Suggestion for add-assigning chars to u32 could be improved

[VoleurVolant]

Given the following code:

https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=005f59d63a84412a73cfe88146c11f39

fn main() {
 let greeting = "Hello, World!";
 let mut output = 0;
 for c in greeting.chars() {
  output += c;
 }
 println!("The values of the characters in \"{greeting}\" sum to {output}");
}

The current output is:

error[E0277]: cannot add-assign `char` to `{integer}`
 --> src/main.rs:5:10
  |
5 |   output += c;
  |          ^^ no implementation for `{integer} += char`
  |
  = help: the trait `AddAssign<char>` is not implemented for `{integer}`

For more information about this error, try `rustc --explain E0277`.

Ideally the output should look like:

error[E0277]: cannot add-assign `char` to `{integer}`
 --> src/main.rs:5:10
  |
5 |   output += c;
  |          ^^ no implementation for `{integer} += char`
  |
  = help: the trait `AddAssign<char>` is not implemented for `{integer}`. You may have meant to use `as u32` or `to_digit` on the `char`.

For more information about this error, try `rustc --explain E0277`.

[SamuelBonilla]

I think that it's not an issue

https://users.rust-lang.org/c/community/8

[inquisitivecrystal]

I think that it's not an issue

Could you elaborate on the reason you think so? In general, rust tries to provide helpful diagnostics for errors that someone might reasonably make. It's not that there aren't other solutions, such as asking for help, but simply that it saves time and makes the language more approachable for newcomers.

[workingjubilee]

I agree that the suggestion can be improved.

error[E0277]: cannot add `char` to `{integer}`
 --> src/main.rs:2:15
  |
2 |     let x = 5 + '0';
  |               ^ no implementation for `{integer} + char`
  |
  = help: the trait `Add<char>` is not implemented for `{integer}`
  
error[E0369]: cannot add `u32` to `char`
 --> src/main.rs:2:17
  |
2 |     let x = '5' + 5u32 + '0';
  |             --- ^ ---- u32
  |             |
  |             char

We also don't offer useful advice for the cases of simple LHS or RHS Add either (a type error is good, but we can do better). The diagnostics we provide for those cases, however, will have to be different and more general: it is not clear from context that adding an integer and a char is intended to be an integer or a String, unless it is AddAssigned, at which point we can simply look at the type of the LHS and suggest applicable conversions to that type.

It is particularly worth offering special diagnostics for the cases of u32 with char: Our char type offers an easy conversion to/from u32, and our char is explicitly a USV, backed by a u32 (what other languages call a "rune", which is sometimes a type alias for u32 or i32), so it is easy to assume they wanted that.

People used to some languages may be used to working with an 8-bit char, and might make an incorrect assumption about our char type, but I become less and less confident in that direction as it requires making more and more assumptions.

[estebank]

We already provide suggestions for casting between numeric literals, it could be extended to also account for char:

rust/compiler/rustc_typeck/src/check/demand.rs

Line 808 in 09e4a75

pub fn check_for_cast(

[cameron1024]

Hi, I'm looking at implementing @estebank 's solution, my current plan is to check for the case of "expected u32 or i32, got char", should I submit a PR for this?

[workingjubilee]

Go for it!

[BGR360]

PR merged, issue can be closed?

[workingjubilee]

Hm, should it be? Diagnostic seems unchanged for the example given, even with a type annotation:

error[E0277]: cannot add-assign `char` to `u32`
 --> src/main.rs:5:10
  |
5 |   output += c;
  |          ^^ no implementation for `u32 += char`
  |
  = help: the trait `AddAssign<char>` is not implemented for `u32`

[inquisitivecrystal]

Hm, should it be? Diagnostic seems unchanged for the example given, even with a type annotation:

error[E0277]: cannot add-assign `char` to `u32`
 --> src/main.rs:5:10
  |
5 |   output += c;
  |          ^^ no implementation for `u32 += char`
  |
  = help: the trait `AddAssign<char>` is not implemented for `u32`

I apologize for the early close. It looked like the PR should have taken care of this.

[BGR360]

I should have done some amount of due diligence before suggesting we close it, my b

[estebank]

Current output:

error[E0277]: cannot add-assign `char` to `{integer}`
 --> src/main.rs:5:10
  |
5 |   output += c;
  |          ^^ no implementation for `{integer} += char`
  |
  = help: the trait `AddAssign<char>` is not implemented for `{integer}`
  = help: the following other types implement trait `AddAssign<Rhs>`:
            <isize as AddAssign>
            <isize as AddAssign<&isize>>
            <i8 as AddAssign>
            <i8 as AddAssign<&i8>>
            <i16 as AddAssign>
            <i16 as AddAssign<&i16>>
            <i32 as AddAssign>
            <i32 as AddAssign<&i32>>
          and 20 others