> What edition 2018 has right now is migrate mode NLL which emits an error if either NLL borrowck or AST borrowck think there is an issue with your code.

If that were true, then this example would fail to compile, since it fails with the old AST borrowck: https://play.rust-lang.org/?version=stable&mode=debug&editio...

See: https://rust-lang-nursery.github.io/edition-guide/rust-2018/...

This is incorrect. NLL is on for Rust 2018 code.

The transition mode is for Rust 2015 code.

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