Efficiently adding to an existing `SyntaxSet`

[9999years]

In Zola, there's a set of builtin syntax definitions and support for loading your own from a directory. To avoid re-linking all the contexts in the builtin syntax definitions, these two SyntaxSets are kept separate.

However, this means that if you write a new syntax definition, you can't reference or embed any syntax from builtin set (getzola/zola#1678). The naïve approach of loading the serialized dump of the SyntaxSet, calling into_builder(), adding the new syntaxes, and build()ing it again causes an 8x slowdown (getzola/zola#2738).

It would be really nice if it were possible to efficiently add to an existing syntax set.

[keith-hall]

To avoid re-linking all the contexts in the builtin syntax definitions, these two SyntaxSets are kept separate.

Perhaps it would be possible to do the re-linking and then dump the resulting syntax set, and only load that dump the next time? And only re-parse/dump when something changes in the user's custom syntax definition directory?

Otherwise I'm not sure how to make it more efficient to add to an existing syntax set...