We want to migrate all assists to use the new SyntaxEditor/SyntaxFactory abstraction. This will unblock moving to immutable syntax trees (#15710). In doing so, we can choose a flat, contiguous representation that should yield significant performance wins (#17491).

@DropDemBits has outlined the process for migrating most assists:

• Create a SyntaxFactory, optionally named make to ease migration
• Create a SyntaxEditor using builder.make_editor with a node from the file
• Replace all make:: with make
  • Add any missing methods to syntax::ast::syntax_factory::constructors as needed
• Replace all ted:: with editor.
• After all changes are complete:
  • editor.add_mappings(make.finish_with_mappings()) to add the generated mappings
  • builder.add_file_edits(ctx.file_id(), editor) to add the edits
    For assists that use snippets, check the snippet capability, and then use editor.add_annotation to associate the snippets with the elements.

They have also already migrated the extract_variable assist in #18196 which should be a useful reference.

The assists are located in crates/ide-assists/src/handlers. They should all have tests which you can use to verify that your change works properly by running:

cargo test --package ide-assists -- handlers::<ASSIST>::tests  

replacing <ASSIST> with the assist name, e.g., add_braces.