Implement indent conversion editor commands (#32340)

## Description of Feature or Change

Zed currently lacks a built-in way to convert a file’s indentation style
on the fly. While it's possible to change indentation behavior via
global or language-specific settings, these changes are persistent and
broad in scope as they apply to all files or all files of a given
language. We believe this could be improved for quick one-off
adjustments to specific files.

This PR introduces two new editor commands:
`Editor::convert_indentation_to_spaces` and
`Editor::convert_indentation_to_tabs`. These commands allow users to
convert the indentation of either the entire buffer or a selection of
lines, to spaces or tabs. Indentation levels are preserved, and any
mixed whitespace lines are properly normalized.

This feature is inspired by VS Code’s "Convert Indentation to
Tabs/Spaces" commands, but offers faster execution and supports
selection-based conversion, making it more flexible for quick formatting
changes.

## Implementation Details

To enable selection-based indentation conversion, we initially
considered reusing the existing `Editor::manipulate_lines` function,
which handles selections for line-based manipulations. However, this
method was designed specifically for operations like sorting or
reversing lines, and does not allow modifications to the line contents
themselves.

To address this limitation, we refactored the method into a more
flexible version: `Editor::manipulate_generic_lines`. This new method
passes a reference to the selected text directly into a callback, giving
the callback full control over how to process and construct the
resulting lines. The callback returns a `String` containing the modified
text, as well as the number of lines before and after the
transformation. These counts are computed using `.len()` on the line
vectors during manipulation, which is more efficient than calculating
them after the fact.


```rust
fn manipulate_generic_lines<M>(
  &mut self,
  window: &mut Window,
  cx: &mut Context<Self>,
  mut manipulate: M,
) where
   M: FnMut(&str) -> (String, usize, usize),
 {
   // ... Get text from buffer.text_for_range() ...
   let (new_text, lines_before, lines_after) = manipulate(&text);
   // ...
``` 

We now introduce two specialized methods:
`Editor::manipulate_mutable_lines` and
`Editor::manipulate_immutable_lines`. Each editor command selects the
appropriate method based on whether it needs to modify line contents or
simply reorder them. This distinction is important for performance: when
line contents remain unchanged, working with an immutable reference as
`&mut Vec<&str>` is both faster and more memory-efficient than using an
owned `&mut Vec<String>`.

## Demonstration


https://github.com/user-attachments/assets/e50b37ea-a128-4c2a-b252-46c3c4530d97



Release Notes:

- Added `editor::ConvertIndentationToSpaces` and
`editor::ConvertIndentationToTabs` actions to change editor indents

---------

Co-authored-by: Pedro Silveira <pedroruanosilveira@tecnico.ulisboa.pt>

crates/editor/src/editor.rs

@@ -10080,7 +10080,7 @@ impl Editor {
         window: &mut Window,
         cx: &mut Context<Self>,
     ) {
-        self.manipulate_lines(window, cx, |lines| lines.sort())
+        self.manipulate_immutable_lines(window, cx, |lines| lines.sort())
     }
 
     pub fn sort_lines_case_insensitive(
@@ -10089,7 +10089,7 @@ impl Editor {
         window: &mut Window,
         cx: &mut Context<Self>,
     ) {
-        self.manipulate_lines(window, cx, |lines| {
+        self.manipulate_immutable_lines(window, cx, |lines| {
             lines.sort_by_key(|line| line.to_lowercase())
         })
     }
@@ -10100,7 +10100,7 @@ impl Editor {
         window: &mut Window,
         cx: &mut Context<Self>,
     ) {
-        self.manipulate_lines(window, cx, |lines| {
+        self.manipulate_immutable_lines(window, cx, |lines| {
             let mut seen = HashSet::default();
             lines.retain(|line| seen.insert(line.to_lowercase()));
         })
@@ -10112,7 +10112,7 @@ impl Editor {
         window: &mut Window,
         cx: &mut Context<Self>,
     ) {
-        self.manipulate_lines(window, cx, |lines| {
+        self.manipulate_immutable_lines(window, cx, |lines| {
             let mut seen = HashSet::default();
             lines.retain(|line| seen.insert(*line));
         })
@@ -10555,20 +10555,20 @@ impl Editor {
     }
 
     pub fn reverse_lines(&mut self, _: &ReverseLines, window: &mut Window, cx: &mut Context<Self>) {
-        self.manipulate_lines(window, cx, |lines| lines.reverse())
+        self.manipulate_immutable_lines(window, cx, |lines| lines.reverse())
     }
 
     pub fn shuffle_lines(&mut self, _: &ShuffleLines, window: &mut Window, cx: &mut Context<Self>) {
-        self.manipulate_lines(window, cx, |lines| lines.shuffle(&mut thread_rng()))
+        self.manipulate_immutable_lines(window, cx, |lines| lines.shuffle(&mut thread_rng()))
     }
 
