give a specific error message for the wrong return type

[sourcefrog]

This can be confusing for people new to rust and perhaps trying to copy some example code from https://doc.rust-lang.org/std/fs/struct.File.html:

use std::fs::File;

fn main() {
    let mut f = try!(File::create("foo.txt"));
}

https://play.rust-lang.org/?gist=2201429be6f0833e2161&version=stable

At the moment this produces

<std macros>:5:8: 6:42 error: mismatched types:
 expected `()`,
    found `core::result::Result<_, _>`
(expected (),
    found enum `core::result::Result`) [E0308]
<std macros>:5 return $ crate:: result:: Result:: Err (
<std macros>:6 $ crate:: convert:: From:: from ( err ) ) } } )
<std macros>:1:1: 6:48 note: in expansion of try!
<anon>:4:17: 4:46 note: expansion site
<std macros>:5:8: 6:42 help: see the detailed explanation for E0308
error: aborting due to previous error
playpen: application terminated with error code 101

https://doc.rust-lang.org/error-index.html#E0308 is OK but may be hard to see how it applies.

Perhaps the friendliest thing would be to suggest using unwrap() rather than try!, though perhaps that is too specialized.

I think there was another bug about main returning a Result?

[steveklabnik]

We have decided, generally, to assume that when reading API docs, you already know Rust. If you click the link to show the example in the playpen, it will insert the stuff needed to make it compile.

That said, E0308 could use a specific call-out of this situation.

And yes, there's an RFC repo issue about main and Result.

[sourcefrog]

Yes, I wasn't suggesting doing this in the File documentation, which would be unscalable.

E0308 says "This error occurs when the compiler was unable to infer the concrete type of a variable", but there's no variable involved here, at least at the user-visible level.

Could we have a more specific error code for 'returning X in a function declared to return Y'? Or, otherwise, mention in the documentation for E0308 that it covers that case too?

[steveklabnik]

/cc #27112

[sourcefrog]

Given #27112 exists, I think the real heart remaining part of this is https://doc.rust-lang.org/error-index.html#E0308 says "wrong type for a variable" but it would be clearer with "wrong return type."

[sourcefrog]

yay, thanks!