odoc crashes when processing comments enriched with ocaml-rpc ppx

I'm trying to generate docs for all the xs-opam packages using odig but odoc spectacularly crashes when processing rpc-enhanced comments in xapi-idl, xapi-storage, and xapi-xenopsd.

Example:

$ odig doc
Updating documentation, this may take some time...
[FAILED][spawn:22191]
 Illegal exit status: exited [2] expected: 0
 cmd: ['/home/paus/.local/share/opam/toolstack/bin/odoc' 'compile' '--pkg' 'xapi-idl' '-o' '/home/paus/.local/share/opam/toolstack/var/cache/odig/odoc/xapi-idl/varstore/privileged/varstore_privileged_interface.odoc' '/home/paus/.local/share/opam/toolstack/lib/xapi-idl/varstore/privileged/varstore_privileged_interface.cmt' '-I' '/home/paus/.local/share/opam/toolstack/var/cache/odig/odoc/ocaml/' '-I' '/home/paus/.local/share/opam/toolstack/var/cache/odig/odoc/result/' '-I' '/home/paus/.local/share/opam/toolstack/var/cache/odig/odoc/rpclib/core/' '-I' '/home/paus/.local/share/opam/toolstack/var/cache/odig/odoc/rresult/' '-I' '/home/paus/.local/share/opam/toolstack/var/cache/odig/odoc/uuidm/' '-I' '/home/paus/.local/share/opam/toolstack/var/cache/odig/odoc/xapi-idl/']
 env: 
 relevant-env: 
 cwd: '/home/paus/code/toolstack/xcp-idl'
 success-exits: 0
 stdin: <none>
 stdout: <ui>
 stderr: <ui>
 stdo-ui: odoc: internal error, uncaught exception:
      File "src/loader/doc_attr.ml", line 71, characters 29-35: Assertion failed
      Raised at file "src/loader/doc_attr.ml", line 71, characters 29-41
      Called from file "src/loader/doc_attr.ml", line 80, characters 4-32
      Called from file "src/loader/cmti.ml", line 255, characters 12-59
      Called from file "src/loader/cmti.ml", line 269, characters 20-57
      Called from file "list.ml", line 117, characters 24-34
      Called from file "src/loader/cmti.ml", line 264, characters 4-383
      Called from file "src/loader/cmt.ml", line 515, characters 25-62
      Called from file "list.ml", line 117, characters 24-34
      Called from file "src/loader/cmt.ml", line 513, characters 4-136
      Called from file "src/loader/cmt.ml", line 359, characters 38-69
      Called from file "src/loader/cmt.ml", line 376, characters 16-70
      Called from file "src/loader/cmt.ml", line 409, characters 22-70
      Called from file "src/loader/cmt.ml", line 469, characters 27-60
      Called from file "src/loader/cmt.ml", line 515, characters 25-62
      Called from file "list.ml", line 117, characters 24-34
      Called from file "src/loader/cmt.ml", line 513, characters 4-136
      Called from file "src/loader/cmt.ml", line 522, characters 14-46
      Called from file "src/loader/odoc_loader.ml", line 137, characters 31-69
      Called from file "src/model/error.ml", line 65, characters 9-15
      Called from file "src/odoc/compile.ml", line 23, characters 8-36
      Called from file "cmdliner_term.ml", line 25, characters 19-24
      Called from file "cmdliner.ml", line 117, characters 32-39

