Graphical handler panics when span endpoints are not aligned to char boundaries

[Benjamin-L]

This is another regression introduced by #202.

Steps to reproduce

Source: source\n 👼🏼text\n here
Span: (10, 5)

Result:

thread 'single_line_with_wide_char_unaligned_span_start' panicked at 'byte index 3 is not a char boundary; it is inside '👼' (bytes 2..6) of `  👼🏼text`', library/core/src/str/mod.rs:127:5

Expected Behavior

When the span boundaries are inside a char, the rendered highlight should extend to include that char instead of panicking.

This is almost the same as the current behavior of the narratable report handler. The current narratable report handler behaves differently when the start of the span is not char-aligned. In this case, the reported span does not include the char that the span start is inside. I think it makes more sense for the start of the span to be extended backwards, and and then end to be extended forwards. This way, whenever a span endpoint lies inside a char, that char is always included.

Unit tests for this

#[test]
fn single_line_with_wide_char_unaligned_span_start() -> Result<(), MietteError> {
  #[derive(Debug, Diagnostic, Error)]
  #[error("oops!")]
  #[diagnostic(code(oops::my::bad), help("try doing it better next time?"))]
  struct MyBad {
      #[source_code]
      src: NamedSource,
      #[label("this bit here")]
      highlight: SourceSpan,
  }

  let src = "source\n  👼🏼text\n    here".to_string();
  let err = MyBad {
      src: NamedSource::new("bad_file.rs", src),
      highlight: (10, 5).into(),
  };
  let out = fmt_report(err.into());
  println!("Error: {}", out);
  let expected = r#"oops::my::bad

× oops!
 ╭─[bad_file.rs:1:1]
1 │ source
2 │   👼🏼text
 ·   ────┬───
 ·       ╰── this bit here
3 │     here
 ╰────
help: try doing it better next time?
"#
  .trim_start()
  .to_string();
  assert_eq!(expected, out);
  Ok(())
}

#[test]
fn single_line_with_wide_char_unaligned_span_end() -> Result<(), MietteError> {
  #[derive(Debug, Diagnostic, Error)]
  #[error("oops!")]
  #[diagnostic(code(oops::my::bad), help("try doing it better next time?"))]
  struct MyBad {
      #[source_code]
      src: NamedSource,
      #[label("this bit here")]
      highlight: SourceSpan,
  }

  let src = "source\n  text 👼🏼\n    here".to_string();
  let err = MyBad {
      src: NamedSource::new("bad_file.rs", src),
      highlight: (9, 6).into(),
  };
  let out = fmt_report(err.into());
  println!("Error: {}", out);
  let expected = r#"oops::my::bad

× oops!
 ╭─[bad_file.rs:1:1]
1 │ source
2 │   text 👼🏼
 ·   ───┬───
 ·      ╰── this bit here
3 │     here
 ╰────
help: try doing it better next time?
"#
  .trim_start()
  .to_string();
  assert_eq!(expected, out);
  Ok(())
}

[Benjamin-L]

Originally reported in #221 (comment).