Offload safe mutable args with Region and PartitioningStrategy

[Sa4dUs]

Now Region are lang items and mapped as slices.
needs #156620 to work

r? @ZuseZ4

[ZuseZ4]

I don't think copy/clone are sound here. After all we intentionally made the members private to prevent this:

#[offload_kernel]
   fn k(mut a: Region<f32, Linear1D>) {
       let mut b = a;                                   // Copy
       if let (Some(x), Some(y)) = (a.get_mut(), b.get_mut()) { *x = 1.0; *y = 2.0; }  // two live &mut f32 to the same element
   }

We also don't really need or use them that way anywhere. Can you drop them and add a test to make sure it doesn't compile?
For convenience you can instead probably add something like
fn reborrow(&mut self) -> Region<'_, T, S> to reuse it accross launches

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[ZuseZ4]

wait a second :D #124509

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[Sa4dUs]

lol

[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[ZuseZ4]

Next round. I think we should actually refuse all cases (for now) where we have a Region within any ADT/struct/slice, etc. not just &Region. Most of them would be transfered incorrectly, and I don't think we gain much. The other direction is something like Region<Vec<f32>, S>, where we also transfer T incorrectly. But to be fair, I think that direction is already unchecked on main. I assume that offloading a vec will still do the wrong thing, so I'm fine if we accept that here as well till we implement a proper handling. But also feel free to already reject it already if you want.

[ZuseZ4]

I think this is only correct by default, not under -Zrandomize-layout, so you should either mark Region as repr(C), or properly look up the order. I'd to the later, just out of principle so we give fewer guarantees to the user.

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[ZuseZ4]

It should be safe to promise NonNull<T> here and below.

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