Modify around add_trait_assoc_items_to_impl to migrate add_missing_impl_members

Signed-off-by: Hayashi Mikihiro <34ttrweoewiwe28@gmail.com>

crates/syntax/src/ast/make.rs

@@ -237,7 +237,7 @@ pub fn assoc_item_list(
     let body_indent = if is_break_braces { "    ".to_owned() } else { String::new() };
 
     let body = match body {
-        Some(bd) => bd.iter().map(|elem| elem.to_string()).join(""),
+        Some(bd) => bd.iter().map(|elem| elem.to_string()).join("\n\n    "),
         None => String::new(),
     };
     ast_from_text(&format!("impl C for D {{{body_newline}{body_indent}{body}{body_newline}}}"))

crates/syntax/src/syntax_editor/edits.rs

@@ -92,6 +92,42 @@ fn get_or_insert_comma_after(editor: &mut SyntaxEditor, syntax: &SyntaxNode) ->
     }
 }
 
+impl ast::AssocItemList {
+    /// Adds a new associated item after all of the existing associated items.
+    ///
+    /// Attention! This function does align the first line of `item` with respect to `self`,
+    /// but it does _not_ change indentation of other lines (if any).
+    pub fn add_items(&self, editor: &mut SyntaxEditor, items: Vec<ast::AssocItem>) {
+        let (indent, position, whitespace) = match self.assoc_items().last() {
+            Some(last_item) => (
+                IndentLevel::from_node(last_item.syntax()),
+                Position::after(last_item.syntax()),
+                "\n\n",
+            ),
+            None => match self.l_curly_token() {
+                Some(l_curly) => {
+                    normalize_ws_between_braces(editor, self.syntax());
+                    (IndentLevel::from_token(&l_curly) + 1, Position::after(&l_curly), "\n")
+                }
+                None => (IndentLevel::single(), Position::last_child_of(self.syntax()), "\n"),
+            },
+        };
+
+        let elements: Vec<SyntaxElement> = items
+            .into_iter()
+            .enumerate()
+            .flat_map(|(i, item)| {
+                let whitespace = if i != 0 { "\n\n" } else { whitespace };
+                vec![
+                    make::tokens::whitespace(&format!("{whitespace}{indent}")).into(),
+                    item.syntax().clone().into(),
+                ]
+            })
+            .collect();
+        editor.insert_all(position, elements);
+    }
+}
+
 impl ast::VariantList {
     pub fn add_variant(&self, editor: &mut SyntaxEditor, variant: &ast::Variant) {
         let make = SyntaxFactory::without_mappings();