Strip comment and list prefixes in vim: join lines (#49295)

Closes #ISSUE

Before you mark this PR as ready for review, make sure that you have:
- [ ] Added a solid test coverage and/or screenshots from doing manual
testing
- [ ] Done a self-review taking into account security and performance
aspects
- [ ] Aligned any UI changes with the [UI
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)

Release Notes:

- Improved the behavior of the `vim: join lines` and `editor: join lines`. Comment and other language specific line prefixes are now stripped from the join line

crates/editor/src/editor.rs

@@ -11334,14 +11334,66 @@ impl Editor {
                     let end_of_line = Point::new(row.0, snapshot.line_len(row));
                     let next_line_row = row.next_row();
                     let indent = snapshot.indent_size_for_line(next_line_row);
-                    let start_of_next_line = Point::new(next_line_row.0, indent.len);
+                    let mut join_start_column = indent.len;
 
-                    let replace =
-                        if snapshot.line_len(next_line_row) > indent.len && insert_whitespace {
-                            " "
-                        } else {
-                            ""
-                        };
+                    if let Some(language_scope) =
+                        snapshot.language_scope_at(Point::new(next_line_row.0, indent.len))
+                    {
+                        let line_end =
+                            Point::new(next_line_row.0, snapshot.line_len(next_line_row));
+                        let line_text_after_indent = snapshot
+                            .text_for_range(Point::new(next_line_row.0, indent.len)..line_end)
+                            .collect::<String>();
+
+                        if !line_text_after_indent.is_empty() {
+                            let block_prefix = language_scope
+                                .block_comment()
+                                .map(|c| c.prefix.as_ref())
+                                .filter(|p| !p.is_empty());
+                            let doc_prefix = language_scope
+                                .documentation_comment()
+                                .map(|c| c.prefix.as_ref())
+                                .filter(|p| !p.is_empty());
+                            let all_prefixes = language_scope
+                                .line_comment_prefixes()
+                                .iter()
+                                .map(|p| p.as_ref())
+                                .chain(block_prefix)
+                                .chain(doc_prefix)
+                                .chain(language_scope.unordered_list().iter().map(|p| p.as_ref()));
+
+                            let mut longest_prefix_len = None;
+                            for prefix in all_prefixes {
+                                let trimmed = prefix.trim_end();
+                                if line_text_after_indent.starts_with(trimmed) {
+                                    let candidate_len =
+                                        if line_text_after_indent.starts_with(prefix) {
+                                            prefix.len()
+                                        } else {
+                                            trimmed.len()
+                                        };
+                                    if longest_prefix_len.map_or(true, |len| candidate_len > len) {
+                                        longest_prefix_len = Some(candidate_len);
+                                    }
+                                }
+                            }
+
+                            if let Some(prefix_len) = longest_prefix_len {
+                                join_start_column =
+                                    join_start_column.saturating_add(prefix_len as u32);
+                            }
+                        }
+                    }
+
+                    let start_of_next_line = Point::new(next_line_row.0, join_start_column);
+
+                    let replace = if snapshot.line_len(next_line_row) > join_start_column
+                        && insert_whitespace
+                    {
+                        " "
+                    } else {
+                        ""
+                    };
 
                     this.buffer.update(cx, |buffer, cx| {
                         buffer.edit([(end_of_line..start_of_next_line, replace)], None, cx)

crates/editor/src/editor_tests.rs

@@ -5014,6 +5014,163 @@ async fn test_join_lines_with_git_diff_base(executor: BackgroundExecutor, cx: &m
     );
 }
 
