Offload safe mutable args with Region and PartitioningStrategy

[Sa4dUs]

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

r? @ZuseZ4

[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]

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

View changes since the review

[ZuseZ4]

Can you add a test for when we try to pass &Region? I think I did that accidentally when writing some of our benchmarks and iirc it didn't get caught anywhere but resulted in a buggy runtime.
A tuple element isn't a location where Rust auto-dereferences, and I don't think there's value in teaching our mapper to look through &Region, so we should just nudge users to pass it by value by rejecting this.

View changes since the review

[ZuseZ4]

Also, can you add a test to show that the borrowchecker rejects writing into x here while the Region is alive? Similar to the preload test here (which I need to clean up too) main...ZuseZ4:rust:offload-explicit-datatransfer3#diff-f828186e6b7ed6e21e5a04161c08c8aa3d03f107a62d728e516c5741b6d6a21a

[ZuseZ4]

wait a second :D #124509

View changes since the review

[Sa4dUs]

lol