Potentially misleading compiler error for unfinished line with Option<T>

[DoumanAsh]

Next code:

use std::io::{Read, stdin};

fn main() {
    stdin().bytes().next()
}

Gives this compile error:

E:\Downloads\Git\collectionScripts\rust
 [master +2 ~1 -0 !] > rustc.exe .\temp.rs
.\temp.rs:4:5: 4:27 error: mismatched types:
 expected `()`,
    found `core::option::Option<core::result::Result<u8, std::io::error::Error>>`
(expected (),
    found enum `core::option::Option`) [E0308]
.\temp.rs:4     stdin().bytes().next()
                ^~~~~~~~~~~~~~~~~~~~~~
error: aborting due to previous error

Which is a bit misleading as there is actually we're missing either ; or unwrap() i think?

Rust version:

rustc 1.0.0-beta.4 (850151a75 2015-04-30) (built 2015-05-01)
binary: rustc
commit-hash: 850151a75709f146addd30bbbf1f23d384f0b381
commit-date: 2015-04-30
build-date: 2015-05-01
host: x86_64-pc-windows-gnu
release: 1.0.0-beta.4

[frewsxcv]

fn main() { } is a function that expects nothing (()) to be returned. In your code block, since that statement doesn't end in a ;, it is going to try to return the result of it to the function.

[sinistersnare]

I agree, error messages for wrong function return types should be better.

I would expect something like

fn main is marked as returning (), however returns Option

or something like that.

[DoumanAsh]

Oh.. i actually did not notice it at the beginning. Originally i had this problem in a bit more bigger code block.
I think it would be good to point that it is being return type of function so that it would be more clear to user why compiler is angry

[steveklabnik]

Triage: main's diagnostics have a long history, #35946 for example, and all of the try! stuff before that. Looks like there's a PR for this issue.

[Mark-Simulacrum]

I think the semicolon suggestion we give today is a good start, but indicating that the cause of the expected type is the function's return type would be even better.

error[E0308]: mismatched types
 --> test.rs:4:5
  |
4 |     stdin().bytes().next()
  |     ^^^^^^^^^^^^^^^^^^^^^^- help: did you mean to add a semicolon here?: `;`
  |     |
  |     expected (), found enum `std::option::Option`
  |
  = note: expected type `()`
             found type `std::option::Option<std::result::Result<u8, std::io::Error>>`

error: aborting due to previous error