stir the hash state a little to avoid prefix collisions

Author: the8472

std/src/path.rs

@@ -3098,15 +3098,19 @@ impl Hash for Path {
         let bytes = &bytes[prefix_len..];
 
         let mut component_start = 0;
-        let mut bytes_hashed = 0;
+        // track some extra state to avoid prefix collisions.
+        // ["foo", "bar"] and ["foobar"], will have the same payload bytes
+        // but result in different chunk_bits
+        let mut chunk_bits: usize = 0;
 
         for i in 0..bytes.len() {
             let is_sep = if verbatim { is_verbatim_sep(bytes[i]) } else { is_sep_byte(bytes[i]) };
             if is_sep {
                 if i > component_start {
                     let to_hash = &bytes[component_start..i];
+                    chunk_bits = chunk_bits.wrapping_add(to_hash.len());
+                    chunk_bits = chunk_bits.rotate_right(2);
                     h.write(to_hash);
-                    bytes_hashed += to_hash.len();
                 }
 
                 // skip over separator and optionally a following CurDir item
@@ -3127,11 +3131,12 @@ impl Hash for Path {
 
         if component_start < bytes.len() {
             let to_hash = &bytes[component_start..];
+            chunk_bits = chunk_bits.wrapping_add(to_hash.len());
+            chunk_bits = chunk_bits.rotate_right(2);
             h.write(to_hash);
-            bytes_hashed += to_hash.len();
         }
 
-        h.write_usize(bytes_hashed);
+        h.write_usize(chunk_bits);
     }
 }