Label fails to render when using emojis as source

[GrandChaman]

I noticed that when using an emoji in the source, depending on the range I specify to associate a label to it, I get a label that make sense or not.

Code to reproduce:

use ariadne::{Label, Report, ReportKind, Source};

fn main() {
    for i in 1..=4 {
        println!("Using range {:?} for a 4-bytes emoji", 0..i);
        let mut report = Report::build(ReportKind::Error, (), 0);
        report = report.with_label(Label::new(0..i).with_message("That is very haha 😂"));
        report.finish().print(Source::from("😂")).unwrap();
    }
}

The output:

Using range 0..1 for a 4-bytes emoji
Error:
   ╭─[<unknown>:1:1]
   │
 1 │ 😂
   · ┬
   · ╰── That is very haha 😂
───╯
Using range 0..2 for a 4-bytes emoji
Error:
   ╭─[<unknown>:1:1]
   │
 1 │ 😂
   · ─┬
   ·  ╰── That is very haha 😂
───╯
Using range 0..3 for a 4-bytes emoji
Error:
   ╭─[<unknown>:1:1]
   │
   ·
───╯
Using range 0..4 for a 4-bytes emoji
Error:
   ╭─[<unknown>:1:1]
   │
 1 │ ╭─▶ 😂
───╯

I think the versions with the ranges 0..1, 0..2 and 0..3 don't make sense, but I would expect the last range (0..4) to display the label correctly.

[zesterer]

Ariadne currently uses character indices, not byte indices. I consider this to be a mistake that I want to fix eventually. I'm going to close this in favour of #57, but I'm happy to reopen it if you believe there's more going on here.