Reduce memory usage of cached matches for RegionsHighlighter

This adds a limitation that capture matching on regions only works
if the regions start/end/recurse match is less than 65635 byte long.
With this limitation we can reduce the RegexMatch struct size to 16
bytes instead of 32.

This is still not good enough,but should slightly improve high memory
usage as reported in #2454

src/highlighters.cc

@@ -1623,10 +1623,18 @@ struct RegexMatch
     LineCount line;
     ByteCount begin;
     ByteCount end;
-    StringView capture;
+    uint16_t capture_pos;
+    uint16_t capture_len;
 
     BufferCoord begin_coord() const { return { line, begin }; }
     BufferCoord end_coord() const { return { line, end }; }
+
+    StringView capture(const Buffer& buffer) const
+    {
+        if (capture_len == 0)
+            return {};
+        return buffer[line].substr(begin + capture_pos, ByteCount{capture_len});
+    }
 };
 using RegexMatchList = Vector<RegexMatch, MemoryDomain::Highlight>;
 
@@ -1639,10 +1647,15 @@ void find_matches(const Buffer& buffer, RegexMatchList& matches, const Regex& re
         for (RegexIterator<const char*> it{l.begin(), l.end(), regex}, end{}; it != end; ++it)
         {
             auto& m = *it;
-            ByteCount b = (int)(m[0].first - l.begin());
-            ByteCount e = (int)(m[0].second - l.begin());
-            auto cap = (capture and m[1].matched) ? StringView{m[1].first, m[1].second} : StringView{};
-            matches.push_back({ line, b, e, cap });
+            const bool with_capture = capture and m[1].matched and
+                                      m[0].second - m[0].first > std::numeric_limits<uint16_t>::max();
+            matches.push_back({
+                line,
+                (int)(m[0].first - l.begin()),
+                (int)(m[0].second - l.begin()),
+                (uint16_t)(with_capture ? m[1].first - m[0].first : 0),
+                (uint16_t)(with_capture ? m[1].second - m[1].first : 0)
+            });
         }
     }
 }
@@ -1689,10 +1702,15 @@ void update_matches(const Buffer& buffer, ConstArrayView<LineModification> modif
             for (RegexIterator<const char*> it{l.begin(), l.end(), regex}, end{}; it != end; ++it)
             {
                 auto& m = *it;
-                ByteCount b = (int)(m[0].first - l.begin());
-                ByteCount e = (int)(m[0].second - l.begin());
-                auto cap = (capture and m[1].matched) ? StringView{m[1].first, m[1].second} : StringView{};
-                matches.push_back({ line, b, e, cap });
+                const bool with_capture = capture and m[1].matched and
+                                          m[0].second - m[0].first > std::numeric_limits<uint16_t>::max();
+                matches.push_back({
+                    line,
+                    (int)(m[0].first - l.begin()),
+                    (int)(m[0].second - l.begin()),
+                    (uint16_t)(with_capture ? m[1].first - m[0].first : 0),
+                    (uint16_t)(with_capture ? m[1].second - m[1].first : 0)
+                });
             }
         }
     }
@@ -1719,7 +1737,7 @@ struct RegionMatches
                                 pos, compare_to_begin);
     }
 
-    RegexMatchList::const_iterator find_matching_end(BufferCoord beg_pos, Optional<StringView> capture) const
+    RegexMatchList::const_iterator find_matching_end(const Buffer& buffer, BufferCoord beg_pos, Optional<StringView> capture) const
     {
         auto end_it = end_matches.begin();
         auto rec_it = recurse_matches.begin();
@@ -1737,12 +1755,12 @@ struct RegionMatches
             while (rec_it != recurse_matches.end() and
                    rec_it->end_coord() <= end_it->end_coord())
             {
-                if (not capture or rec_it->capture == *capture)
+                if (not capture or rec_it->capture(buffer) == *capture)
                     ++recurse_level;
                 ++rec_it;
             }
 
-            if (not capture or *capture == end_it->capture)
+            if (not capture or *capture == end_it->capture(buffer))
             {
                 if (recurse_level == 0)
                     return end_it;
@@ -1997,8 +2015,8 @@ struct RegionsHighlighter : public Highlighter
             const RegionMatches& matches = cache.matches[begin.first];
             auto& region = m_regions.item(begin.first);
             auto beg_it = begin.second;
-            auto end_it = matches.find_matching_end(beg_it->end_coord(),
-                                                    region.value->match_capture() ? beg_it->capture : Optional<StringView>{});
+            auto end_it = matches.find_matching_end(buffer, beg_it->end_coord(),
+                                                    region.value->match_capture() ? beg_it->capture(buffer) : Optional<StringView>{});
 
             if (end_it == matches.end_matches.end() or end_it->end_coord() >= range.end)
             {