Auto merge of #14752 - ponyii:fix/generate_derive_breaks_indentation, r=Veykril

fix: assists no longer break indentation

Fixes https://github.com/rust-lang/rust-analyzer/issues/14674

These are _ad hoc_ patches for a number of assists that can produce incorrectly indented code, namely:
- generate_derive
- add_missing_impl_members
- add_missing_default_members

Some general solution is required in future, as the same problem arises in many other assists, e.g.
- replace_derive_with...
- generate_default_from_enum...
- generate_default_from_new
- generate_delegate_methods
(the list is incomplete)

crates/syntax/src/ast/edit_in_place.rs

@@ -465,6 +465,8 @@ impl ast::Impl {
 }
 
 impl ast::AssocItemList {
+    /// 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_item(&self, item: ast::AssocItem) {
         let (indent, position, whitespace) = match self.assoc_items().last() {
             Some(last_item) => (