Fix strict provenance polyfills (#34)

* Fix strict provenance polyfills

* Make sure we pass Miri without the nightly feature too

.github/workflows/ci.yml

@@ -54,4 +54,6 @@ jobs:
           rustup override set nightly
           cargo miri setup
       - name: Test with Miri
-        run: cargo miri test --features nightly
+        run: |
+          cargo miri test
+          cargo miri test --features nightly

src/sptr.rs

@@ -21,16 +21,16 @@ mod implementation {
     }
 
     pub fn without_provenance_mut<T>(addr: usize) -> *mut T {
-        addr as _
+        unsafe { core::mem::transmute(addr) }
     }
 
     pub fn with_metadata_of<T: ?Sized, U: ?Sized>(ptr: *const T, meta: *const U) -> *const U {
         with_metadata_of_mut(cast_to_mut(ptr), meta)
     }
 
     pub fn with_metadata_of_mut<T: ?Sized, U: ?Sized>(ptr: *mut T, mut meta: *const U) -> *mut U {
-        let meta_ptr = addr_of_mut!(meta).cast::<usize>();
-        unsafe { meta_ptr.write(ptr.cast::<u8>() as usize) }
+        let meta_ptr = addr_of_mut!(meta).cast::<*mut u8>();
+        unsafe { meta_ptr.write(ptr.cast::<u8>()) }
         cast_to_mut(meta)
     }
 }