New parser based on new version of the Coq backend of Menhir (#276)

What's new:

1. A rewrite of the Coq interpreter of Menhir automaton, with
   dependent types removing the need for runtime checks for the
   well-formedness of the LR stack. This seem to cause some speedup on
   the parsing time (~10% for lexing + parsing).

2. Thanks to 1., it is now possible to avoid the use of int31 for
   comparing symbols: Since this is only used for validation,
   positives are enough.

3. Speedup of Validation: on my machine, the time needed for compiling
   Parser.v goes from about 2 minutes to about 1 minute. This seem to
   be related to a performance bug in the completeness validator and
   to the use of positive instead of int31.

3. Menhir now generates a dedicated inductive type for
   (semantic-value-carrying) tokens (in addition to the already
   existing inductive type for (non-semantic-value-carrying)
   terminals. The end result is that the OCaml support code for the
   parser no longer contain calls to Obj.magic. The bad side of this
   change is that the formal specification of the parser is perhaps
   harder to read.

4. The parser and its library are now free of axioms (I used to use
   axiom K and proof irrelevance for easing proofs involving dependent
   types).

5. Use of a dedicated custom negative coinductive type for the input
   stream of tokens, instead of Coq stdlib's `Stream`. `Stream` is a
   positive coinductive type, which are now deprecated by Coq.

6. The fuel of the parser is now specified using its logarithm instead
   of its actual value. This makes it possible to give large fuel
   values instead of using the `let rec fuel = S fuel` hack.

7. Some refactoring in the lexer, the parser and the Cabs syntax tree.

The corresponding changes in Menhir have been released as part of
version 20190626. The `MenhirLib` directory is identical to the
content of the `src` directory of the corresponding `coq-menhirlib`
opam package except that:
   - In order to try to make CompCert compatible with several Menhir
     versions without updates, we do not check the version of menhir
     is compatible with the version of coq-menhirlib. Hence the
     `Version.v` file is not present in CompCert's copy.
   - Build-system related files have been removed.

1 files changed, 74 insertions, 0 deletions

diff --git a/MenhirLib/Validator_classes.v b/MenhirLib/Validator_classes.v
new file mode 100644
index 00000000..c043cb73
--- /dev/null
+++ b/MenhirLib/Validator_classes.v
@@ -0,0 +1,74 @@
+(****************************************************************************)
+(*                                                                          *)
+(*                                   Menhir                                 *)
+(*                                                                          *)
+(*           Jacques-Henri Jourdan, CNRS, LRI, Université Paris Sud         *)
+(*                                                                          *)
+(*  Copyright Inria. All rights reserved. This file is distributed under    *)
+(*  the terms of the GNU Lesser General Public License as published by the  *)
+(*  Free Software Foundation, either version 3 of the License, or (at your  *)
+(*  option) any later version, as described in the file LICENSE.            *)
+(*                                                                          *)
+(****************************************************************************)
+
+From Coq Require Import List.
+From Coq.ssr Require Import ssreflect.
+Require Import Alphabet.
+
+Class IsValidator (P : Prop) (b : bool) :=
+  is_validator : b = true -> P.
+Hint Mode IsValidator + -.
+
+Instance is_validator_true : IsValidator True true.
+Proof. done. Qed.
+
+Instance is_validator_false : IsValidator False false.
+Proof. done. Qed.
+
+Instance is_validator_eq_true b :
+  IsValidator (b = true) b.
+Proof. done. Qed.
+
+Instance is_validator_and P1 b1 P2 b2 `{IsValidator P1 b1} `{IsValidator P2 b2}:
+  IsValidator (P1 /\ P2) (if b1 then b2 else false).
+Proof. split; destruct b1, b2; auto using is_validator. Qed.
+
+Instance is_validator_comparable_leibniz_eq A (C:Comparable A) (x y : A) :
+  ComparableLeibnizEq C ->
+  IsValidator (x = y) (compare_eqb x y).
+Proof. intros ??. by apply compare_eqb_iff. Qed.
+
+Instance is_validator_comparable_eq_impl A `(Comparable A) (x y : A) P b :
+  IsValidator P b ->
+  IsValidator (x = y -> P) (if compare_eqb x y then b else true).
+Proof.
+  intros Hval Val ->. rewrite /compare_eqb compare_refl in Val. auto.
+Qed.
+
+Lemma is_validator_forall_finite A P b `(Finite A) :
+  (forall (x : A), IsValidator (P x) (b x)) ->
+  IsValidator (forall (x : A), P x) (forallb b all_list).
+Proof.
+  move=> ? /forallb_forall.
+  auto using all_list_forall, is_validator, forallb_forall.
+Qed.
+(* We do not use an instance directly here, because we need somehow to
+   force Coq to instantiate b with a lambda. *)
+Hint Extern 2 (IsValidator (forall x : ?A, _) _) =>
+    eapply (is_validator_forall_finite _ _ (fun (x:A) => _))
+  : typeclass_instances.
+
+(* Hint for synthetizing pattern-matching. *)
+Hint Extern 2 (IsValidator (match ?u with _ => _ end) ?b0) =>
+    let b := fresh "b" in
+    unshelve notypeclasses refine (let b : bool := _ in _);
+      [destruct u; intros; shelve|];    (* Synthetize `match .. with` in the validator. *)
+    unify b b0;
+    unfold b; destruct u; clear b
+  : typeclass_instances.
+
+(* Hint for unfolding definitions. This is necessary because many
+  hints for IsValidator use [Hint Extern], which do not automatically
+  unfold identifiers. *)
+Hint Extern 100 (IsValidator ?X _) => unfold X
+  : typeclass_instances.