feat(frontends/lean/elaborator): include number of unsolved goals

See #509

src/frontends/lean/elaborator.cpp

@@ -1506,6 +1506,19 @@ void elaborator::display_tactic_exception(tactic_exception const & ex, proof_sta
         out << ps.pp(env(), ios()) << "\n";
 }
 
+void elaborator::display_unsolved_subgoals(expr const & mvar, proof_state const & ps, expr const & pos) {
+    unsigned ngoals = length(ps.get_goals());
+    sstream s;
+    s << ngoals << " unsolved subgoal";
+    if (ngoals > 1) s << "s";
+    display_unsolved_proof_state(mvar, ps, s.str().c_str(), pos);
+}
+
+void elaborator::display_unsolved_subgoals(expr const & mvar, proof_state const & ps) {
+    display_unsolved_subgoals(mvar, ps, mvar);
+}
+
+
 /** \brief Try to instantiate meta-variable \c mvar (modulo its state ps) using the given tactic.
     If it succeeds, then update subst with the solution.
     Return true iff the metavariable \c mvar has been assigned.
@@ -1530,7 +1543,7 @@ bool elaborator::try_using(substitution & subst, expr const & mvar, proof_state
         } else if (!empty(r->first.get_goals())) {
             // tactic contains unsolved subgoals
             if (show_failure)
-                display_unsolved_proof_state(mvar, r->first, "unsolved subgoals");
+                display_unsolved_subgoals(mvar, r->first);
             return false;
         } else {
             subst = r->first.get_subst();
@@ -1612,7 +1625,7 @@ bool elaborator::try_using_begin_end(substitution & subst, expr const & mvar, pr
     }
 
     if (!empty(ps.get_goals())) {
-        display_unsolved_proof_state(mvar, ps, "unsolved subgoals", pre_tac);
+        display_unsolved_subgoals(mvar, ps, pre_tac);
         return false;
     } else {
         subst = ps.get_subst();

src/frontends/lean/elaborator.h

@@ -144,6 +144,8 @@ class elaborator : public coercion_info_manager {
     unify_result_seq solve(constraint_seq const & cs);
     void display_unsolved_proof_state(expr const & mvar, proof_state const & ps, char const * msg, expr const & pos);
     void display_unsolved_proof_state(expr const & mvar, proof_state const & ps, char const * msg);
+    void display_unsolved_subgoals(expr const & mvar, proof_state const & ps, expr const & pos);
+    void display_unsolved_subgoals(expr const & mvar, proof_state const & ps);
     void display_tactic_exception(tactic_exception const & ex, proof_state const & ps, expr const & pre_tac);
     optional<expr> get_pre_tactic_for(expr const & mvar);
     optional<tactic> pre_tactic_to_tactic(expr const & pre_tac);

tests/lean/487.hlean.expected.out

@@ -1,11 +1,11 @@
-487.hlean:18:56: error: unsolved subgoals
+487.hlean:18:56: error: 1 unsolved subgoal
 A : Type,
 B : Type,
 f : A → B,
 g : B → A,
 ε : Π (b : B), f (g b) = b,
 b b' : B
-⊢ (ε b) ⁻¹ ⬝ refl (f (g b)) ⬝ ε b = refl b
+⊢ (ε b)⁻¹ ⬝ refl (f (g b)) ⬝ ε b = refl b
 487.hlean:19:0: error: failed to add declaration 'foo' to environment, value has metavariables
 remark: set 'formatter.hide_full_terms' to false to see the complete term
   λ (A : Type) (B : Type) (f : …) (g : …) (ε : …) (b b' : B),

tests/lean/goals1.lean.expected.out

@@ -1,4 +1,4 @@
-goals1.lean:9:0: error: unsolved subgoals
+goals1.lean:9:0: error: 1 unsolved subgoal
 A : Type,
 a b c : A,
 Hab : eq a b,

tests/lean/inv_del.lean.expected.out

@@ -1,4 +1,4 @@
-inv_del.lean:15:2: error: unsolved subgoals
+inv_del.lean:15:2: error: 1 unsolved subgoal
 A : Type,
 P : vec A 1 → Type,
 H : Π (a : A), P (vone a),