CODEWITHSUNDEEP
rustiteratortype-conversiontraits
Nick
Nick

Reputation: 10499

Type mismatch with iterator of traits as function argument

I have a function that accepts a trait as argument wrapped in a Box

use std::fmt::Display;

fn push(e: Box<dyn Display>) {}

push(Box::new(0));

Now I'd like to create another function that accepts an iterator where each Item is of the same type

fn push_batch<I>(batch: I)
where
    I: IntoIterator<Item = Box<dyn Display>>,
{
}

push_batch((0..10).map(|i| Box::new(i)));

But doing so results in the following error:

error[E0271]: type mismatch resolving `<[closure@src/main.rs:13:28: 13:43] as FnOnce<({integer},)>>::Output == Box<(dyn std::fmt::Display + 'static)>`
  --> src/main.rs:13:5
   |
5  | fn push_batch<I>(batch: I)
   |    ---------- required by a bound in this
6  | where
7  |     I: IntoIterator<Item = Box<dyn Display>>,
   |                     ----------------------- required by this bound in `push_batch`
...
13 |     push_batch((0..10).map(|i| Box::new(i)));
   |     ^^^^^^^^^^ expected integer, found trait object `dyn std::fmt::Display`
   |
   = note: expected struct `Box<{integer}>`
              found struct `Box<(dyn std::fmt::Display + 'static)>`
   = note: required because of the requirements on the impl of `Iterator` for `Map<std::ops::Range<{integer}>, [closure@src/main.rs:13:28: 13:43]>`

Why does the first example compile while the second doesn't? And how can I create an iterator of boxed traits to pass to the push_batch function?

Upvotes: 1

Views: 324

Answers (1)

Aplet123
Aplet123

Reputation: 35512

Rust does not automatically cast Box<T> to Box<dyn Trait>, you must do it explicitly:

push_batch((0..10).map(|i| Box::new(i) as Box<dyn Display>));

Upvotes: 1

Related Questions