std::fs::File functions panic on valid return type

[jhpratt]

The following code taken directly from documentation will not compile.

fn main() {
    use std::fs::File;
    use std::io::prelude::*;

    let mut file = File::create("foo.txt")?;
    file.write_all(b"Hello, world!")?;
}

The error message is as follows:

error[E0277]: the `?` operator can only be used in a function that returns `Result` (or another type that implements `std::ops::Try`)
 --> example.rs:5:20
  |
5 |     let mut file = File::create("foo.txt")?;
  |                    ^^^^^^^^^^^^^^^^^^^^^^^^ cannot use the `?` operator in a function that returns `()`
  |
  = help: the trait `std::ops::Try` is not implemented for `()`
  = note: required by `std::ops::Try::from_error`

error[E0277]: the `?` operator can only be used in a function that returns `Result` (or another type that implements `std::ops::Try`)
 --> example.rs:6:5
  |
6 |     file.write_all(b"Hello, world!")?;
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ cannot use the `?` operator in a function that returns `()`
  |
  = help: the trait `std::ops::Try` is not implemented for `()`
  = note: required by `std::ops::Try::from_error`

error: aborting due to 2 previous errors

The documentation for create() indicates that the function does return Result<File>, but the compiler believes it returns (). write_all() indicates it returns Result<()>.

Clearly, both functions implement std::ops::Try via Result, but an example as basic as this fails on that test.

Version

Fails on both stable and nightly.

rustc --version
→ rustc 1.25.0 (84203cac6 2018-03-25)
→ rustc 1.26.0-nightly (e5277c145 2018-03-28)

[mattico]

Notice the error message:

cannot use the ? operator in a function that returns ()

main is a function that returns (), so you can't use the ? operator inside of it.

This restriction is changing soon, but in the mean time I'd suggest using a separate function:

use std::fs::File;
use std::io::{self, prelude::*};

fn run() -> io::Result<File> {
    let mut file = File::create("foo.txt")?;
    file.write_all(b"Hello, world!")?;
    
    Ok(file)
}

fn main() {
    run().unwrap();
}

[jhpratt]

Ah, ok. English can be so finicky — a single word changes everything.

I certainly look forward to the issue you've linked to.

Thanks!

[mattico]

Yeah, I was confused when I read the error message too - and I've been speaking English for a while...

[jhpratt]

Oh, I'm a native speaker. That's probably why I missed it.