Off-by-one error in multiline error highlights

[digama0]

In both cases, the highlight begins at letter e and ends at letter l. In the one-line case, it is correct:

error: oops
 --> <current file>:1:6
  |
1 | abcd efgh ijkl mnop
  |      ^^^^^^^^^ oops
  |

But in multiple lines it is off by one - the end pointer points to one past the end rather than the last character of the highlight:

error: oops
 --> <current file>:1:6
  |
1 |   abcd efgh
  |  ______^
2 | | ijkl mnop
  | |_____^ oops
  |

I could propose code but I'm not sure I'll get all the details of continuations right since I haven't used those and am not certain of the desired behavior. Here's the issue presented as a test case.

use annotate_snippets::{display_list::DisplayList, snippet};
use snippet::Snippet;

#[test]
fn with_space_works() {
    let snippets = Snippet {
        title: Some(snippet::Annotation {
            id: None,
            label: Some("oops"),
            annotation_type: snippet::AnnotationType::Error,
        }),
        footer: vec![],
        slices: vec![snippet::Slice {
            source: "abcd efgh ijkl mnop",
            line_start: 1,
            origin: Some("<current file>"),
            annotations: vec![snippet::SourceAnnotation {
                range: (5, 14),
                label: "oops",
                annotation_type: snippet::AnnotationType::Error,
            }],
            fold: true,
        }],
        opt: Default::default(),
    };
    let expected = r#"error: oops
 --> <current file>:1:6
  |
1 | abcd efgh ijkl mnop
  |      ^^^^^^^^^ oops
  |"#;
    assert_eq!(DisplayList::from(snippets).to_string(), expected);
}

#[test]
fn with_newline_fails() {
    let snippets = Snippet {
        title: Some(snippet::Annotation {
            id: None,
            label: Some("oops"),
            annotation_type: snippet::AnnotationType::Error,
        }),
        footer: vec![],
        slices: vec![snippet::Slice {
            source: "abcd efgh\nijkl mnop",
            line_start: 1,
            origin: Some("<current file>"),
            annotations: vec![snippet::SourceAnnotation {
                range: (5, 14),
                label: "oops",
                annotation_type: snippet::AnnotationType::Error,
            }],
            fold: true,
        }],
        opt: Default::default(),
    };
    let expected = r#"error: oops
 --> <current file>:1:6
  |
1 |   abcd efgh
  |  ______^
2 | | ijkl mnop
  | |____^ oops
  |"#;

    assert_eq!(DisplayList::from(snippets).to_string(), expected);
}

[digama0]

A possible fix is to change

annotate-snippets-rs/src/display_list/from_snippet.rs

Lines 430 to 433 in 46c5588

	let range = (
	(end - line_start) - margin_left,
	(end - line_start + 1) - margin_left,
	);

to

                        let end_mark = (end - line_start).saturating_sub(1);
                        let range = (
                            end_mark - margin_left,
                            (end_mark + 1) - margin_left,
                        );

The question here is what to do about annotations that end on a newline, like "efgh\n", since there is no final character to point to. The suggestion here highlights it as

error: oops
 --> <current file>:1:6
  |
1 |   abcd efgh
  |  ______^
2 | | ijkl mnop
  | |_^ oops
  |

which is the same as the current behavior, but if the annotation is extended to "efgh\ni" then you get the exact same result, and thereafter it will point at the last character in the highlight, so that the original example with `"efgh\nijkl" is highlighted as

error: oops
 --> <current file>:1:6
  |
1 |   abcd efgh
  |  ______^
2 | | ijkl mnop
  | |____^ oops
  |

I don't know how to test what rustc does if the error highlight ends on a newline; it generally tries to avoid this and it also doesn't come up in my application so it doesn't seem very important.

[digama0]

Ah, I found the relevant code in rustc.

            start_col: self.end_col.saturating_sub(1),
            end_col: self.end_col,

Using end_col.saturating_sub(1) for the start and end_col for the end leads to the somewhat odd situation that the marker disappears completely if the annotation ends on a newline, like this:

error: oops
 --> <current file>:1:6
  |
1 |   abcd efgh
  |  ______^
2 | | ijkl mnop
  | |_ oops
  |

But this is also an option if that seems reasonable to folks.