-    fn manipulate_lines<Fn>(
+    fn manipulate_lines<M>(
         &mut self,
         window: &mut Window,
         cx: &mut Context<Self>,
-        mut callback: Fn,
+        mut manipulate: M,
     ) where
-        Fn: FnMut(&mut Vec<&str>),
+        M: FnMut(&str) -> LineManipulationResult,
     {
         self.hide_mouse_cursor(HideMouseCursorOrigin::TypingAction, cx);
 
@@ -10601,18 +10601,18 @@ impl Editor {
                 .text_for_range(start_point..end_point)
                 .collect::<String>();
 
-            let mut lines = text.split('\n').collect_vec();
+            let LineManipulationResult {
+                new_text,
+                line_count_before,
+                line_count_after,
+            } = manipulate(&text);
 
-            let lines_before = lines.len();
-            callback(&mut lines);
-            let lines_after = lines.len();
-
-            edits.push((start_point..end_point, lines.join("\n")));
+            edits.push((start_point..end_point, new_text));
 
             // Selections must change based on added and removed line count
             let start_row =
                 MultiBufferRow(start_point.row + added_lines as u32 - removed_lines as u32);
-            let end_row = MultiBufferRow(start_row.0 + lines_after.saturating_sub(1) as u32);
+            let end_row = MultiBufferRow(start_row.0 + line_count_after.saturating_sub(1) as u32);
             new_selections.push(Selection {
                 id: selection.id,
                 start: start_row,
@@ -10621,10 +10621,10 @@ impl Editor {
                 reversed: selection.reversed,
             });
 
-            if lines_after > lines_before {
-                added_lines += lines_after - lines_before;
-            } else if lines_before > lines_after {
-                removed_lines += lines_before - lines_after;
+            if line_count_after > line_count_before {
+                added_lines += line_count_after - line_count_before;
+            } else if line_count_before > line_count_after {
+                removed_lines += line_count_before - line_count_after;
             }
         }
 
