Added From<Vec<NonZeroU8>> for CString

Updated tracking issue number

Added safeguards for transmute_vec potentially being factored out elsewhere

Clarified comment about avoiding mem::forget

Removed unneeded unstable guard

Added back a stability annotation for CI

Minor documentation improvements

Thanks to @Centril's code review

Co-Authored-By: Mazdak Farrokhzad <twingoow@gmail.com>

Improved layout checks, type annotations and removed unaccurate comment

Removed unnecessary check on array layout

Adapt the stability annotation to the new 1.41 milestone

Co-Authored-By: Mazdak Farrokhzad <twingoow@gmail.com>

Simplify the implementation.

Use `Vec::into_raw_parts` instead of a manual implementation of
`Vec::transmute`.

If `Vec::into_raw_parts` uses `NonNull` instead, then the code here
will need to be adjusted to take it into account (issue #65816)

Reduce the whitespace of safety comments

src/libstd/ffi/c_str.rs

@@ -6,6 +6,7 @@ use crate::fmt::{self, Write};
 use crate::io;
 use crate::mem;
 use crate::memchr;
+use crate::num::NonZeroU8;
 use crate::ops;
 use crate::os::raw::c_char;
 use crate::ptr;
@@ -741,6 +742,32 @@ impl From<Box<CStr>> for CString {
     }
 }
 
+#[stable(feature = "cstring_from_vec_of_nonzerou8", since = "1.43.0")]
+impl From<Vec<NonZeroU8>> for CString {
+    /// Converts a [`Vec`]`<`[`NonZeroU8`]`>` into a [`CString`] without
+    /// copying nor checking for inner null bytes.
+    ///
+    /// [`CString`]: ../ffi/struct.CString.html
+    /// [`NonZeroU8`]: ../num/struct.NonZeroU8.html
+    /// [`Vec`]: ../vec/struct.Vec.html
+    #[inline]
+    fn from(v: Vec<NonZeroU8>) -> CString {
+        unsafe {
+            // Transmute `Vec<NonZeroU8>` to `Vec<u8>`.
+            let v: Vec<u8> = {
+                // Safety:
+                //   - transmuting between `NonZeroU8` and `u8` is sound;
+                //   - `alloc::Layout<NonZeroU8> == alloc::Layout<u8>`.
+                let (ptr, len, cap): (*mut NonZeroU8, _, _) = Vec::into_raw_parts(v);
+                Vec::from_raw_parts(ptr.cast::<u8>(), len, cap)
+            };
+            // Safety: `v` cannot contain null bytes, given the type-level
+            // invariant of `NonZeroU8`.
+            CString::from_vec_unchecked(v)
+        }
+    }
+}
+
 #[stable(feature = "more_box_slice_clone", since = "1.29.0")]
 impl Clone for Box<CStr> {
     #[inline]

src/libstd/lib.rs

@@ -310,6 +310,7 @@
 #![feature(unboxed_closures)]
 #![feature(untagged_unions)]
 #![feature(unwind_attributes)]
+#![feature(vec_into_raw_parts)]
 // NB: the above list is sorted to minimize merge conflicts.
 #![default_lib_allocator]