expected type parameter, found struct

Question

I've encountered a problem which I reduced to the following code:

trait Logger {}

struct DefaultLogger;

impl Logger for DefaultLogger {}

struct A<L> where L: Logger {
    logger: Box<L>,
}

impl<L> A<L> where L: Logger {
    fn new() -> Self {
        let logger = DefaultLogger;

        A {
            logger: Box::new(logger),
                          // ^^^^^^ Here is the problem
        }
    }
}

fn main() {
    let a = A::new();
}

which produces the following error:

error[E0308]: mismatched types
  --> src/main.rs:16:30
   |
16 |             logger: Box::new(logger),
   |                              ^^^^^^ expected type parameter, found struct `DefaultLogger`
   |
   = note: expected type `L`
              found type `DefaultLogger`

When I construct trait A in a normal function (like main), like I expected it to work. Example:

trait Logger {}

struct DefaultLogger;

impl Logger for DefaultLogger {}

struct A<L> where L: Logger {
    logger: Box<L>,
}

fn main() {
        let logger = DefaultLogger;

        let _a = A {
            logger: Box::new(logger),
        };
}

Answer 1

The problem is here:

impl<L> A<L> where L: Logger {
    fn new() -> Self {
        let logger = DefaultLogger;

        A {
            logger: Box::new(logger),
        }
    }
}

You want to return A<L> but you return A<DefaultLogger> where you cannot be sure that L == DefaultLogger.

To fix that you can provide specialisation that will create method A::new only for DefaultLogger:

impl A<DefaultLogger> {
    fn new() -> Self {
        let logger = DefaultLogger;

        A {
            logger: Box::new(logger),
        }
    }
}