Auto merge of #1725 - RalfJung:zst-reborrow, r=RalfJung

fix reborrowing of tagged ZST references

`@SkiFire13` [pointed out](rust-lang/rust#82554 (comment)) that Miri fails to detect illegal use of empty slices. This PR fixes that. In so doing, it uncovers a flaw of Stacked Borrows: it is incompatible with how the formatting machinery uses `extern type`, so for now we skip reborrowing when we cannot determine the exact size of the pointee.

src/stacked_borrows.rs

@@ -586,15 +586,23 @@ trait EvalContextPrivExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
  let place = this.ref_to_mplace(val)?;
  let size = this
  .size_and_align_of_mplace(&place)?
- .map(|(size, _)| size)
- .unwrap_or_else(|| place.layout.size);
+ .map(|(size, _)| size);
+ // FIXME: If we cannot determine the size (because the unsized tail is an `extern type`),
+ // bail out -- we cannot reasonably figure out which memory range to reborrow.
+ // See https://github.com/rust-lang/unsafe-code-guidelines/issues/276.
+ let size = match size {
+ Some(size) => size,
+ None => return Ok(*val),
+ };
  // `reborrow` relies on getting a `Pointer` and everything being in-bounds,
  // so let's ensure that. However, we do not care about alignment.
  // We can see dangling ptrs in here e.g. after a Box's `Unique` was
  // updated using "self.0 = ..." (can happen in Box::from_raw) so we cannot ICE; see miri#1050.
  let place = this.mplace_access_checked(place, Some(Align::from_bytes(1).unwrap()))?;
- // Nothing to do for ZSTs.
- if size == Size::ZERO {
+ // Nothing to do for ZSTs. We use `is_bits` here because we *do* need to retag even ZSTs
+ // when there actually is a tag (to avoid inheriting a tag that would let us access more
+ // than 0 bytes).
+ if size == Size::ZERO && place.ptr.is_bits() {
  return Ok(*val);
  }
 

tests/compile-fail/stacked_borrows/zst_slice.rs

@@ -0,0 +1,11 @@
+// compile-flags: -Zmiri-track-raw-pointers
+// error-pattern: does not have an appropriate item in the borrow stack
+
+fn main() {
+ unsafe {
+ let a = [1, 2, 3];
+ let s = &a[0..0];
+ assert_eq!(s.len(), 0);
+ assert_eq!(*s.get_unchecked(1), 2);
+ }
+}