Remove int2ptr cast in bevy_ptr::dangling_with_align and remove -Zmiri-permissive-provenance in CI (#15311)

# Objective

- Remove an int2ptr cast in `bevy_ptr::dangling_with_align`
- This is flagged by MIRI unless `-Zmiri-permissive-provenance` is used
(like in CI)
- Remove `-Zmiri-permissive-provenance` in CI

## Solution

- Create the raw pointer like
[`std::ptr::without_provenance`](https://doc.rust-lang.org/stable/std/ptr/fn.without_provenance_mut.html)
does, i.e. by starting from a null pointer.

---------

Co-authored-by: Alice Cecile <alice.i.cecile@gmail.com>
Co-authored-by: François Mockers <francois.mockers@vleue.com>

.github/workflows/ci.yml

@@ -92,16 +92,15 @@ jobs:
           components: miri
       - name: CI job
         # To run the tests one item at a time for troubleshooting, use
-        # cargo --quiet test --lib -- --list | sed 's/: test$//' | MIRIFLAGS="-Zmiri-disable-isolation -Zmiri-permissive-provenance -Zmiri-disable-weak-memory-emulation" xargs -n1 cargo miri test -p bevy_ecs --lib -- --exact
+        # cargo --quiet test --lib -- --list | sed 's/: test$//' | MIRIFLAGS="-Zmiri-disable-isolation -Zmiri-disable-weak-memory-emulation" xargs -n1 cargo miri test -p bevy_ecs --lib -- --exact
         run: cargo miri test -p bevy_ecs
         env:
           # -Zrandomize-layout makes sure we dont rely on the layout of anything that might change
           RUSTFLAGS: -Zrandomize-layout
           # https://github.com/rust-lang/miri#miri--z-flags-and-environment-variables
           # -Zmiri-disable-isolation is needed because our executor uses `fastrand` which accesses system time.
-          # -Zmiri-permissive-provenance disables warnings against int2ptr casts (since those are used by once_cell)
           # -Zmiri-ignore-leaks is necessary because a bunch of tests don't join all threads before finishing.
-          MIRIFLAGS: -Zmiri-ignore-leaks -Zmiri-disable-isolation -Zmiri-permissive-provenance
+          MIRIFLAGS: -Zmiri-ignore-leaks -Zmiri-disable-isolation
 
   check-compiles:
     runs-on: ubuntu-latest

crates/bevy_ptr/src/lib.rs

@@ -13,7 +13,7 @@ use core::{
     marker::PhantomData,
     mem::ManuallyDrop,
     num::NonZeroUsize,
-    ptr::NonNull,
+    ptr::{self, NonNull},
 };
 
 /// Used as a type argument to [`Ptr`], [`PtrMut`] and [`OwningPtr`] to specify that the pointer is aligned.
@@ -539,7 +539,8 @@ pub const fn dangling_with_align(align: NonZeroUsize) -> NonNull<u8> {
     debug_assert!(align.is_power_of_two(), "Alignment must be power of two.");
     // SAFETY: The pointer will not be null, since it was created
     // from the address of a `NonZero<usize>`.
-    unsafe { NonNull::new_unchecked(align.get() as *mut u8) }
+    // TODO: use https://doc.rust-lang.org/std/ptr/struct.NonNull.html#method.with_addr once stabilised
+    unsafe { NonNull::new_unchecked(ptr::null_mut::<u8>().wrapping_add(align.get())) }
 }
 
 mod private {