ADT-extended protobuf preprocessor

Input is approximately protobuf syntax, with a few extensions and a few missing parts. Output is (a) protobuf syntax (b) C++ code that checks structures for well-formedness.

In particular:

1. Fields are "required" by default and are automatically given tags based on their order in the file. So instead of:

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    required int foo = 1;
    optional string bar = 2;
   

   You write:

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    int foo;
    optional string bar;
   

   These are arguably misfeatures.

2. Adds a "data" construct to define an algebraic data type / variant / tagged union. Inside of "data" you define "branches". Branches can have associated data, which can either be a message, another type, or nothing.

   Example:

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    data Example {
        branch Branch1 {
            // A message body goes here.
            int foo;
            string blah;
            repeated Example children;
        };
   
        // this branch is just an int, no message type
        branch Branch2 = int;
   
        // this branch has no associated data
        branch Branch3;
    };
   

   This turns into the following protobuf code:

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    message Example {
        enum ExampleType {
            BRANCH1 = 0;
            BRANCH2 = 1;
            BRANCH3 = 2;
        };
        required ExampleType type = 1;
        message Branch1 {
            required int foo = 1;
            required string blah = 2;
            repeated Example children = 3;
        };
        optional Branch1 branch1 = 2;
        optional int branch2 = 3;
    };
   

   And the following C++ checker code:

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    bool well_formed(const Example& self) {
        if (self.type == Example::BRANCH1 && !self.has_branch1())
            return false;
   
        if (self.has_branch1() && self.type != Example::BRANCH1)
            return false;
   
        if (self.type == Example::BRANCH2 && !self.has_branch2())
            return false;
   
        if (self.has_branch2() && self.type != Example::BRANCH2)
            return false;
   
        return true;
    }
   

Bugs

Doesn't recursively check for well-formedness yet.