automata: fix unintended panic in max_haystack_len

This fixes a bug where the bounded backtracker's `max_haystack_len`
could panic if its bitset capacity ended up being smaller than the total
number of NFA states. Under a default configuration this seems unlikely
to happen due to the default limits on the size of a compiled regex. But
if the compiled regex size limit is increased to a large number, then
the likelihood of this panicking increases. Of course, one can provoke
this even easier by just setting the visited capacity to a small number.
Indeed, this is how we provoke it in a regression test.

regex-automata/src/meta/wrappers.rs

@@ -212,7 +212,10 @@ impl BoundedBacktrackerEngine {
                 .configure(backtrack_config)
                 .build_from_nfa(nfa.clone())
                 .map_err(BuildError::nfa)?;
-            debug!("BoundedBacktracker built");
+            debug!(
+                "BoundedBacktracker built (max haystack length: {:?})",
+                engine.max_haystack_len()
+            );
             Ok(Some(BoundedBacktrackerEngine(engine)))
         }
         #[cfg(not(feature = "nfa-backtrack"))]

regex-automata/src/nfa/thompson/backtrack.rs

@@ -820,8 +820,11 @@ impl BoundedBacktracker {
         // bytes to the capacity in bits.
         let capacity = 8 * self.get_config().get_visited_capacity();
         let blocks = div_ceil(capacity, Visited::BLOCK_SIZE);
-        let real_capacity = blocks * Visited::BLOCK_SIZE;
-        (real_capacity / self.nfa.states().len()) - 1
+        let real_capacity = blocks.saturating_mul(Visited::BLOCK_SIZE);
+        // It's possible for `real_capacity` to be smaller than the number of
+        // NFA states for particularly large regexes, so we saturate towards
+        // zero.
+        (real_capacity / self.nfa.states().len()).saturating_sub(1)
     }
 }
 
@@ -1882,3 +1885,24 @@ fn div_ceil(lhs: usize, rhs: usize) -> usize {
         (lhs / rhs) + 1
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    // This is a regression test for the maximum haystack length computation.
+    // Previously, it assumed that the total capacity of the backtracker's
+    // bitset would always be greater than the number of NFA states. But there
+    // is of course no guarantee that this is true. This regression test
+    // ensures that not only does `max_haystack_len` not panic, but that it
+    // should return `0`.
+    #[cfg(feature = "syntax")]
+    #[test]
+    fn max_haystack_len_overflow() {
+        let re = BoundedBacktracker::builder()
+            .configure(BoundedBacktracker::config().visited_capacity(10))
+            .build(r"[0-9A-Za-z]{100}")
+            .unwrap();
+        assert_eq!(0, re.max_haystack_len());
+    }
+}