Error diagnostics for `?` regressed over `try!`

[skade]

This is a thing that occured to me while giving a training on Rust: the diagnostics of ? over try! are worse, despite it being special syntax.

Compare:

fn main() {
    
}

try() {
    try!(std::fs::File::open("foo"));
}

rustc 1.19.0-nightly (258ae6dd9 2017-06-15)
error[E0308]: mismatched types
 --> <anon>:6:5
  |
6 |     try!(std::fs::File::open("foo"));
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected (), found enum `std::result::Result`
  |
  = note: expected type `()`
             found type `std::result::Result<_, _>`
  = note: this error originates in a macro outside of the current crate

error: aborting due to previous error(s)

over:

fn main() {
    
}

fn question() {
    std::fs::File::open("foo")?;
}

rustc 1.19.0-nightly (258ae6dd9 2017-06-15)
error[E0277]: the trait bound `(): std::ops::Try` is not satisfied
 --> <anon>:6:5
  |
6 |     std::fs::File::open("foo")?;
  |     ---------------------------
  |     |
  |     the trait `std::ops::Try` is not implemented for `()`
  |     in this macro invocation
  |
  = note: required by `std::ops::Try::from_error`

While the second makes perfect sense to advanced Rust developers, it is confusing to newcomers and leaks a lot of internal details. People have to know about traits, about the ops module, and similar. Also, it reports a macro invocation, where - from a user perspective - there is none. To my knowledge, this is the only place where we do this.

Considering that ? raises a macro to syntax, I would also think it gives opportunity for better diagnostics. It is a crucial part of the language with non-trivial semantics and making a pass on giving better errors here would help newcomers a lot.

[steveklabnik]

We have #35946 tracking this sorta

[estebank]

Current output on nightly after #43001:

error[E0277]: the trait bound `(): std::ops::Try` is not satisfied
 --> file.rs:6:5
  |
6 |     std::fs::File::open("foo")?;
  |     ---------------------------
  |     |
  |     the `?` operator can only be used in a function that returns `Result` (or another type that implements `std::ops::Try`)
  |     in this macro invocation
  |
  = help: the trait `std::ops::Try` is not implemented for `()`
  = note: required by `std::ops::Try::from_error`