merge_tools: take diff editor instruction as Option<&str>

This clarifies that the instructions text can be omitted. All callers appear to
pass non-empty instructions, though.

cli/src/cli_util.rs

@@ -1751,7 +1751,7 @@ impl WorkspaceCommandTransaction<'_> {
         left_tree: &MergedTree,
         right_tree: &MergedTree,
         matcher: &dyn Matcher,
-        instructions: &str,
+        instructions: Option<&str>,
     ) -> Result<MergedTreeId, CommandError> {
         let base_ignores = self.helper.base_ignores()?;
         let settings = &self.helper.settings;
@@ -1772,7 +1772,7 @@ impl WorkspaceCommandTransaction<'_> {
         left_tree: &MergedTree,
         right_tree: &MergedTree,
         matcher: &dyn Matcher,
-        instructions: &str,
+        instructions: Option<&str>,
         interactive: bool,
     ) -> Result<MergedTreeId, CommandError> {
         if interactive {

cli/src/commands/commit.rs

@@ -66,7 +66,7 @@ new working-copy commit.
         &base_tree,
         &commit.tree()?,
         matcher.as_ref(),
-        &instructions,
+        Some(&instructions),
         args.interactive,
     )?;
     let middle_tree = tx.repo().store().get_root_tree(&tree_id)?;

cli/src/commands/diffedit.rs

@@ -90,7 +90,13 @@ don't make any changes, then the operation will be aborted.",
     );
     let base_tree = merge_commit_trees(tx.repo(), base_commits.as_slice())?;
     let tree = target_commit.tree()?;
-    let tree_id = tx.edit_diff(ui, &base_tree, &tree, &EverythingMatcher, &instructions)?;
+    let tree_id = tx.edit_diff(
+        ui,
+        &base_tree,
+        &tree,
+        &EverythingMatcher,
+        Some(&instructions),
+    )?;
     if tree_id == *target_commit.tree_id() {
         writeln!(ui.stderr(), "Nothing changed.")?;
     } else {

cli/src/commands/move.rs

@@ -93,7 +93,7 @@ from the source will be moved into the destination.
         &parent_tree,
         &source_tree,
         matcher.as_ref(),
-        &instructions,
+        Some(&instructions),
         args.interactive,
     )?;
     if args.interactive && new_parent_tree_id == parent_tree.id() {

cli/src/commands/split.rs

@@ -81,7 +81,7 @@ don't make any changes, then the operation will be aborted.
         &base_tree,
         &end_tree,
         matcher.as_ref(),
-        &instructions,
+        Some(&instructions),
         interactive,
     )?;
     if &selected_tree_id == commit.tree_id() && interactive {

cli/src/commands/squash.rs

@@ -90,7 +90,7 @@ from the source will be moved into the parent.
         &parent_tree,
         &tree,
         matcher.as_ref(),
-        &instructions,
+        Some(&instructions),
         args.interactive,
     )?;
     if &new_parent_tree_id == parent.tree_id() {

cli/src/commands/unsquash.rs

@@ -86,7 +86,7 @@ aborted.
             &parent_base_tree,
             &parent_tree,
             &EverythingMatcher,
-            &instructions,
+            Some(&instructions),
         )?;
         if new_parent_tree_id == parent_base_tree.id() {
             return Err(user_error("No changes selected"));

cli/src/merge_tools/external.rs

@@ -427,7 +427,7 @@ pub fn edit_diff_external(
     left_tree: &MergedTree,
     right_tree: &MergedTree,
     matcher: &dyn Matcher,
-    instructions: &str,
+    instructions: Option<&str>,
     base_ignores: Arc<GitIgnoreFile>,
     settings: &UserSettings,
 ) -> Result<MergedTreeId, DiffEditError> {
@@ -452,10 +452,13 @@ pub fn edit_diff_external(
     let instructions_path_to_cleanup = output_wc_path.join("JJ-INSTRUCTIONS");
     // In the unlikely event that the file already exists, then the user will simply
     // not get any instructions.
-    let add_instructions = settings.diff_instructions()
-        && !instructions.is_empty()
-        && !instructions_path_to_cleanup.exists();
-    if add_instructions {
+    let add_instructions = if settings.diff_instructions() && !instructions_path_to_cleanup.exists()
+    {
+        instructions
+    } else {
+        None
+    };
+    if let Some(instructions) = add_instructions {
         let mut output_instructions_file =
             File::create(&instructions_path_to_cleanup).map_err(ExternalToolError::SetUpDir)?;
         if diff_wc.right_working_copy_path() != output_wc_path {
@@ -513,7 +516,7 @@ diff editing in mind and be a little inaccurate.
             exit_status,
         }));
     }
-    if add_instructions {
+    if add_instructions.is_some() {
         std::fs::remove_file(instructions_path_to_cleanup).ok();
     }
 

cli/src/merge_tools/mod.rs

@@ -132,7 +132,7 @@ pub fn edit_diff(
     left_tree: &MergedTree,
     right_tree: &MergedTree,
     matcher: &dyn Matcher,
-    instructions: &str,
+    instructions: Option<&str>,
     base_ignores: Arc<GitIgnoreFile>,
     settings: &UserSettings,
 ) -> Result<MergedTreeId, DiffEditError> {