I use some for e.g. YAML [0] and JSON [1], and they're nice [2], but these are comparatively simple languages.

I'm particularly curious because just plain diffing ASTs is more on the "syntax-aware diffing" side rather than the "semantic diffing" side, yet most semantic tooling descriptions stop at saying they use ASTs.

ASTs are not necessarily in a minimal / optimized form by construction I believe, so I'm pretty sure you'll have situations where a "semantic" differ will report a difference, whereas a compiler would still compile the given translation unit to the same machine bytecode after all the optimization passes during later levels. Not even necessarily for target platform dependent reasons.

But maybe this doesn't matter much or would be more confusing than helpful?

[0] dyff: https://github.com/homeport/dyff

[1] jd: https://github.com/josephburnett/jd

[2] they allow me to ignore ordering differences within arrays (arrays are ordered in YAML and JSON as per the standard), which I found to be a surprisingly rare and useful capability; the programs that consume the YAMLs and JSONs I use these on are not sensitive to these ordering differences

Still, my two cents: Beagle the AST-level version control system, experimental

https://github.com/gritzko/librdx/tree/master/be#readme

It genuinely stores AST trees in (virtually any) key-value database (RocksDB at the moment). In fact, it is a versioned database for the code with very open format and complete freedom to build on top of it.

be is in fact, more of a format/protocol, like in the good old days (HTTP, SMTP, XML, JSON - remember those?)

In fact, I fully expected a use of LLM to derive those meaningful descriptions before I checked the repo.

Anyway I definitely see this as a useful thing to try out as a potential addition to the armoury, but as we go further along the route to AI-coding I expect the diffs to be abstracted even further (derived using AI), for use by both agentic and human contributors.