The only difference between migration mode and NLL-only mode is that errors produced by the NLL checker are downgraded to warnings if the old borrow checker doe

The only difference between migration mode and NLL-only mode is that errors produced by the NLL checker are downgraded to warnings if the old borrow checker doesn't produce any errors on that code[1]. This is strictly more permissive than NLL-only and completely equivalent if you treat warnings as errors. You can completely take advantage of any NLL feature, you just might be able to write code that should be an error but is instead a warning if you hit an edge case (I think this would be considered a bug in one of the borrow checkers since NLL is supposed to allow anything that the old borrow checker allows). [1] https://github.com/rust-lang/rust/issues/46908 https://github.com/rust-lang/rust/issues/46908