Too much large error span for sum()

[leonardo-m]

This code:

fn sum_and_show(data: &[u32]) -> String {
    data.iter().sum().to_string()
}
fn main() {
    let s: String = sum_and_show(&[1, 2, 3]);
}

In rustc 1.20.0-nightly (b2c0707 2017-07-13) gives the error message:

error[E0619]: the type of this value must be known in this context
 --> ...\test.rs:2:5
  |
2 |     data.iter().sum().to_string()
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

The error message doesn't point to the exact location of the problem. The correct code is:

data.iter().sum::<u32>().to_string()

So probably I'd like an error message like:

error[E0619]: the type of this value must be known in this context
 --> ...\test.rs:2:5
  |
2 |     data.iter().sum().to_string()
  |                 ^^^^^

Even better: I'd like Rustc to infer the right type for sum(), not require a type hint there and compile the original code.

[kennytm]

I think rustc cannot confidently infer the type of sum(), because one could legally impl Sum<&u32> for MyType which produces a different to_string()

[zackmdavis]

Maybe related? #42234

[Mark-Simulacrum]

Going to go ahead and close in favor of #36598. Not, perhaps, an exact duplicate but I think it's the same problem.