how to generalize from `File` to `Read`? - rust

Question

I have some working code that reads a file, but I need to generalize it to pull data from additional sources other than simple disk files.
Is Read the correct generalization I should work with in order to replace File?
If so, how can I fix example2 in the following sample code? As is, it fails with the compile error dyn async_std::io::Read cannot be unpinned at the commented line. If not, what type should I return instead from get_read and are there any corresponding changes required in example2?
//! [dependencies]
//! tokio = { version = "1.0.1", features = ["full"] }
//! async-std = "1.8.0"
//! anyhow = "1.0.32"
use async_std::io::prelude::*;
use async_std::fs::File;
use anyhow::Result;
#[tokio::main]
async fn main() -> Result<()> {
example1().await?;
example2().await?;
Ok(())
}
// Example of consuming `File` ... works great!
async fn example1() -> Result<()> {
let mut file = get_file().await?;
let mut contents = String::new();
let _ = file.read_to_string(&mut contents).await?;
println!("read {} characters", contents.len());
Ok(())
}
// Example of consuming `Read` ... does not compile?
async fn example2() -> Result<()> {
let mut read = get_read().await?;
let mut contents = String::new();
// ERROR: `dyn async_std::io::Read` cannot be unpinned
let _ = read.read_to_string(&mut contents).await?;
println!("read {} characters", contents.len());
Ok(())
}
async fn get_read() -> Result<Box<dyn Read>> {
let file = get_file().await?;
Ok(Box::new(file))
}
async fn get_file() -> Result<File> {
let file = File::open("/etc/hosts").await?;
Ok(file)
}

Answer 1

You need to pin:
async fn get_read() -> Result<Pin<Box<dyn Read>>> {
let file = get_file().await?;
Ok(Box::pin(file))
}
Box<File> (without Pin) works because File implements Unpin. Box<dyn Read + Unpin> would work too.