RegexSet: fix literal optimization bug

When combining multiple regexes in a set where some are anchored and
others aren't, it's possible to wind up in a situation where prefix
scanning is used. This is bad, because it can lead to some of the
anchored regexes matching where they shouldn't be allowed to match.
As a result, we disable all literal optimizations for regex sets if
*any* regex in the set is anchored.

Fixes rust-lang#358

src/exec.rs

@@ -210,6 +210,9 @@ impl ExecBuilder {
         let mut prefixes = Some(Literals::empty());
         let mut suffixes = Some(Literals::empty());
         let mut bytes = false;
+        let is_set = self.options.pats.len() > 1;
+        // If we're compiling a regex set and that set has any anchored
+        // expressions, then disable all literal optimizations.
         for pat in &self.options.pats {
             let parser =
                 ExprBuilder::new()
@@ -227,6 +230,10 @@ impl ExecBuilder {
                 // Partial anchors unfortunately make it hard to use prefixes,
                 // so disable them.
                 prefixes = None;
+            } else if is_set && expr.is_anchored_start() {
+                // Regex sets with anchors do not go well with literal
+                // optimizations.
+                prefixes = None;
             }
             prefixes = prefixes.and_then(|mut prefixes| {
                 if !prefixes.union_prefixes(&expr) {
@@ -240,6 +247,10 @@ impl ExecBuilder {
                 // Partial anchors unfortunately make it hard to use suffixes,
                 // so disable them.
                 suffixes = None;
+            } else if is_set && expr.is_anchored_end() {
+                // Regex sets with anchors do not go well with literal
+                // optimizations.
+                prefixes = None;
             }
             suffixes = suffixes.and_then(|mut suffixes| {
                 if !suffixes.union_suffixes(&expr) {

tests/set.rs

@@ -19,6 +19,7 @@ matset!(set17, &[".*a"], "a", 0);
 nomatset!(nset1, &["a", "a"], "b");
 nomatset!(nset2, &["^foo", "bar$"], "bar foo");
 nomatset!(nset3, { let xs: &[&str] = &[]; xs }, "a");
+nomatset!(nset4, &[r"^rooted$", r"\.log$"], "notrooted");
 
 // See: https://github.com/rust-lang/regex/issues/187
 #[test]