Agfdhyk

I'm building a library that implements string joins; that is, printing all the elements of a container separated by a separator. My basic design looks like this:

use std::fmt;



#[derive(Debug, Clone, PartialEq, Eq)]

pub struct Join<Container, Sep> {

    container: Container,

    sep: Sep,

}



impl<Container, Sep> fmt::Display for Join<Container, Sep>

where

    for<'a> &'a Container: IntoIterator,

    for<'a> <&'a Container as IntoIterator>::Item: fmt::Display,

    Sep: fmt::Display,

{

    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {

        let mut iter = self.container.into_iter();



        match iter.next() {

            None => Ok(()),

            Some(first) => {

                first.fmt(f)?;



                iter.try_for_each(move |element| {

                    self.sep.fmt(f)?;

                    element.fmt(f)

                })

            }

        }

    }

}

This trait implementation compiles without complaint. Notice the bound on &'a C: IntoIterator. Many containers implement IntoIterator for a reference to themselves, to allow for iterating over references to the contained items (for instance, Vec implements it here).

However, when I actually try to use my Join struct, I get an unsatisfied trait bound:

fn main() {

    let data = vec!["Hello", "World"];

    let join = Join {

        container: data,

        sep: ", ",

    };

    println!("{}", join);

}

This code produces a compilation error:

error[E0277]: `<&'a std::vec::Vec<&str> as std::iter::IntoIterator>::Item` doesn't implement `std::fmt::Display`

  --> src/main.rs:38:20

   |

38 |     println!("{}", join);

   |                    ^^^^ `<&'a std::vec::Vec<&str> as std::iter::IntoIterator>::Item` cannot be formatted with the default formatter

   |

   = help: the trait `for<'a> std::fmt::Display` is not implemented for `<&'a std::vec::Vec<&str> as std::iter::IntoIterator>::Item`

   = note: in format strings you may be able to use `{:?}` (or {:#?} for pretty-print) instead

   = note: required because of the requirements on the impl of `std::fmt::Display` for `Join<std::vec::Vec<&str>, &str>`

   = note: required by `std::fmt::Display::fmt`

The key line seems to be this:

the trait `for<'a> std::fmt::Display` is not implemented for `<&'a std::vec::Vec<&str> as std::iter::IntoIterator>::Item`

Unfortunately, the compiler doesn't actually tell me what the Item type is, but based on my reading of the docs, it appears to be &T, which in this case means &&str.

Why doesn't the compiler think that &&str implements Display? I've tried this with many other types, like usize and String, and none of them work; they all fail with the same error. I know that these reference type don't directly implement Display, but the implementation should be picked up automatically through deref coercion, right?

Seems like a compiler limitation. You can work around it for now by writing the impl bound in terms of a private helper trait that represents "display with lifetime". This enables the compiler to see that for<'a> private::Display<'a> implies fmt::Display.

use std::fmt;



pub struct Join<Container, Sep> {

    container: Container,

    sep: Sep,

}



mod private {

    use std::fmt;

    pub trait Display<'a>: fmt::Display {}

    impl<'a, T> Display<'a> for T where T: fmt::Display {}

}



impl<Container, Sep> fmt::Display for Join<Container, Sep>

where

    for<'a> &'a Container: IntoIterator,

    for<'a> <&'a Container as IntoIterator>::Item: private::Display<'a>,

    Sep: fmt::Display,

{

    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {

        let mut iter = self.container.into_iter();



        match iter.next() {

            None => Ok(()),

            Some(first) => {

                first.fmt(f)?;



                iter.try_for_each(move |element| {

                    self.sep.fmt(f)?;

                    element.fmt(f)

                })

            }

        }

    }

}



fn main() {

    println!("{}", Join {

        container: vec!["Hello", "World"],

        sep: ", ",

    });

}