ASTs typically discard comments, and that's usually what you want.
The only time (AFAICS) that preserving comments is useful is for writing a code formatter.
Could you write a formatter in terms of a list of lexemes? A CST is a non-trivial bit of code for one use case.
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I think you could build an interesting IDE with a tiny embedded LLM in addition to the usual tooling.
Features like 'extract method' would be much nicer if an LLM could provide a name. Choosing a good name is virtually impossible from just a typed AST.
I've been using "Expected Int, but got String" for my type error messages, but I've been wondering if I could do better.
"Expected Int here, but this value has type String" or "This expression requires Int, but the value is String".
Do you have a favourite?
Shower thought: using a tool like cargo-semver, could you build a package registry where the uploader never chooses the version number?
E.g. your last release was 5.2025-09-13 and you've just changed a type, so today's release is 6.2025-11-05.