 result: exited [2]
 writes: {'/home/paus/.local/share/opam/toolstack/var/cache/odig/odoc/xapi-idl/varstore/privileged/varstore_privileged_interface.odoc'}
 reads: {'/home/paus/.local/share/opam/toolstack/lib/xapi-idl/varstore/privileged/varstore_privileged_interface.cmt'
         '/home/paus/.local/share/opam/toolstack/var/cache/odig/odoc/ocaml/camlinternalFormatBasics.odoc'
         '/home/paus/.local/share/opam/toolstack/var/cache/odig/odoc/rpclib/core/idl.odoc'
         '/home/paus/.local/share/opam/toolstack/var/cache/odig/odoc/result/result.odoc'
         '/home/paus/.local/share/opam/toolstack/var/cache/odig/odoc/rpclib/core/rpc.odoc'
         '/home/paus/.local/share/opam/toolstack/var/cache/odig/odoc/rresult/rresult.odoc'
         '/home/paus/.local/share/opam/toolstack/var/cache/odig/odoc/ocaml/stdlib.odoc'
         '/home/paus/.local/share/opam/toolstack/var/cache/odig/odoc/ocaml/stdlib__arg.odoc'
         '/home/paus/.local/share/opam/toolstack/var/cache/odig/odoc/ocaml/stdlib__bigarray.odoc'
         '/home/paus/.local/share/opam/toolstack/var/cache/odig/odoc/ocaml/stdlib__buffer.odoc'
         '/home/paus/.local/share/opam/toolstack/var/cache/odig/odoc/ocaml/stdlib__complex.odoc'
         '/home/paus/.local/share/opam/toolstack/var/cache/odig/odoc/ocaml/stdlib__format.odoc'
         '/home/paus/.local/share/opam/toolstack/var/cache/odig/odoc/ocaml/stdlib__int32.odoc'
         '/home/paus/.local/share/opam/toolstack/var/cache/odig/odoc/ocaml/stdlib__int64.odoc'
         '/home/paus/.local/share/opam/toolstack/var/cache/odig/odoc/ocaml/stdlib__nativeint.odoc'
         '/home/paus/.local/share/opam/toolstack/var/cache/odig/odoc/ocaml/stdlib__printexc.odoc'
         '/home/paus/.local/share/opam/toolstack/var/cache/odig/odoc/ocaml/stdlib__printf.odoc'
         '/home/paus/.local/share/opam/toolstack/var/cache/odig/odoc/ocaml/stdlib__random.odoc'
         '/home/paus/.local/share/opam/toolstack/var/cache/odig/odoc/ocaml/stdlib__seq.odoc'
         '/home/paus/.local/share/opam/toolstack/var/cache/odig/odoc/ocaml/stdlib__string.odoc'
         '/home/paus/.local/share/opam/toolstack/var/cache/odig/odoc/ocaml/stdlib__uchar.odoc'
         '/home/paus/.local/share/opam/toolstack/var/cache/odig/odoc/xapi-idl/syslog.odoc'
         '/home/paus/.local/share/opam/toolstack/var/cache/odig/odoc/ocaml/unix.odoc'
         '/home/paus/.local/share/opam/toolstack/var/cache/odig/odoc/uuidm/uuidm.odoc'
         '/home/paus/.local/share/opam/toolstack/var/cache/odig/odoc/xapi-idl/xcp_service.odoc'}
 created: 23.7s (23646028022ns)
 start: 23.7s (23665400912ns)
 duration: 12.8ms (12715397ns)
 hash: adfb5b3ffdfe71a6
 kind: spawn
[ABORTED][spawn:22197] ['/home/paus/.local/share/opam/toolstack/bin/odoc' 'compile' '--pkg' 'xapi-idl' '-o' '/home/paus/.local/share/opam/toolstack/var/cache/odig/odoc/xapi-idl/varstore/privileged/varstore_privileged_client.odoc' '/home/paus/.local/share/opam/toolstack/lib/xapi-idl/varstore/privileged/varstore_privileged_client.cmt' '-I' '/home/paus/.local/share/opam/toolstack/var/cache/odig/odoc/base/' '-I' '/home/paus/.local/share/opam/toolstack/var/cache/odig/odoc/base/caml/' '-I' '/home/paus/.local/share/opam/toolstack/var/cache/odig/odoc/base/shadow_stdlib/' '-I' '/home/paus/.local/share/opam/toolstack/var/cache/odig/odoc/cohttp/' '-I' '/home/paus/.local/share/opam/toolstack/var/cache/odig/odoc/fieldslib/' '-I' '/home/paus/.local/share/opam/toolstack/var/cache/odig/odoc/message-switch-core/' '-I' '/home/paus/.local/share/opam/toolstack/var/cache/odig/odoc/message-switch-unix/' '-I' '/home/paus/.local/share/opam/toolstack/var/cache/odig/odoc/ocaml/' '-I' '/home/paus/.local/share/opam/toolstack/var/cache/odig/odoc/ppx_sexp_conv/runtime-lib/' '-I' '/home/paus/.local/share/opam/toolstack/var/cache/odig/odoc/re/' '-I' '/home/paus/.local/share/opam/toolstack/var/cache/odig/odoc/result/' '-I' '/home/paus/.local/share/opam/toolstack/var/cache/odig/odoc/rpclib/core/' '-I' '/home/paus/.local/share/opam/toolstack/var/cache/odig/odoc/rpclib/json/' '-I' '/home/paus/.local/share/opam/toolstack/var/cache/odig/odoc/rpclib/xml/' '-I' '/home/paus/.local/share/opam/toolstack/var/cache/odig/odoc/rresult/' '-I' '/home/paus/.local/share/opam/toolstack/var/cache/odig/odoc/sexplib0/' '-I' '/home/paus/.local/share/opam/toolstack/var/cache/odig/odoc/uri/' '-I' '/home/paus/.local/share/opam/toolstack/var/cache/odig/odoc/uuidm/' '-I' '/home/paus/.local/share/opam/toolstack/var/cache/odig/odoc/xapi-backtrace/' '-I' '/home/paus/.local/share/opam/toolstack/var/cache/odig/odoc/xapi-idl/' '-I' '/home/paus/.local/share/opam/toolstack/var/cache/odig/odoc/xapi-idl/varstore/privileged/' '-I' '/home/paus/.local/share/opam/toolstack/var/cache/odig/odoc/xapi-stdext-pervasives/' '-I' '/home/paus/.local/share/opam/toolstack/var/cache/odig/odoc/xmlm/']

I've noticed that the code where odoc crashes is similar to code existing in ppx/common but I'm still very unfamiliar with preprocessors and comments.

The crash happens on a branched marked as todo: https://github.com/ocaml/odoc/blob/master/src/loader/doc_attr.ml#L71

See https://github.com/mirage/ocaml-rpc/issues/139