It's pretty easy to do it in unsafe code, or to use an existing safe function (implemented internally with unsafe code) like the standard library function split_at_mut to do it.
Here's a doc on how to write such a function yourself, and includes the (actual) source code for split_at_mut:
As others have mentioned, there's nothing special about the standard library in this regard; you can just write the method yourself and it'll work equally well. (The standard library is moderately special in that it gets to use nightly features because dealing with breaking changes is easy for code that literally lives inside Rust's own source repository, but it's not special in terms of its ability to use unsafe code or anything.)
Well, you can use the existing split_at_mut if you're worried about getting your hand-rolled unsafe implementation wrong. It's definitely reasonable if you're splitting on rows; it may or may not be what you want if you're trying to get a slice-like thing that splits on columns.
(There's no reason to avoid safe functions that happen to have unsafe implementations. For instance, the standard-library implementation of indexing a slice, as in b"hello"[3], is just a bounds check plus an unsafe dereference.)
