Fix miri failures by preserving provenance in BitSpan::new_unchecked

Fix the miri failures in doctests, see issue ferrilab#135. The issue is that miri doesn't guess
correct provenance in the int-to-ptr cast in `BitSpan::new_unchecked`, as was found by
@tavianator [here](rust-lang/miri#1866 (comment)).

The solution is to preserve provenance and was proposed by  @tavianator
[here](rust-lang/miri#1866 (comment)).
With this change the entire test suite passes under miri.

src/devel.rs

@@ -103,3 +103,9 @@ where
 {
 	TypeId::of::<T>() == TypeId::of::<U>()
 }
+
+/// Converts `addr` to a pointer using the provenance of `prov`.
+pub fn int_to_ptr_with_provenance<T>(addr: usize, prov: *const T) -> *const T {
+	let ptr = prov.cast::<u8>();
+	ptr.wrapping_add(addr.wrapping_sub(ptr as usize)).cast()
+}

src/ptr/span.rs

@@ -39,6 +39,7 @@ use super::{
 	MisalignError,
 };
 use crate::{
+	devel::int_to_ptr_with_provenance,
 	index::{
 		BitEnd,
 		BitIdx,
@@ -205,12 +206,16 @@ where
 		let head = head.into_inner() as usize;
 		let ptr_data = addr.to_const() as usize & Self::PTR_ADDR_MASK;
 		let ptr_head = head >> Self::LEN_HEAD_BITS;
+		// We need to preserve pointer provenance for miri, see miri issue 1866
+		// <https://github.com/rust-lang/miri/issues/1866#issuecomment-985770125>
+		let ptr =
+			int_to_ptr_with_provenance(ptr_data | ptr_head, addr.to_const());
 
 		let len_head = head & Self::LEN_HEAD_MASK;
 		let len_bits = bits << Self::LEN_HEAD_BITS;
 
 		Self {
-			ptr: NonNull::new_unchecked((ptr_data | ptr_head) as *mut ()),
+			ptr: NonNull::new_unchecked(ptr as *mut ()),
 			len: len_bits | len_head,
 			..Self::EMPTY
 		}