Rollup merge of rust-lang#127444 - Sky9x:cstr-bytes-iter, r=dtolnay

`impl Send + Sync` and override `count` for the `CStr::bytes` iterator

cc tracking issue rust-lang#112115

library/core/src/ffi/c_str.rs

@@ -781,8 +781,15 @@ const unsafe fn strlen(ptr: *const c_char) -> usize {
 pub struct Bytes<'a> {
  // since we know the string is nul-terminated, we only need one pointer
  ptr: NonNull<u8>,
- phantom: PhantomData<&'a u8>,
+ phantom: PhantomData<&'a [c_char]>,
 }
+
+#[unstable(feature = "cstr_bytes", issue = "112115")]
+unsafe impl Send for Bytes<'_> {}
+
+#[unstable(feature = "cstr_bytes", issue = "112115")]
+unsafe impl Sync for Bytes<'_> {}
+
 impl<'a> Bytes<'a> {
  #[inline]
  fn new(s: &'a CStr) -> Self {
@@ -815,7 +822,7 @@ impl Iterator for Bytes<'_> {
  if ret == 0 {
  None
  } else {
- self.ptr = self.ptr.offset(1);
+ self.ptr = self.ptr.add(1);
  Some(ret)
  }
  }
@@ -825,6 +832,12 @@ impl Iterator for Bytes<'_> {
  fn size_hint(&self) -> (usize, Option<usize>) {
  if self.is_empty() { (0, Some(0)) } else { (1, None) }
  }
+
+ #[inline]
+ fn count(self) -> usize {
+ // SAFETY: We always hold a valid pointer to a C string
+ unsafe { strlen(self.ptr.as_ptr().cast()) }
+ }
 }
 
 #[unstable(feature = "cstr_bytes", issue = "112115")]