regex: switch RegexSet to use WhichCaptures::None

And this finally resolves the memory usage problem, as the PikeVM cache
used by the RegexSet in #1059 no longer allocates MBs of memory because
of the existence of impossible-to-use capturing groups.

Fixes #1059

Author: BurntSushi

src/builders.rs

@@ -107,7 +107,7 @@ impl Builder {
             .clone()
             .match_kind(MatchKind::All)
             .utf8_empty(true)
-            .which_captures(WhichCaptures::Implicit);
+            .which_captures(WhichCaptures::None);
         let syntaxc = self.syntaxc.clone().utf8(true);
         let patterns = Arc::from(self.pats.as_slice());
         meta::Builder::new()
@@ -124,7 +124,7 @@ impl Builder {
             .clone()
             .match_kind(MatchKind::All)
             .utf8_empty(false)
-            .which_captures(WhichCaptures::Implicit);
+            .which_captures(WhichCaptures::None);
         let syntaxc = self.syntaxc.clone().utf8(false);
         let patterns = Arc::from(self.pats.as_slice());
         meta::Builder::new()