Are you talking about runtime complexity or programming-time complexity?

If the runtime, then I bet almost no one will notice, especially if the appropriate caching is used.

If the programming-time - sure, but it's not like you can avoid parsers altogether. If the parsers are not in the tools, they must be in IDE. Factor out that parsing logic, and make it a library all the tools can use (or a one-shot LSP server if you are in the language that has hard-to-use bindings).

Note even with AST-in-file approach, you _still_ need the library to read and write that AST, it's not like you can have a shared AST schema for multiple languages. So either way, tools like diff will need to have a wide variety of libraries linked in, one for each language they support. And at that point, there is not much difference between AST reader and code parser.

I meant programming-time, but runtime is also a good point.

Cross-language libraries don't seem to be super common for this. The recovering-sense-from-text tools I named all use different parsers in their respective languages.

Again, reading (and yes, technically that's also parsing) from an AST from a data-exchange formatted file is mags simpler. And for parsing these schemes there are battle-tested cross-language solutions, e.g. protobuf.