@@ -10669,6 +10669,171 @@ impl Editor {
         })
     }
 
+    fn manipulate_immutable_lines<Fn>(
+        &mut self,
+        window: &mut Window,
+        cx: &mut Context<Self>,
+        mut callback: Fn,
+    ) where
+        Fn: FnMut(&mut Vec<&str>),
+    {
+        self.manipulate_lines(window, cx, |text| {
+            let mut lines: Vec<&str> = text.split('\n').collect();
+            let line_count_before = lines.len();
+
+            callback(&mut lines);
+
+            LineManipulationResult {
+                new_text: lines.join("\n"),
+                line_count_before,
+                line_count_after: lines.len(),
+            }
+        });
+    }
+
+    fn manipulate_mutable_lines<Fn>(
+        &mut self,
+        window: &mut Window,
+        cx: &mut Context<Self>,
+        mut callback: Fn,
+    ) where
+        Fn: FnMut(&mut Vec<Cow<'_, str>>),
+    {
+        self.manipulate_lines(window, cx, |text| {
+            let mut lines: Vec<Cow<str>> = text.split('\n').map(Cow::from).collect();
+            let line_count_before = lines.len();
+
+            callback(&mut lines);
+
+            LineManipulationResult {
+                new_text: lines.join("\n"),
+                line_count_before,
+                line_count_after: lines.len(),
+            }
+        });
+    }
+
+    pub fn convert_indentation_to_spaces(
+        &mut self,
+        _: &ConvertIndentationToSpaces,
+        window: &mut Window,
+        cx: &mut Context<Self>,
+    ) {
+        let settings = self.buffer.read(cx).language_settings(cx);
+        let tab_size = settings.tab_size.get() as usize;
+
+        self.manipulate_mutable_lines(window, cx, |lines| {
+            // Allocates a reasonably sized scratch buffer once for the whole loop
+            let mut reindented_line = String::with_capacity(MAX_LINE_LEN);
+            // Avoids recomputing spaces that could be inserted many times
+            let space_cache: Vec<Vec<char>> = (1..=tab_size)
+                .map(|n| IndentSize::spaces(n as u32).chars().collect())
+                .collect();
+
+            for line in lines.iter_mut().filter(|line| !line.is_empty()) {
+                let mut chars = line.as_ref().chars();
+                let mut col = 0;
+                let mut changed = false;
+
+                while let Some(ch) = chars.next() {
+                    match ch {
+                        ' ' => {
+                            reindented_line.push(' ');
+                            col += 1;
+                        }
+                        '\t' => {
+                            // \t are converted to spaces depending on the current column
+                            let spaces_len = tab_size - (col % tab_size);
+                            reindented_line.extend(&space_cache[spaces_len - 1]);
+                            col += spaces_len;
+                            changed = true;
+                        }
+                        _ => {
+                            // If we dont append before break, the character is consumed
+                            reindented_line.push(ch);
+                            break;
+                        }
+                    }
+                }
+
+                if !changed {
+                    reindented_line.clear();
+                    continue;
+                }
+                // Append the rest of the line and replace old reference with new one
+                reindented_line.extend(chars);
+                *line = Cow::Owned(reindented_line.clone());
+                reindented_line.clear();
+            }
+        });
+    }
+
+    pub fn convert_indentation_to_tabs(
+        &mut self,
+        _: &ConvertIndentationToTabs,
+        window: &mut Window,
+        cx: &mut Context<Self>,
+    ) {
+        let settings = self.buffer.read(cx).language_settings(cx);
+        let tab_size = settings.tab_size.get() as usize;
+
+        self.manipulate_mutable_lines(window, cx, |lines| {
+            // Allocates a reasonably sized buffer once for the whole loop
+            let mut reindented_line = String::with_capacity(MAX_LINE_LEN);
+            // Avoids recomputing spaces that could be inserted many times
+            let space_cache: Vec<Vec<char>> = (1..=tab_size)
+                .map(|n| IndentSize::spaces(n as u32).chars().collect())
+                .collect();
+
+            for line in lines.iter_mut().filter(|line| !line.is_empty()) {
+                let mut chars = line.chars();
+                let mut spaces_count = 0;
+                let mut first_non_indent_char = None;
+                let mut changed = false;
+
+                while let Some(ch) = chars.next() {
+                    match ch {
+                        ' ' => {
+                            // Keep track of spaces. Append \t when we reach tab_size
+                            spaces_count += 1;
+                            changed = true;
+                            if spaces_count == tab_size {
+                                reindented_line.push('\t');
+                                spaces_count = 0;
+                            }
+                        }
+                        '\t' => {
+                            reindented_line.push('\t');
+                            spaces_count = 0;
+                        }
+                        _ => {
+                            // Dont append it yet, we might have remaining spaces
+                            first_non_indent_char = Some(ch);
+                            break;
+                        }
+                    }
+                }
+
+                if !changed {
+                    reindented_line.clear();
+                    continue;
+                }
+                // Remaining spaces that didn't make a full tab stop
+                if spaces_count > 0 {
+                    reindented_line.extend(&space_cache[spaces_count - 1]);
+                }
+                // If we consume an extra character that was not indentation, add it back
+                if let Some(extra_char) = first_non_indent_char {
+                    reindented_line.push(extra_char);
+                }
+                // Append the rest of the line and replace old reference with new one
+                reindented_line.extend(chars);
+                *line = Cow::Owned(reindented_line.clone());
+                reindented_line.clear();
+            }
+        });
+    }
+
     pub fn convert_to_upper_case(
         &mut self,
         _: &ConvertToUpperCase,
@@ -22941,6 +23106,12 @@ pub struct LineHighlight {
     pub type_id: Option<TypeId>,
 }
 
+struct LineManipulationResult {
+    pub new_text: String,
+    pub line_count_before: usize,
+    pub line_count_after: usize,
+}
+
 fn render_diff_hunk_controls(
     row: u32,
     status: &DiffHunkStatus,