core: make documentation clearer, rename slice comparison specialization trait

Author: joboet

Diff for: core/src/slice/cmp.rs

@@ -183,30 +183,40 @@ impl<A: Ord> SliceOrd for A {
     }
 }
 
-// The type should be treated as an unsigned byte for comparisons.
+/// Marks that a type should be treated as an unsigned byte for comparisons.
+///
+/// # Safety
+/// * The type must be readable as an `u8`, meaning it has to have the same
+///   layout as `u8` and always be initialized.
+/// * For every `x` and `y` of this type, `Ord(x, y)` must return the same
+///   value as `Ord::cmp(transmute::<_, u8>(x), transmute::<_, u8>(y))`.
 #[rustc_specialization_trait]
-unsafe trait UnsignedByte {}
+unsafe trait UnsignedBytewiseOrd {}
 
-unsafe impl UnsignedByte for bool {}
-unsafe impl UnsignedByte for u8 {}
-unsafe impl UnsignedByte for NonZero<u8> {}
-unsafe impl UnsignedByte for Option<NonZero<u8>> {}
-unsafe impl UnsignedByte for ascii::Char {}
+unsafe impl UnsignedBytewiseOrd for bool {}
+unsafe impl UnsignedBytewiseOrd for u8 {}
+unsafe impl UnsignedBytewiseOrd for NonZero<u8> {}
+unsafe impl UnsignedBytewiseOrd for Option<NonZero<u8>> {}
+unsafe impl UnsignedBytewiseOrd for ascii::Char {}
 
-// `compare_bytes` compares a sequence of unsigned bytes lexicographically.
-impl<A: Ord + UnsignedByte> SliceOrd for A {
+// `compare_bytes` compares a sequence of unsigned bytes lexicographically, so
+// use it if the requirements for `UnsignedBytewiseOrd` are fulfilled.
+impl<A: Ord + UnsignedBytewiseOrd> SliceOrd for A {
     #[inline]
     fn compare(left: &[Self], right: &[Self]) -> Ordering {
-        // Since the length of a slice is always less than or equal to isize::MAX, this never underflows.
+        // Since the length of a slice is always less than or equal to
+        // isize::MAX, this never underflows.
         let diff = left.len() as isize - right.len() as isize;
-        // This comparison gets optimized away (on x86_64 and ARM) because the subtraction updates flags.
+        // This comparison gets optimized away (on x86_64 and ARM) because the
+        // subtraction updates flags.
         let len = if left.len() < right.len() { left.len() } else { right.len() };
         let left = left.as_ptr().cast();
         let right = right.as_ptr().cast();
-        // SAFETY: `left` and `right` are references and are thus guaranteed to be valid.
-        // `UnsignedByte` is only implemented for types that are valid u8s. We use the
-        // minimum of both lengths which guarantees that both regions are valid for reads
-        // in that interval.
+        // SAFETY: `left` and `right` are references and are thus guaranteed to
+        // be valid. `UnsignedBytewiseOrd` is only implemented for types that
+        // are valid u8s and can be compared the same way. We use the minimum
+        // of both lengths which guarantees that both regions are valid for
+        // reads in that interval.
         let mut order = unsafe { compare_bytes(left, right, len) as isize };
         if order == 0 {
             order = diff;