Merge branch 'ralf/atomic' into 'master'

add a strong form of atomicity, for weak forms of weakest-pre

Closes #107

See merge request FP/iris-coq!77

theories/program_logic/language.v

@@ -52,9 +52,22 @@ Section language.
   Definition irreducible (e : expr Λ) (σ : state Λ) :=
     ∀ e' σ' efs, ¬prim_step e σ e' σ' efs.
 
+  (* This (weak) form of atomicity is enough to open invariants when WP ensures
+     safety, i.e., programs never can get stuck.  We have an example in
+     lambdaRust of an expression that is atomic in this sense, but not in the
+     stronger sense defined below, and we have to be able to open invariants
+     around that expression.  See `CasStuckS` in
+     [lambdaRust](https://gitlab.mpi-sws.org/FP/LambdaRust-coq/blob/master/theories/lang/lang.v). *)
   Definition atomic (e : expr Λ) : Prop :=
     ∀ σ e' σ' efs, prim_step e σ e' σ' efs → irreducible e' σ'.
 
+  (* To open invariants with a WP that does not ensure safety, we need a
+     stronger form of atomicity.  With the above definition, in case `e` reduces
+     to a stuck non-value, there is no proof that the invariants have been
+     established again. *)
+  Definition strongly_atomic (e : expr Λ) : Prop :=
+    ∀ σ e' σ' efs, prim_step e σ e' σ' efs → is_Some (to_val e').
+
   Inductive step (ρ1 ρ2 : cfg Λ) : Prop :=
     | step_atomic e1 σ1 e2 σ2 efs t1 t2 :
        ρ1 = (t1 ++ e1 :: t2, σ1) →
@@ -74,6 +87,10 @@ Section language.
   Global Instance of_val_inj : Inj (=) (=) (@of_val Λ).
   Proof. by intros v v' Hv; apply (inj Some); rewrite -!to_of_val Hv. Qed.
 
+  Lemma strongly_atomic_atomic e :
+    strongly_atomic e → atomic e.
+  Proof. unfold strongly_atomic, atomic. eauto using val_irreducible. Qed.
+
   Lemma reducible_fill `{LanguageCtx Λ K} e σ :
     to_val e = None → reducible (K e) σ → reducible e σ.
   Proof.