanter
where
    Self: std::fmt::Debug,
{
}

    In short, it requires any type that wants to implement Enchanter to also implement std::fmt::Debug. Otherwise, an error will be raised:

    error[E0277]: `S` doesn't implement `Debug`
 --> src/lib.rs:4:6
  |
4 | impl Enchanter for S {}
  |      ^^^^^^^^^ `S` cannot be formatted using `{:?}`
  |
  = help: the trait `Debug` is not implemented for `S`
  = note: add `#[derive(Debug)]` to `S` or manually `impl Debug for S`
note: required by a bound in `Enchanter`
 --> src/lib.rs:1:18
  |
1 | trait Enchanter: std::fmt::Debug {}
  |                  ^^^^^^^^^^^^^^^ required by this bound in `Enchanter`