[hashes] Disable fixed-time equality cmp when building for fuzzers

Fuzzers want to break memcmp calls into separate comparisons for
coverage monitoring, allowing them to not-quite-brute-force find
inputs that fully match. Thus, we disable our fancy fixed-time
comparison when built with the `hashes_fuzz` cfg.

hashes/src/cmp.rs

@@ -15,6 +15,14 @@
 /// As of rust 1.31.0 disassembly looks completely within reason for this, see
 /// <https://godbolt.org/z/mMbGQv>.
 pub fn fixed_time_eq(a: &[u8], b: &[u8]) -> bool {
+    #[cfg(hashes_fuzz)]
+    {
+        // Fuzzers want to break memcmp calls into separate comparisons for coverage monitoring,
+        // so we avoid our fancy fixed-time comparison below for fuzzers.
+        a == b
+    }
+    #[cfg(not(hashes_fuzz))]
+    {
         assert!(a.len() == b.len());
         let count = a.len();
         let lhs = &a[..count];
@@ -51,6 +59,7 @@ pub fn fixed_time_eq(a: &[u8], b: &[u8]) -> bool {
         }
         unsafe { (::core::ptr::read_volatile(&r) & 1) == 0 }
     }
+}
 
 #[cfg(test)]
 mod tests {