[econstr] Forbid calling `to_constr` in open terms.

We forbid calling `EConstr.to_constr` on terms that are not evar-free,
as to progress towards enforcing the invariant that `Constr.t` is
evar-free. [c.f. #6308]

Due to compatibility constraints we provide an optional parameter to
`to_constr`, `abort` which can be used to overcome this restriction
until we fix all parts of the code.

Now, grepping for `~abort:false` should return the questionable
parts of the system.

Not a lot of places had to be fixed, some comments:

- problems with the interface due to `Evd/Constr` [`Evd.define` being
  the prime example] do seem real!
- inductives also look bad with regards to `Constr/EConstr`.
- code in plugins needs work.

A notable user of this "feature" is `Obligations/Program` that seem to
like to generate kernel-level entries with free evars, then to scan
them and workaround this problem by generating constants.

1 files changed, 2 insertions, 2 deletions

diff --git a/pretyping/pretyping.ml b/pretyping/pretyping.ml
index 4962b89a0..2c371d5cf 100644
--- a/pretyping/pretyping.ml
+++ b/pretyping/pretyping.ml
@@ -117,7 +117,7 @@ open ExtraEnv
 exception Found of int array
 
 let nf_fix sigma (nas, cs, ts) =
-  let inj c = EConstr.to_constr sigma c in
+  let inj c = EConstr.to_constr ~abort_on_undefined_evars:false sigma c in
   (nas, Array.map inj cs, Array.map inj ts)
 
 let search_guard ?loc env possible_indexes fixdefs =
@@ -1150,7 +1150,7 @@ and pretype_type k0 resolve_tc valcon (env : ExtraEnv.t) evdref lvar c = match D
            (* Correction of bug #5315 : we need to define an evar for *all* holes *)
            let evkt = e_new_evar env evdref ~src:(loc, knd) ~naming (mkSort s) in
            let ev,_ = destEvar !evdref evkt in
-           evdref := Evd.define ev (to_constr !evdref v) !evdref;
+           evdref := Evd.define ev (to_constr ~abort_on_undefined_evars:false !evdref v) !evdref;
            (* End of correction of bug #5315 *)
            { utj_val = v;
 	     utj_type = s }