add safety comments to String::insert_str

and update safety comments in `String::insert` for consistency

library/alloc/src/string.rs

@@ -1726,8 +1726,9 @@ impl String {
         let ch_len = ch.len_utf8();
         self.reserve(ch_len);
 
-        // SAFETY: Shift data `ch_len` bytes to the right,
-        // capacity was just reserved for at least that many bytes.
+        // SAFETY: Move the bytes starting from `idx` to their new location `ch_len`
+        // bytes ahead. This is safe because sufficient capacity was reserved, and `idx`
+        // is a char boundary.
         unsafe {
             ptr::copy(
                 self.vec.as_ptr().add(idx),
@@ -1736,13 +1737,13 @@ impl String {
             );
         }
 
-        // SAFETY: Encode the character into the space left after the shift
-        // if `idx != len`, or into the uninitialized spare capacity otherwise.
+        // SAFETY: Encode the character into the vacated region if `idx != len`,
+        // or into the uninitialized spare capacity otherwise.
         unsafe {
             core::char::encode_utf8_raw_unchecked(ch as u32, self.vec.as_mut_ptr().add(idx));
         }
 
-        // SAFETY: `ch_len` initialized bytes have been added.
+        // SAFETY: Update the length to include the newly added bytes.
         unsafe {
             self.vec.set_len(len + ch_len);
         }
@@ -1778,9 +1779,26 @@ impl String {
         let amt = string.len();
         self.reserve(amt);
 
+        // SAFETY: Move the bytes starting from `idx` to their new location `amt` bytes
+        // ahead. This is safe because sufficient capacity was just reserved, and `idx`
+        // is a char boundary.
+        unsafe {
+            ptr::copy(
+                self.vec.as_ptr().add(idx),
+                self.vec.as_mut_ptr().add(idx + amt),
+                len - idx,
+            );
+        }
+
+        // SAFETY: Copy the new string slice into the vacated region if `idx != len`,
+        // or into the uninitialized spare capacity otherwise. The borrow checker
+        // ensures that the source and destination do not overlap.
         unsafe {
-            ptr::copy(self.vec.as_ptr().add(idx), self.vec.as_mut_ptr().add(idx + amt), len - idx);
             ptr::copy_nonoverlapping(string.as_ptr(), self.vec.as_mut_ptr().add(idx), amt);
+        }
+
+        // SAFETY: Update the length to include the newly added bytes.
+        unsafe {
             self.vec.set_len(len + amt);
         }
     }