Update doc examples that use ? (Carrier) to use Termination

[frewsxcv]

Blocked on #49162

Example:

rust/src/libstd/fs.rs

Lines 40 to 49 in c19264f

	/// ```no_run
	/// use std::fs::File;
	/// use std::io::prelude::*;
	///
	/// # fn foo() -> std::io::Result<()> {
	/// let mut file = File::create("foo.txt")?;
	/// file.write_all(b"Hello, world!")?;
	/// # Ok(())
	/// # }
	/// ```

Instead of:

use std::fs::File;
use std::io::prelude::*;

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

We can now do:

use std::fs::File;
use std::io::Error;
use std::io::prelude::*;

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

Which can be copied/pasted straight from the docs such that it will compile, unlike the old example

[steveklabnik]

Big 👍 from me

[GuillaumeGomez]

Seems a good idea to me as well.

[QuietMisdreavus]

I think this will be a good thing for combating the most common error of pasting the trimmed examples into fn main() without seeing or considering the ?. Being able to have main return the error like that will greatly help cut down the boilerplate these currently have, and allow us to stop hiding most of the test.

[frewsxcv]

cool, i'll start working on this then 🤸‍♂️

[frewsxcv]

opened a pr for this: #49357