Cannot compile: using try_buffer_unordered with StreamExt map - rust

Question

I'm trying to use StreamExt and TryStreamExt as follows:
use tokio::fs::File;
use tokio::io::{self, AsyncReadExt};
use futures::stream::{self, StreamExt, TryStreamExt};
async fn read_file(name: &str) -> io::Result<[u8; 10]> {
let mut f = File::open(name).await?;
let mut buffer = [0; 10];
// read up to 10 bytes... yes I know this is not safe but...
let n = f.read(&mut buffer[..]).await?;
Ok(buffer)
}
#[tokio::main]
async fn main() -> io::Result<()> {
let files = vec!["foo.txt", "bar.txt"];
let headers = stream::iter(files)
.map(|f| {
tokio::spawn(
read_file(f)
)
})
.try_buffer_unordered(8)
.try_collect()?;
println!("Got results: {:?}", headers);
Ok(())
}
A link to the playground is here:
https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=3edf39926c3f046f89f2d925f1a93115
The result of the async_fn and the tokio::spawn is a Result<Result<...., MyError>, JoinError>. The try_buffer_unordered does not compile after a map, which I cannot figure out. I get:
Compiling playground v0.0.1 (/playground)
error[E0599]: no method named `try_buffer_unordered` found for struct `futures::stream::Map<futures::stream::Iter<std::vec::IntoIter<&str>>, [closure#src/main.rs:19:14: 23:10]>` in the current scope
--> src/main.rs:24:10
|
24 | .try_buffer_unordered(8)
| ^^^^^^^^^^^^^^^^^^^^ method not found in `futures::stream::Map<futures::stream::Iter<std::vec::IntoIter<&str>>, [closure#src/main.rs:19:14: 23:10]>`
|
::: /playground/.cargo/registry/src/github.com-1ecc6299db9ec823/futures-util-0.3.13/src/stream/stream/map.rs:12:1
|
12 | / pin_project! {
13 | | /// Stream for the [`map`](super::StreamExt::map) method.
14 | | #[must_use = "streams do nothing unless polled"]
15 | | pub struct Map<St, F> {
... |
19 | | }
20 | | }
| | -
| | |
| |_doesn't satisfy `_: TryStreamExt`
| doesn't satisfy `_: TryStream`
|
= note: the method `try_buffer_unordered` exists but the following trait bounds were not satisfied:
`futures::stream::Map<futures::stream::Iter<std::vec::IntoIter<&str>>, [closure#src/main.rs:19:14: 23:10]>: TryStream`
which is required by `futures::stream::Map<futures::stream::Iter<std::vec::IntoIter<&str>>, [closure#src/main.rs:19:14: 23:10]>: TryStreamExt`
`&futures::stream::Map<futures::stream::Iter<std::vec::IntoIter<&str>>, [closure#src/main.rs:19:14: 23:10]>: TryStream`
which is required by `&futures::stream::Map<futures::stream::Iter<std::vec::IntoIter<&str>>, [closure#src/main.rs:19:14: 23:10]>: TryStreamExt`
`&mut futures::stream::Map<futures::stream::Iter<std::vec::IntoIter<&str>>, [closure#src/main.rs:19:14: 23:10]>: TryStream`
which is required by `&mut futures::stream::Map<futures::stream::Iter<std::vec::IntoIter<&str>>, [closure#src/main.rs:19:14: 23:10]>: TryStreamExt`
I could use buffer_unordered() but then I'm left with a stream with a double nested Result
and I was hoping to use try_buffer_unordered and try_collect().... any thoughts?
(Yes I know the read code has problems, it's just to illustrate the compile problem with StreamExt
(The alternative is something like a series of collects to try to get rid of the Results, which is ugly and uses up extra collections)

Answer 1

FWIW, what I was told from Tokio discord is that one cannot use try_buffer_unordered with a regular Future returned from a map. The try_* methods are intended for when the creation of the Future might not succeed, in other words something like a Result<Future, _>.