Generate .proto file from desc.FileDescriptor

[jkinkead]

Generate .proto file from desc.FileDescriptor

[jhump]

I have thought about writing out proto files from a descriptor. I agree it would be a neat feature.

Making it lossless, for cases where source_code_info is present, is actually very complicated because of the way the source_code_info models language elements and file locations. Also, the way extra comments are represented, like inline comments between elements, makes it really hard to preserve all comments in the output. For example:

message Foo {
  /* comment 1 */ optional /* comment 2 */ /* comment 3 */ string
    // yet another comment
    fieldNameBar = /* more comments */ 3 [ /* some option comments! */ default = 
    // another line comment, inside options
     "baz" ]/* another one! */ ; // trailer
}

I believe there is enough info in source_code_info to be aware of all of the comments, but not enough to actually preserve formatting and placement.

---

I have seriously looked at preserving comments in the protoparse package. It is a hefty change that would involve quite a lot of boiler-plate -- wrapping every value in the parse tree with a new struct that includes position information and comments. There would also be some non-trivial work to make sure comments are attributed to elements correctly (to match protoc behavior).

I'll add this as a separate issue.

[jhump]

Feature implemented in #84