Is there a way to find out what cel value names are required by a cel expression? about cel-cpp HOT 1 OPEN

Hi @arghness, what you're looking for would typically come out of a CEL type-checker and the CheckedExpr proto. Currently, C++ only implements a parser and not a type-checker; however, if you want to look at the https://github.com/google/cel-go/checker implementation for how the reference_map is constructed you could presumably implement the variable reference tracking logic without implementing the full type-checking behavior.

A rough sketch of what you're looking for functionality-wise would be as follow:

1. Walk the AST
2. For each Ident and Select expr node, determine whether the expression node could be a qualified name.
3. An expression node may be a qualified name if it is composed of only Ident and Select expr nodes and none of the Select expr nodes is TestOnly.
4. Join expr nodes into a qualified name, e.g. Select(Ident('first'), 'second') -> first.second
5. Test whether the qualified name is declared as a variable, if not continue the AST walk. In the example, the more qualified name first.second would be checked as a variable before first.

The CEL-Go PR google/cel-go#341 introduced some changes to perform AST rewrites that would consolidate all possible variables into Ident nodes as qualified names if you're looking for an example to follow.

from cel-cpp.