Add dev tool to generate dummy migration

This tool is inspired from the omp version of old.

It is invoked as follows:
```
dune exec -- dev/gencopy/gencopy.exe astlib/ast_X.ml astlib/ast_Y.ml > migrate_X_Y.ml
```
It will generates deep "copy" functions from ast version X to version Y.
These are dummy function that are generated under the assumption that
the ast types in X and Y are identical.

When adding support for new compiler versions, this should be used as a
base to be editted, following the compiler errors that will point to AST
nodes that actually differ between the two versions.

Signed-off-by: Nathan Rebours <[email protected]>

Author: NathanReb

astlib/ast_503.ml

@@ -1,3 +1,5 @@
+module Longident = Longident
+
 module Asttypes = struct
   type constant (*IF_CURRENT = Asttypes.constant *) =
       Const_int of int

astlib/migrate_504_503.ml

@@ -1264,7 +1264,7 @@ and copy_variance : Ast_504.Asttypes.variance -> Ast_503.Asttypes.variance =
   | Ast_504.Asttypes.Covariant -> Ast_503.Asttypes.Covariant
   | Ast_504.Asttypes.Contravariant -> Ast_503.Asttypes.Contravariant
   | Ast_504.Asttypes.NoVariance -> Ast_503.Asttypes.NoVariance
-  | Ast_504.Asttypes.Bivariant -> migration_error Location.none "bivarance"
+  | Ast_504.Asttypes.Bivariant -> migration_error Location.none "bivariance"
 
 and copy_value_description :
     Ast_504.Parsetree.value_description -> Ast_503.Parsetree.value_description =

dev/README.md

@@ -1,3 +1,14 @@
+## gencopy
+
+`gencopy` is a script used to generate identity migrations. When adding support
+for a new version of ocaml, it's used to generate the copy functions for each
+compiler types. It simply generates identity copies so this likely won't compile
+because it does not account for changes in the parsetree type. Once you have
+those identity migrations, you should be able to follow the compilation errors
+to find out which part need proper migrations.
+
+The tool is further documented [here](gencopy/README.md).
+
 ## rev-deps.sh
 
 You can use this script to fetch ppxlib's rev-deps and clone them locally to test them

dev/gencopy/README.md

@@ -0,0 +1,5 @@
+(executable
+ (name gencopy)
+ (libraries ppxlib astlib)
+ (preprocess
+  (pps ppxlib.metaquot)))