`?` desugaring leaks into error messages when `Ok` is forgotten on final expr

[nikomatsakis]

Given this example:

fn foo() -> Result<(), ()> {
   bar()? 
}

fn bar() -> Result<(), ()> {
}

fn main() { }

I get this error (playground):

error[E0308]: match arms have incompatible types
 --> src/main.rs:2:4
  |
2 |    bar()? 
  |    ^^^^^^
  |    |
  |    expected enum `std::result::Result`, found ()
  |    match arm with an incompatible type
  |
  = note: expected type `std::result::Result<(), ()>`
             found type `()`

It seems like it should not be talking about match =)

[nikomatsakis]

Ideally, I would think it should say something like "missing Ok", but that may be a separate bug -- i.e., maybe we should strive to just produce the same error you would get if you did let x = bar(); x