unikernel/duniverse/ppxlib/dev/gencopy/README.md

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+gencopy is a dev utility tool that we use to generate migration functions
+skeletons when adding support for new compiler versions.
+
+It generates deep copy functions from the types defined in the provided
+ast_X.ml to what it assumes is in ast_Y.ml. The latter is only used to for
+type annotations and qualified record fiels/variant names. Those functions
+are generated assuming types are symmetrical across the two files.
+
+One would first generate `migrate_X_Y.ml` and `migrate_Y_X.ml` by running
+respectively:
+```
+gencopy ast_X.ml ast_Y.ml > migrate_X_Y.ml
+```
+and
+```
+gencopy ast_Y.ml ast_X.ml > migrate_Y_X.ml
+```
+from the `astlib/` folder.
+
+Trying to build ppxlib should give you compile errors for those two files
+corresponding to the specific parts of the AST types that changed across the two
+versions.
+
+Properly fixing those until it all builds should give you a valid set of
+migration functions.