Avoid type checking private_constants (side_eff) again during Qed (#4357).

Side effects are now an opaque data type, called private_constant, you can
only obtain from safe_typing.  When add_constant is called on a
definition_entry that contains private constants, they are either
- inlined in the main proof term but not re-checked
- declared globally without re-checking them
As a safety measure, the opaque data type contains a pointer to the
revstruct (an internal field of safe_env that changes every time a new
constant is added), and such pointer is compared with the current value
store in safe_env when the private_constant is inlined.  Only when the
comparison is successful the private_constant is not re-checked.  Otherwise
else it is.  In short, we accept into the kernel private constant only
when they arrive in the very same order and on top of the very same env
they arrived when we fist checked them.

Note: private_constants produced by workers never pass the safety
measure (the revstruct pointer is an Ephemeron).  Sending back the
entire revstruct is possible but: 1. we lack a way to quickly compare
two revstructs, 2. it can be large.

1 files changed, 4 insertions, 1 deletions

diff --git a/library/heads.ml b/library/heads.ml
index 5c153b067..73d2aa053 100644
--- a/library/heads.ml
+++ b/library/heads.ml
@@ -68,7 +68,10 @@ let kind_of_head env t =
         | None -> NotImmediatelyComputableHead)
   | Const (cst,_) ->
       (try on_subterm k l b (constant_head cst)
-       with Not_found -> assert false)
+       with Not_found ->
+         Errors.anomaly
+           Pp.(str "constant not found in kind_of_head: " ++
+               str (Names.Constant.to_string cst)))
   | Construct _ | CoFix _ ->
       if b then NotImmediatelyComputableHead else ConstructorHead
   | Sort _ | Ind _ | Prod _ -> RigidHead RigidType