[exn] Forbid raising in exn printers, make them return Pp.t option

Raising inside exception printers is quite tricky as the order of
registration for printers will indeed depend on the linking order.

We thus forbid this, and make our API closer to the upstream
`Printexn` by having printers return an option type.

1 files changed, 6 insertions, 0 deletions

diff --git a/dev/doc/changes.md b/dev/doc/changes.md
index 42dd2dd052..d5938713d6 100644
--- a/dev/doc/changes.md
+++ b/dev/doc/changes.md
@@ -15,6 +15,12 @@ Exception handling:
   `Exninfo.capture` and `iraise` when re-raising inside an exception
   handler.
 
+- Registration of exception printers now follows more closely OCaml's
+  API, thus:
+
+  + printers are of type `exn -> Pp.t option` [`None` == not handled]
+  + it is forbidden for exception printers to raise.
+
 Printers:
 
 - Functions such as Printer.pr_lconstr_goal_style_env have been