[gpm]

IANAL - But I think it depends on how much it is a "translation", and how much it is a reimplementation of an identical algorithm.

As I understand it copyright doesn't apply to the algorithm. It does apply to things like code organization, variable naming, and comments.

[UncleEntity]

> Original elisp algorithm. While using the original elisp source as a guideline might result in less idiomatic Rust code...

Seems to be a pretty direct translation (without reading the sources of both).

Now, if they took the original source and used it to implement a grammar then based the rust code off said grammar they might have a case -- which, I believe, is what a "clean room" implementation would do, preferably with two different people doing either task.

IMHO this is a clear copyright violation else you could just "reimplement" your way out of any license.

[gpm]

AIUI clean room implementation isn't required, just good practice to avoid

- Accidentally copying copyrighted material (past what fair use allows)

- Expensive lawsuits where you have to actually argue that "no we didn't copy copyrighted elements"

We're talking about two open source projects here, the latter anyways doesn't really matter.