Auto merge of #53804 - RalfJung:ptr-invalid, r=nagisa

fix some uses of pointer intrinsics with invalid pointers

[Found by miri](https://github.com/solson/miri/pull/446):

* `Vec::into_iter` calls `ptr::read` (and the underlying `copy_nonoverlapping`) with an unaligned pointer to a ZST. [According to LLVM devs](https://bugs.llvm.org/show_bug.cgi?id=38583), this is UB because it contradicts the metadata we are attaching to that pointer.
* `HashMap` creation calls `ptr:.write_bytes` on a NULL pointer with a count of 0. This is likely not currently UB *currently*, but it violates the rules we are setting in https://github.com/rust-lang/rust/pull/53783, and we might want to exploit those rules later (e.g. with more `nonnull` attributes for LLVM).

    Probably what `HashMap` really should do is use `NonNull::dangling()` instead of 0 for the empty case, but that would require a more careful analysis of the code.

It seems like ideally, we should do a review of usage of such intrinsics all over libstd to ensure that they use valid pointers even when the size is 0. Is it worth opening an issue for that?

1 files changed, 4 insertions, 6 deletions

diff --git a/src/liballoc/vec.rs b/src/liballoc/vec.rs
index e9c1a3df518..7fc4453fec5 100644
--- a/src/liballoc/vec.rs
+++ b/src/liballoc/vec.rs
@@ -2410,9 +2410,8 @@ impl<T> Iterator for IntoIter<T> {
                     // same pointer.
                     self.ptr = arith_offset(self.ptr as *const i8, 1) as *mut T;
 
-                    // Use a non-null pointer value
-                    // (self.ptr might be null because of wrapping)
-                    Some(ptr::read(1 as *mut T))
+                    // Make up a value of this ZST.
+                    Some(mem::zeroed())
                 } else {
                     let old = self.ptr;
                     self.ptr = self.ptr.offset(1);
@@ -2451,9 +2450,8 @@ impl<T> DoubleEndedIterator for IntoIter<T> {
                     // See above for why 'ptr.offset' isn't used
                     self.end = arith_offset(self.end as *const i8, -1) as *mut T;
 
-                    // Use a non-null pointer value
-                    // (self.end might be null because of wrapping)
-                    Some(ptr::read(1 as *mut T))
+                    // Make up a value of this ZST.
+                    Some(mem::zeroed())
                 } else {
                     self.end = self.end.offset(-1);