With the solution I imagine, this example wouldn't compile. In the crate bar, edition 2021, the code written as:

    fn foo<T>(input: T)

would actually parse as if:

    fn foo<T: Leak>(input: T)

An implicit Leak bound would be inserted into AST of every trait bound in the 2021 edition. Then all code from all editions would be interpreted the same.

This way, the 2021 code wouldn't compile, because the 2024 trait requires `<T>` bound, and the 2021 code can only express `<T: Leak>` and nothing less.

2021 code could not work at all with any 2024 code that allows !Leak. libstd would have to add `+ Leak` to all existing APIs. Upgrading to 2024 edition would need a migration to change the bounds.