Unboxing and inspecting a trait object - rust

Question

Here's a simple application that duplicates 2 times on stdout the contents of stdin:
use std::{
io,
io::{stdin, stdout, Read, Write},
num::NonZeroUsize,
};
fn dup_input(
input: &mut Box<dyn Read>,
output: &mut Box<dyn Write>,
count: NonZeroUsize,
) -> io::Result<()> {
let mut buf = Vec::new();
input.read_to_end(&mut buf)?;
for _idx in 0..count.get() {
output.write_all(&buf)?;
}
Ok(())
}
fn main() {
let mut input: Box<dyn Read> = Box::new(stdin());
let mut output: Box<dyn Write> = Box::new(stdout());
dup_input(&mut input, &mut output, NonZeroUsize::new(2).unwrap())
.expect("Failed to duplicate input");
}
This part works fine. I want to put a unit test on top of this and this is where the problem lies. The closest I've got to build is with the following attempt:
#[cfg(test)]
mod tests {
use super::*;
use std::{any::Any, io::Cursor};
#[test]
fn test() {
let mut input: Box<dyn Read> = Box::new(Cursor::new([b't', b'e', b's', b't', b'\n']));
let mut output: Box<dyn Write + Any> = Box::new(Vec::<u8>::new());
assert!(dup_input(&mut input, &mut output, NonZeroUsize::new(3).unwrap()).is_ok());
assert_eq!(output.downcast::<Vec<u8>>().unwrap().len(), 15);
}
}
but rust 1.41.0 doesn't agree:
$ cargo test
Compiling unbox-example v0.1.0 (/XXX/unbox-example)
error[E0225]: only auto traits can be used as additional traits in a trait object
--> src/main.rs:39:41
|
39 | let mut output: Box<dyn Write + Any> = Box::new(Vec::<u8>::new());
| ----- ^^^
| | |
| | additional non-auto trait
| | trait alias used in trait object type (additional use)
| first non-auto trait
| trait alias used in trait object type (first use)
error[E0308]: mismatched types
--> src/main.rs:41:39
|
41 | assert!(dup_input(&mut input, &mut output, NonZeroUsize::new(3).unwrap()).is_ok());
| ^^^^^^^^^^^ expected trait `std::io::Write`, found a different trait `std::io::Write`
|
= note: expected mutable reference `&mut std::boxed::Box<(dyn std::io::Write + 'static)>`
found mutable reference `&mut std::boxed::Box<(dyn std::io::Write + 'static)>`
error[E0599]: no method named `downcast` found for type `std::boxed::Box<(dyn std::io::Write + 'static)>` in the current scope
--> src/main.rs:43:27
|
43 | assert_eq!(output.downcast::<Vec<u8>>().unwrap().len(), 15);
| ^^^^^^^^ method not found in `std::boxed::Box<(dyn std::io::Write + 'static)>`
error: aborting due to 3 previous errors
Some errors have detailed explanations: E0225, E0308, E0599.
For more information about an error, try `rustc --explain E0225`.
error: could not compile `unbox-example`.
To learn more, run the command again with --verbose.
Is there a way to change the unit test without changing the main code? Note: I could have used generics while implementing dup_input and make the problem significantly easier to solve but this code is part of a broader application and I have to use Read/Write trait objects.

Answer 1

Passing a mutable reference to a Box to dup_input is unnecessarily complicated. You can simply pass a mutable reference to the trait object.
use std::{
io,
io::{stdin, stdout, Read, Write},
num::NonZeroUsize,
};
fn dup_input(
input: &mut dyn Read,
output: &mut dyn Write,
count: NonZeroUsize,
) -> io::Result<()> {
let mut buf = Vec::new();
input.read_to_end(&mut buf)?;
for _idx in 0..count.get() {
output.write_all(&buf)?;
}
Ok(())
}
fn main() {
let mut input = stdin();
let mut output = stdout();
dup_input(&mut input, &mut output, NonZeroUsize::new(2).unwrap())
.expect("Failed to duplicate input");
}
With this version, the test can be written like this:
#[cfg(test)]
mod tests {
use super::*;
use std::io::Cursor;
#[test]
fn test() {
let mut input = Cursor::new([b't', b'e', b's', b't', b'\n']);
let mut output = Vec::<u8>::new();
assert!(dup_input(&mut input, &mut output, NonZeroUsize::new(3).unwrap()).is_ok());
assert_eq!(output.len(), 15);
}
}
We don't need to use Any at all here: output is simply a Vec<u8>.