Misleading error message with negative range

[vultix]

Given the following code: Playground

fn main() {
  for x in -5..5 {
      do_something(x);
  }    
}

fn do_something(val: usize) {}

The current output is:

error[E0277]: the trait bound `usize: Neg` is not satisfied
 --> src/main.rs:2:12
  |
2 |   for x in -5..5 {
  |            ^^ the trait `Neg` is not implemented for `usize`

error: aborting due to previous error

This created confusion for a new Rust user on our team.

Ideally the range type would be inferred to be i32:

error[E0308]: mismatched types
 --> src/main.rs:3:20
  |
3 |       do_something(x);
  |                    ^ expected `usize`, found `i32`
  |
help: you can convert an `i32` to a `usize` and panic if the converted value doesn't fit
  |
3 |       do_something(x.try_into().unwrap());
  |                    ^^^^^^^^^^^^^^^^^^^^^

Alternatively the output could mention why the type was inferred to be usize:

  |
2 |   for x in -5..5 {
  |            ^^ the trait `Neg` is not implemented for `usize`
  |
help: type `usize` was inferred due to call on line 3
  |
3 |      do_something(x);
  |                   ^

[leonardo-m]

Wow, explanations for problems caused by type inference, nice :)

[vvzen]

As a new starter of Rust, this also caught me off guard. Thanks for the explanation! Was there any progress made here ?

[estebank]

Current output:

error[E0277]: the trait bound `usize: Neg` is not satisfied
 --> src/main.rs:2:12
  |
2 |   for x in -5..5 {
  |            ^^ the trait `Neg` is not implemented for `usize`
  |
  = help: the following other types implement trait `Neg`:
            isize
            i8
            i16
            i32
            i64
            i128
            f32
            f64
          and 8 others

It should likely suggest writing for x in -5i32..5 { instead (which will cause subsequent type errors in the fn call).