Auto merge of #51134 - RalfJung:from_raw_parts, r=SimonSapin

extend from_raw_parts docs for slices and strs to mention alignment requirement

The documentation for `str::from_raw_parts_mut` seems to not be visible because that method is private, bit I figured it could still be fixed. I also removed the reference to the no-longer-existing `str::from_raw_parts` while I was at it.

Alternatively, should I remove `str::from_raw_parts_mut` completely? it is only used in `str::split_at_mut`, where it might as well be inlined.

src/libcore/slice/mod.rs

@@ -3839,10 +3839,9 @@ unsafe impl<'a, T> TrustedRandomAccess for ExactChunksMut<'a, T> {
 /// valid for `len` elements, nor whether the lifetime inferred is a suitable
 /// lifetime for the returned slice.
 ///
-/// `p` must be non-null, even for zero-length slices, because non-zero bits
+/// `p` must be non-null and aligned, even for zero-length slices, as is
-/// are required to distinguish between a zero-length slice within `Some()`
+/// required for all references. However, for zero-length slices, `p` can be
-/// from `None`. `p` can be a bogus non-dereferencable pointer, such as `0x1`,
+/// a bogus non-dereferencable pointer such as [`NonNull::dangling()`].
-/// for zero-length slices, though.
 ///
 /// # Caveat
 ///
@@ -3864,6 +3863,8 @@ unsafe impl<'a, T> TrustedRandomAccess for ExactChunksMut<'a, T> {
 ///     let slice = slice::from_raw_parts(ptr, amt);
 /// }
 /// ```
+///
+/// [`NonNull::dangling()`]: ../../std/ptr/struct.NonNull.html#method.dangling
 #[inline]
 #[stable(feature = "rust1", since = "1.0.0")]
 pub unsafe fn from_raw_parts<'a, T>(data: *const T, len: usize) -> &'a [T] {
@@ -3875,7 +3876,7 @@ pub unsafe fn from_raw_parts<'a, T>(data: *const T, len: usize) -> &'a [T] {
 ///
 /// This function is unsafe for the same reasons as `from_raw_parts`, as well
 /// as not being able to provide a non-aliasing guarantee of the returned
-/// mutable slice. `p` must be non-null even for zero-length slices as with
+/// mutable slice. `p` must be non-null and aligned even for zero-length slices as with
 /// `from_raw_parts`.
 #[inline]
 #[stable(feature = "rust1", since = "1.0.0")]

src/libcore/str/mod.rs

@@ -376,37 +376,6 @@ pub fn from_utf8_mut(v: &mut [u8]) -> Result<&mut str, Utf8Error> {
     Ok(unsafe { from_utf8_unchecked_mut(v) })
 }
 
-/// Forms a str from a pointer and a length.
-///
-/// The `len` argument is the number of bytes in the string.
-///
-/// # Safety
-///
-/// This function is unsafe as there is no guarantee that the given pointer is
-/// valid for `len` bytes, nor whether the lifetime inferred is a suitable
-/// lifetime for the returned str.
-///
-/// The data must be valid UTF-8
-///
-/// `p` must be non-null, even for zero-length strs, because non-zero bits
-/// are required to distinguish between a zero-length str within `Some()`
-/// from `None`. `p` can be a bogus non-dereferencable pointer, such as `0x1`,
-/// for zero-length strs, though.
-///
-/// # Caveat
-///
-/// The lifetime for the returned str is inferred from its usage. To
-/// prevent accidental misuse, it's suggested to tie the lifetime to whichever
-/// source lifetime is safe in the context, such as by providing a helper
-/// function taking the lifetime of a host value for the str, or by explicit
-/// annotation.
-/// Performs the same functionality as `from_raw_parts`, except that a mutable
-/// str is returned.
-///
-unsafe fn from_raw_parts_mut<'a>(p: *mut u8, len: usize) -> &'a mut str {
-    from_utf8_unchecked_mut(slice::from_raw_parts_mut(p, len))
-}
-
 /// Converts a slice of bytes to a string slice without checking
 /// that the string contains valid UTF-8.
 ///
@@ -2602,8 +2571,11 @@ impl str {
             let len = self.len();
             let ptr = self.as_ptr() as *mut u8;
             unsafe {
-                (from_raw_parts_mut(ptr, mid),
+                (from_utf8_unchecked_mut(slice::from_raw_parts_mut(ptr, mid)),
-                 from_raw_parts_mut(ptr.offset(mid as isize), len - mid))
+                 from_utf8_unchecked_mut(slice::from_raw_parts_mut(
+                    ptr.offset(mid as isize),
+                    len - mid
+                 )))
             }
         } else {
             slice_error_fail(self, 0, mid)