Add line numbers in the editor

src/tui.rs

@@ -968,6 +968,11 @@ fn render_editor(frame: &mut Frame, area: Rect, editor: &Editor, focused: bool)
     };
     let block = Block::default().borders(Borders::ALL).title(title);
 
+    // Line numbers occupy a fixed-width left gutter. Width grows with the
+    // largest line number so the body never shifts as lines are added.
+    let gutter_width = line_number_width(editor.buffer.len());
+    let gutter_style = Style::default().fg(Color::DarkGray);
+
     if focused {
         // Highlight the character under the cursor with explicit bg + fg
         // colours (not `REVERSED`) so the cursor cell is filled even when
@@ -981,24 +986,55 @@ fn render_editor(frame: &mut Frame, area: Rect, editor: &Editor, focused: bool)
             .iter()
             .enumerate()
             .map(|(r, line)| {
-                if r == cr {
+                let body = if r == cr {
                     editor_cursor_line(line, cc, cursor_style)
                 } else {
                     Line::from(line.clone())
-                }
+                };
+                prepend_line_number(body, r + 1, gutter_width, gutter_style)
             })
             .collect();
 
         let p = Paragraph::new(lines).block(block).wrap(Wrap { trim: false });
         frame.render_widget(p, area);
     } else {
-        let p = Paragraph::new(editor.text())
-            .block(block)
-            .wrap(Wrap { trim: false });
+        let lines: Vec<Line> = editor
+            .buffer
+            .iter()
+            .enumerate()
+            .map(|(r, line)| {
+                prepend_line_number(
+                    Line::from(line.clone()),
+                    r + 1,
+                    gutter_width,
+                    gutter_style,
+                )
+            })
+            .collect();
+        let p = Paragraph::new(lines).block(block).wrap(Wrap { trim: false });
         frame.render_widget(p, area);
     }
 }
 
+fn line_number_width(line_count: usize) -> usize {
+    let max = line_count.max(1);
+    let mut digits = 1;
+    let mut n = max;
+    while n >= 10 {
+        n /= 10;
+        digits += 1;
+    }
+    digits
+}
+
+fn prepend_line_number(line: Line<'static>, number: usize, width: usize, style: Style) -> Line<'static> {
+    let gutter = format!("{:>width$} │ ", number, width = width);
+    let mut spans = Vec::with_capacity(line.spans.len() + 1);
+    spans.push(Span::styled(gutter, style));
+    spans.extend(line.spans);
+    Line::from(spans)
+}
+
 /// Build the cursor-bearing line as three spans: text before, the
 /// highlighted character at the cursor, text after. If the cursor sits
 /// past the end of the line (or on a whitespace cell), the highlight