Audit the use of unsafe in byte_str.rs (#408)

Add comments describing the invariant that makes the one use of unsafe
sound.

src/byte_str.rs

@@ -4,25 +4,34 @@ use std::{ops, str};
 
 #[derive(Debug, Clone, Eq, PartialEq, Ord, PartialOrd, Hash)]
 pub(crate) struct ByteStr {
+    // Invariant: bytes contains valid UTF-8
     bytes: Bytes,
 }
 
 impl ByteStr {
     #[inline]
     pub fn new() -> ByteStr {
         ByteStr {
+            // Invariant: the empty slice is trivially valid UTF-8.
             bytes: Bytes::new(),
         }
     }
 
     #[inline]
     pub fn from_static(val: &'static str) -> ByteStr {
         ByteStr {
+            // Invariant: val is a str so contains vaid UTF-8.
             bytes: Bytes::from_static(val.as_bytes()),
         }
     }
 
     #[inline]
+    /// ## Panics
+    /// In a debug build this will panic if `bytes` is not valid UTF-8.
+    ///
+    /// ## Safety
+    /// `bytes` must contain valid UTF-8. In a release build it is undefined
+    /// behaviour to call this with `bytes` that is not valid UTF-8.
     pub unsafe fn from_utf8_unchecked(bytes: Bytes) -> ByteStr {
         if cfg!(debug_assertions) {
             match str::from_utf8(&bytes) {
@@ -33,6 +42,7 @@ impl ByteStr {
                 ),
             }
         }
+        // Invariant: assumed by the safety requirements of this function.
         ByteStr { bytes: bytes }
     }
 }
@@ -43,6 +53,7 @@ impl ops::Deref for ByteStr {
     #[inline]
     fn deref(&self) -> &str {
         let b: &[u8] = self.bytes.as_ref();
+        // Safety: the invariant of `bytes` is that it contains valid UTF-8.
         unsafe { str::from_utf8_unchecked(b) }
     }
 }
@@ -51,6 +62,7 @@ impl From<String> for ByteStr {
     #[inline]
     fn from(src: String) -> ByteStr {
         ByteStr {
+            // Invariant: src is a String so contains valid UTF-8.
             bytes: Bytes::from(src),
         }
     }
@@ -60,6 +72,7 @@ impl<'a> From<&'a str> for ByteStr {
     #[inline]
     fn from(src: &'a str) -> ByteStr {
         ByteStr {
+            // Invariant: src is a str so contains valid UTF-8.
             bytes: Bytes::copy_from_slice(src.as_bytes()),
         }
     }