Yup. I’m not claiming anyone has successfully applied the general concept of memoizing expensive compiler operations at the AST level. There’s all sorts of practical complexities that come up with such a design but conceptually I think it should be doable. Using your “the text is the serialized AST” argument, we see primitive tools around this concept that operate just on the text contents and codegen end-result with ccache/sccache/bazel/etc.
However, using a richer AST representation and knowledge about what expensive operations the compiler performs should let you retain cache acceleration even when the high-level text->compiled code fails. Heck, one could imagine that some AST processing could end up being agnostic to a codebase so that the compiler ends up memoizing certain results across all code bases since code is very similar and have similar constructs that should generate very case similarly.
However, using a richer AST representation and knowledge about what expensive operations the compiler performs should let you retain cache acceleration even when the high-level text->compiled code fails. Heck, one could imagine that some AST processing could end up being agnostic to a codebase so that the compiler ends up memoizing certain results across all code bases since code is very similar and have similar constructs that should generate very case similarly.