Poor error message from try! or ? and type-mismatch #36988
Description
For try! we used to give a type mismatch message when the type of the value did not match the function type (see example below). We now give a message about the Carrier trait, which the user should not know about, e.g., error[E0277]: the trait bound(): std::ops::Carrieris not satisfied for the below example.
I removed the following from doc/error-index.md
Another situation in which this occurs is when you attempt to use the `try!`
macro inside a function that does not return a `Result<T, E>`:
```compile_fail,E0308
use std::fs::File;
fn main() {
let mut f = try!(File::create("foo.txt"));
}
This code gives an error like this:
<std macros>:5:8: 6:42 error: mismatched types:
expected `()`,
found `core::result::Result<_, _>`
(expected (),
found enum `core::result::Result`) [E0308]
try! returns a Result<T, E>, and so the function must. But main() has
() as its return type, hence the error.