There's a fair amount of experimental work happening trying different parsing and resolution procedures such that the training data reflects an AST and or predicts nodes in an AST as an in-filling capability.

Do you know if any such experimental work is using a special tokenizer for example in Lisp a special token for left or right parenthesis?

The training data is text code that can be compiled, though, so the training data can also easily be an Abstract Syntax Tree.

But is anyone actually doing this?

It's possible that LLMs build ASTs internally for programming. I have no 1st hand data on this, but it would not surprise me at all.

LLMs don't have memory, so they can't build anything. Insofar as they produce correct results, they have implicit structures corresponding to ASTs built into their networks during training time.

"LLMs don't have memory"

That's interesting. Is there research into adding memory or has it been proven that it provides no pragmatic value over any context it outputs?