Annotating newline characters leads to busy looking multi-line span

[epage]

Take the test case

#[test]
fn span_eol() {
    let source = "#: E112\nif False:\nprint()\n#: E113\nprint()\n";
    let input = &[Group::with_level(Level::ERROR).element(
        Snippet::source(source)
            .line_start(7)
            .fold(false)
            .annotation(AnnotationKind::Primary.span(17..18).label("E112")),
    )];

    let expected_ascii = str![[r#"
   |
 7 |   #: E112
 8 |   if False:
   |  __________^
 9 | | print()
   | |_^ E112
10 |   #: E113
11 |   print()
   |
"#]];
    let renderer = Renderer::plain();
    assert_data_eq!(renderer.render(input), expected_ascii);

    let expected_unicode = str![[r#"
   ╭▸ 
 7 │   #: E112
 8 │   if False:
   │ ┏━━━━━━━━━━┛
 9 │ ┃ print()
   │ ┗━┛ E112
10 │   #: E113
11 │   print()
   ╰╴
"#]];
    let renderer = renderer.decor_style(DecorStyle::Unicode);
    assert_data_eq!(renderer.render(input), expected_unicode);
}

It would be cleaner to annotate the space just after the last character in ascii:

    let expected_ascii = str![[r#"
   |
 7 | #: E112
 8 | if False:
   |          ^ E112
 9 | print()
10 | #: E113
11 | print()
   |
"#]];

or a new-line substitute in unicode

    let expected_unicode = str![[r#"
   ╭▸ 
 7 │ #: E112
 8 │ if False:␤
   │          ━ E112
 9 │ print()
10 │ #: E113
11 │ print()
   ╰╴
"#]];

Identified in astral-sh/ruff#27033

Related issues:

• how to annotate an empty span immediately before the start of a line? #176

[epage]

I think a good set of rules of thumb

• A span starting between \n and a character starts the annotation on the character (how to annotate an empty span immediately before the start of a line? #176)
  • Even if empty (Special way to underline with empty spans #108)
• A span ending between \n and a character ends on the display cell after the last visible character of the line
  • In unicode, we would substitute \r\n / \n with their unicode character
  • Code folding would ensujre the line after to still be visible
  • If someone creates a span for the entire line but doesn't want the newline to be annotated, they should adjust their adapter to not include the newline in the span
  • As for empty spans (Special way to underline with empty spans #108), I am unsure if the newline substitution needs to be shown
• Ideally, all substitutions are colored like ... is.