Display highest severity diagnostic in gutter (helix-editor#2835)

* Display highest severity diagnostic in gutter

* Improve gutter diagnostic performance

Very slight improvement (doesn't really make a difference), iterates over the diagnostics of the line
once instead of twice.

* Add comment justifying unwrap

helix-view/src/gutter.rs

@@ -26,7 +26,18 @@ pub fn diagnostic<'doc>(
     Box::new(move |line: usize, _selected: bool, out: &mut String| {
         use helix_core::diagnostic::Severity;
         if let Ok(index) = diagnostics.binary_search_by_key(&line, |d| d.line) {
-            let diagnostic = &diagnostics[index];
+            let after = diagnostics[index..].iter().take_while(|d| d.line == line);
+
+            let before = diagnostics[..index]
+                .iter()
+                .rev()
+                .take_while(|d| d.line == line);
+
+            let diagnostics_on_line = after.chain(before);
+
+            // This unwrap is safe because the iterator cannot be empty as it contains at least the item found by the binary search.
+            let diagnostic = diagnostics_on_line.max_by_key(|d| d.severity).unwrap();
+
             write!(out, "●").unwrap();
             return Some(match diagnostic.severity {
                 Some(Severity::Error) => error,