Make test ptr::slice_mut compliant with Tree Borrows

[JoJoDeveloping]

Tree Borrows is a new aliasing model for Rust, which features more precise tracking of pointer aliasing. The aim is to be more lenient than Stacked Borrows, while also having less dirty hacks than SB had.

It turns out that the test ptr::slice_mut here relied on one such hack.

The problematic code is here:

soa-derive/tests/ptr.rs

Lines 77 to 93 in ad918be

	let mut slice = particles.as_mut_slice();
	let ptr = slice.as_mut_ptr();
	unsafe {
	*ptr.as_mut().unwrap().name = String::from("Fe");
	*ptr.as_mut().unwrap().mass = 42.0;
	}
	assert_eq!(slice.name[0], "Fe");
	assert_eq!(slice.mass[0], 42.0);
	unsafe {
	let slice = ParticleSliceMut::from_raw_parts_mut(ptr, 2);
	for mass in slice.mass {
	*mass = -1.0;
	}

There, a slice is created, and then a reference to it is obtained using slice::as_mut_ptr(), which (internally) creates a new mutable reference. This means that the resulting pointer can not be mixed with accesses to the original slice.

However, the test does so: It first uses the pointer, then the original slice, and then again the pointer. The proper solution is to re-derive the pointer when it's needed again, which is what this PR does.

[Luthaf]

Thanks for this contribution! Is this all that's required to comply with tree borrow in this project?

[JoJoDeveloping]

Is this all that's required to comply with tree borrow in this project?

I hope so. I did not audit all code, but usually, code that complies with Stacked Borrows also complies with Tree Borrows. Also note that I only tested the deno_core package, the other tests all fail because they use features / have dependencies not supported by miri.