How to implement a custom 'fmt::Debug' trait?

Question

I presume you do something like this:

extern crate uuid;  use uuid::Uuid; use std::fmt::Formatter; use std::fmt::Debug;  #[derive(Debug)] struct BlahLF {     id: Uuid, }  impl BlahLF {     fn new() -> BlahLF {         return BlahLF { id: Uuid::new_v4() };     } }  impl Debug for BlahLF {     fn fmt(&self, &mut f: Formatter) -> Result {         write!(f.buf, "Hi: {}", self.id);     } } 

...but attempting to implement this trait generates:

error[E0243]: wrong number of type arguments   --> src/main.rs:19:41    | 19 |     fn fmt(&self, &mut f: Formatter) -> Result {    |                                         ^^^^^^ expected 2 type arguments, found 0 

However, that seems to be how other implementations do it. What am I doing wrong?

Answer 1

According to the example from the std::fmt docs:

extern crate uuid;  use uuid::Uuid; use std::fmt;  struct BlahLF {     id: Uuid, }  impl fmt::Debug for BlahLF {     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {         write!(f, "Hi: {}", self.id)     } } 

The part to emphasize is the fmt:: in fmt::Result. Without that you're referring to the plain Result type. The plain Result type does have two generic type parameters, fmt::Result has none.