Avoid UB in PySlice::indices

[Amanieu]

Mutating a variables that is not declared with mut is undefined behavior.

[birkenfeld]

Just to check that my understanding is correct: the problem is not the missing let mut but the mutation through a &shared ref, right? And let mut is necessary to get a &mut ref.

[Amanieu]

From the list of undefined behavior in the reference:

Mutating immutable data. All data inside a const item is immutable. Moreover, all data reached through a shared reference or data owned by an immutable binding is immutable, unless that data is contained within an UnsafeCell<U>.

[davidhewitt]

Thank you for submitting this, appreciate the fix! Out of curiosity, did you find this via automated tooling or because you happened to be browsing the PyO3 source?

I might take a moment to check our other pointer casts in case this pattern was also employed elsewhere.

[Amanieu]

I was working on some code which used indices and had a look into the source code to see how it dealt with the isize/usize differences. Then I noticed the bug which immediately stood out.