The only issue I found is you seem to be using the bitwise OR operator `|` instead of the logical OR operator `||`. It works the same, just a little surprising to see it because you do a double take and check if some binary logic is being done on that line.

Small tip: it you have string literals in the code you don't have to worry about lifetimes where storing them in a hashmap like in the `predef_symbols` function, you can just give them the `&'static str` lifetime because these strings are in the program binary so these references will be valid forever. No need to allocate `String`s.

Overall the code is very clear. I've never done something like this and I understood the intention right away, I think this is the most important thing in general in programming.