Certain selector combinations negatively affect compile time

I have ported the openstreetmap-carto style to vector tiles, and am using tessera, which is based on tilelive, to serve the raster tiles (which are created from vectortiles). However it takes a very long time to start up (~4 minutes) and that's from carto's processing of the cartocss files. I initially reported this on the tessera issue tracker, see more background there

Using simple print statement debugging, I can see that some of the longest parts are inside the rules = inheritDefinitions(matching, env); call. For some osm-carto layers, there are thousands of items in the matching variable. (e.g. test-polyg-low-zoom has 6,005. bridges has 1,099, test-poly as 6,064 etc). It takes about 1 second (on my machine) for every 1,000 items. so some of these layers take about 5sec to process. There are lots of layers, so it's about 3½ minutes to parse the whole file.

A weird thing is that I can use the same node_modules to parse the upstream project.mml into mapnik XML in 24sec. This is only a problem when parsing a vector tiles file.

I have patched tilelive-tm{style,source} to use the latest carto (0.17.2).

You can test this yourself by checking out the openstreetmap-carto-vector-tiles project and doing make tessera (which will serve the tiles after a while).