[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/engine/evarutil.ml b/engine/evarutil.ml
index 45760c6b4..c707b37b6 100644
--- a/engine/evarutil.ml
+++ b/engine/evarutil.ml
@@ -72,9 +72,9 @@ let flush_and_check_evars sigma c =
 (** Term exploration up to instantiation. *)
 let kind_of_term_upto = EConstr.kind_upto
 
-let nf_evar0 sigma t = EConstr.to_constr sigma (EConstr.of_constr t)
+let nf_evar0 sigma t = EConstr.to_constr ~abort_on_undefined_evars:false sigma (EConstr.of_constr t)
 let whd_evar = EConstr.whd_evar
-let nf_evar sigma c = EConstr.of_constr (EConstr.to_constr sigma c)
+let nf_evar sigma c = EConstr.of_constr (EConstr.to_constr ~abort_on_undefined_evars:false sigma c)
 
 let j_nf_evar sigma j =
   { uj_val = nf_evar sigma j.uj_val;