UI: the code field draws the equation editor's error mark again

The merge of main brought #471's code_editor_marked, whose mark was drawn by the
layouter inside the code editor; on this branch that layouter lives in code_field,
which had no mark, so the merge did not compile. FieldOptions gains mark (None by
default), code_editor_marked passes it, and code_field underlines it as before.

Test: the_marked_editor_underlines_its_mark (fails when the field ignores the mark).

crates/ui-egui/src/code_editor.rs

@@ -583,7 +583,7 @@ pub fn code_editor_marked(ui: &mut Ui, id: egui::Id, text: &mut String, lang: La
     let max_lines = max_lines.clamp(1, 200);
     let mut response = None;
     egui::Frame::new().fill(t.input_bg).stroke(egui::Stroke::new(1.0, t.input_border)).corner_radius(4.0).inner_margin(4.0).show(ui, |ui| {
-        let options = FieldOptions { max_height: row_h * max_lines as f32, ..FieldOptions::new(lang) };
+        let options = FieldOptions { max_height: row_h * max_lines as f32, mark: mark.clone(), ..FieldOptions::new(lang) };
         response = Some(code_field(ui, id, text, &options).response);
     });
     response.unwrap_or_else(|| ui.allocate_response(egui::Vec2::ZERO, egui::Sense::hover()))
@@ -606,6 +606,8 @@ pub struct FieldOptions<'a> {
     pub caret: Option<usize>,
     /// The field's name for screen readers.
     pub name: &'a str,
+    /// A byte range underlined in the error colour (see [`underline`]).
+    pub mark: Option<Range<usize>>,
 }
 
 impl FieldOptions<'_> {
@@ -621,6 +623,7 @@ impl FieldOptions<'_> {
             // The editor is a generic code field, so the name is not part of the UI's translated
             // strings unless the caller gives one.
             name: "Code Editor",
+            mark: None,
         }
     }
 }
@@ -685,7 +688,7 @@ pub fn code_field(ui: &mut Ui, id: egui::Id, text: &mut String, options: &FieldO
     let danger = t.danger;
     let mut layouter = |ui: &Ui, buf: &dyn egui::TextBuffer, wrap_width: f32| {
         let mut job = layout_job(buf.as_str(), lang, &self::font(), plain, &syntax, wrap_width);
-        if let Some(m) = mark.clone() {
+        if let Some(m) = options.mark.clone() {
             underline(&mut job, m, danger);
         }
         ui.fonts_mut(|f| f.layout_job(job))

crates/ui-egui/src/code_editor/tests.rs

@@ -459,3 +459,24 @@ fn a_repeat_pass_of_the_frame_counts_as_the_next_pass() {
     h.run_steps(1);
     assert_eq!(h.state().0, "    a\n    b");
 }
+
+/// Whether a frame drew text with an underline.
+fn draws_underline(mark: Option<std::ops::Range<usize>>) -> bool {
+    let ctx = egui::Context::default();
+    let mut out = ctx.run_ui(egui::RawInput::default(), |ui| {
+        let mut text = String::from("x^{2");
+        code_editor_marked(ui, egui::Id::unique("marked"), &mut text, Lang::Latex, 3, mark.clone());
+    });
+    out.textures_delta.clear();
+    out.shapes.iter().any(|s| match &s.shape {
+        egui::Shape::Text(t) => t.galley.job.sections.iter().any(|sec| sec.format.underline.width > 0.0),
+        _ => false,
+    })
+}
+
+#[test]
+fn the_marked_editor_underlines_its_mark() {
+    // The equation editor marks an error's position; the code field draws it.
+    assert!(draws_underline(Some(3..4)));
+    assert!(!draws_underline(None));
+}