chore: add safety comments on unsafe blocks (#827)

src/bytes.rs

@@ -489,6 +489,8 @@ impl Bytes {
 
         self.len = at;
 
+        // SAFETY: `at` has been asserted to be <= `self.len()`, and the
+        // `at == self.len()` and `at == 0` cases were handled above.
         unsafe { ret.inc_start(at) };
 
         ret
@@ -537,6 +539,8 @@ impl Bytes {
 
         let mut ret = self.clone();
 
+        // SAFETY: `at` has been asserted to be <= `self.len()`, and the
+        // `at == self.len()` and `at == 0` cases were handled above.
         unsafe { self.inc_start(at) };
 
         ret.len = at;

src/bytes_mut.rs

@@ -324,6 +324,10 @@ impl BytesMut {
             self.capacity(),
         );
         unsafe {
+            // SAFETY: `shallow_clone` increments the reference count (or
+            // promotes to shared) and returns a bitwise copy of the handle.
+            // The caller immediately adjusts both handles so they represent
+            // disjoint regions.
             let mut other = self.shallow_clone();
             // SAFETY: We've checked that `at` <= `self.capacity()` above.
             other.advance_unchecked(at);
@@ -400,6 +404,10 @@ impl BytesMut {
         );
 
         unsafe {
+            // SAFETY: `shallow_clone` increments the reference count (or
+            // promotes to shared) and returns a bitwise copy of the handle.
+            // The caller immediately adjusts both handles so they represent
+            // disjoint regions.
             let mut other = self.shallow_clone();
             // SAFETY: We've checked that `at` <= `self.len()` and we know that `self.len()` <=
             // `self.capacity()`.