Error msg is obtuse when doing `float_fallback += 1;` (or any int literal)

[mdinger]

This may be WONTFIX but I'm reporting it in case it isn't. I'm not sure what the opinion on this is exactly anyway. The idea came from here. This error may come up very easily on rust-lang.org and so is likely very noticeable. This:

fn main() {
    let mut x = 0.0;
    x += 1;
}

Gives this error:

<anon>:3:5: 3:6 error: binary assignment operation `+=` cannot be applied to types `_` and `_` [E0368]
<anon>:3     x += 1;
             ^
error: aborting due to previous error
playpen: application terminated with error code 101

There are three issues, particularly to a new user (first time exploring rust-lang.org):

• The distinction 0 and 0.0 may seem bizarre
• Use of the mysterious type _ will be puzzling and makes the error confusing
• Comparing _ and _ makes it worse.

It would be ideal if 1 could be inferred to the type of x in this case. This would also avoid this confusing error on the rust main landing page.

[daboross]

Just as a note, I think both types are _ because rustc doesn't know exactly what types they actually are. The 1 could be any number type really, from u8 to i64, and the 0.0 could be either f32 or f64. It should have a better way of describing this, yes, but I don't think it's exactly clear what exactly it should be saying instead of _. Since it doesn't know what either of them are, it is surprising that it doesn't assume the 1 to be an f32/f64 as well.

[bluss]

Wishes for new features should go on the RFC repo, and the issue is already present: rust-lang/rfcs#260

[bluss]

Note: The well-defined default type for 0.0 is f64 in Rust (in case it is not inferred to a specific floating point type)

[pnkfelix]

Nonetheless, the error message here is particularly bad ...

[pnkfelix]

(but I agree with @bluss's assessment, and conclude that my title change is not quite right.)

[mdinger]

@bluss It didn't occur to me this would be a feature. I thought it might have just been a bug or old design decision.

What a better error message would look like isn't clear which is why I didn't suggest it though.

[jaredly]

Instead of '_' it would be nice to have "integral type" (that language shows up elsewhere) and "float type"

[leoyvens]

1.14 output:

error[E0277]: the trait bound `{float}: std::ops::AddAssign<{integer}>` is not satisfied
 --> <anon>:3:5
  |
3 |     x += 1;
  |     ^^^^^^ the trait `std::ops::AddAssign<{integer}>` is not implemented for `{float}`
  |
  = help: the following implementations were found:
  = help:   <f64 as std::ops::AddAssign>
  = help:   <f32 as std::ops::AddAssign>
  = help:   <u64 as std::ops::AddAssign>
  = help:   <std::time::Duration as std::ops::AddAssign>
  = help: and 24 others

This bug is fixed.

[bluss]

Thank you. New bugs can be filed if there are problems with the new error message.