+#[gpui::test]
+async fn test_join_lines_strips_comment_prefix(cx: &mut TestAppContext) {
+    init_test(cx, |_| {});
+
+    {
+        let language = Arc::new(Language::new(
+            LanguageConfig {
+                line_comments: vec!["// ".into(), "/// ".into()],
+                documentation_comment: Some(BlockCommentConfig {
+                    start: "/*".into(),
+                    end: "*/".into(),
+                    prefix: "* ".into(),
+                    tab_size: 1,
+                }),
+                ..LanguageConfig::default()
+            },
+            None,
+        ));
+
+        let mut cx = EditorTestContext::new(cx).await;
+        cx.update_buffer(|buffer, cx| buffer.set_language(Some(language), cx));
+
+        // Strips the comment prefix (with trailing space) from the joined-in line.
+        cx.set_state(indoc! {"
+            // ˇfoo
+            // bar
+        "});
+        cx.update_editor(|e, window, cx| e.join_lines(&JoinLines, window, cx));
+        cx.assert_editor_state(indoc! {"
+            // fooˇ bar
+        "});
+
+        // Strips the longer doc-comment prefix when both `//` and `///` match.
+        cx.set_state(indoc! {"
+            /// ˇfoo
+            /// bar
+        "});
+        cx.update_editor(|e, window, cx| e.join_lines(&JoinLines, window, cx));
+        cx.assert_editor_state(indoc! {"
+            /// fooˇ bar
+        "});
+
+        // Does not strip when the second line is a regular line (no comment prefix).
+        cx.set_state(indoc! {"
+            // ˇfoo
+            bar
+        "});
+        cx.update_editor(|e, window, cx| e.join_lines(&JoinLines, window, cx));
+        cx.assert_editor_state(indoc! {"
+            // fooˇ bar
+        "});
+
+        // No-whitespace join also strips the comment prefix.
+        cx.set_state(indoc! {"
+            // ˇfoo
+            // bar
+        "});
+        cx.update_editor(|e, window, cx| e.join_lines_impl(false, window, cx));
+        cx.assert_editor_state(indoc! {"
+            // fooˇbar
+        "});
+
+        // Strips even when the joined-in line is just the bare prefix (no trailing space).
+        cx.set_state(indoc! {"
+            // ˇfoo
+            //
+        "});
+        cx.update_editor(|e, window, cx| e.join_lines(&JoinLines, window, cx));
+        cx.assert_editor_state(indoc! {"
+            // fooˇ
+        "});
+
+        // Mixed line comment prefix types: the longer matching prefix is stripped.
+        cx.set_state(indoc! {"
+            // ˇfoo
+            /// bar
+        "});
+        cx.update_editor(|e, window, cx| e.join_lines(&JoinLines, window, cx));
+        cx.assert_editor_state(indoc! {"
+            // fooˇ bar
+        "});
+
+        // Strips block comment body prefix (`* `) from the joined-in line.
+        cx.set_state(indoc! {"
+             * ˇfoo
+             * bar
+        "});
+        cx.update_editor(|e, window, cx| e.join_lines(&JoinLines, window, cx));
+        cx.assert_editor_state(indoc! {"
+             * fooˇ bar
+        "});
+
+        // Strips bare block comment body prefix (`*` without trailing space).
+        cx.set_state(indoc! {"
+             * ˇfoo
+             *
+        "});
+        cx.update_editor(|e, window, cx| e.join_lines(&JoinLines, window, cx));
+        cx.assert_editor_state(indoc! {"
+             * fooˇ
+        "});
+    }
+
+    {
+        let markdown_language = Arc::new(Language::new(
+            LanguageConfig {
+                unordered_list: vec!["- ".into(), "* ".into(), "+ ".into()],
+                ..LanguageConfig::default()
+            },
+            None,
+        ));
+
+        let mut cx = EditorTestContext::new(cx).await;
+        cx.update_buffer(|buffer, cx| buffer.set_language(Some(markdown_language), cx));
+
+        // Strips the `- ` list marker from the joined-in line.
+        cx.set_state(indoc! {"
+            - ˇfoo
+            - bar
+        "});
+        cx.update_editor(|e, window, cx| e.join_lines(&JoinLines, window, cx));
+        cx.assert_editor_state(indoc! {"
+            - fooˇ bar
+        "});
+
+        // Strips the `* ` list marker from the joined-in line.
+        cx.set_state(indoc! {"
+            * ˇfoo
+            * bar
+        "});
+        cx.update_editor(|e, window, cx| e.join_lines(&JoinLines, window, cx));
+        cx.assert_editor_state(indoc! {"
+            * fooˇ bar
+        "});
+
+        // Strips the `+ ` list marker from the joined-in line.
+        cx.set_state(indoc! {"
+            + ˇfoo
+            + bar
+        "});
+        cx.update_editor(|e, window, cx| e.join_lines(&JoinLines, window, cx));
+        cx.assert_editor_state(indoc! {"
+            + fooˇ bar
+        "});
+
+        // No-whitespace join also strips the list marker.
+        cx.set_state(indoc! {"
+            - ˇfoo
+            - bar
+        "});
+        cx.update_editor(|e, window, cx| e.join_lines_impl(false, window, cx));
+        cx.assert_editor_state(indoc! {"
+            - fooˇbar
+        "});
+    }
+}
+
 #[gpui::test]
 async fn test_custom_newlines_cause_no_false_positive_diffs(
     executor: BackgroundExecutor,