feat(library/simplifier): improve error message when simplifier is looping

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>

src/library/simplifier/simplifier.cpp

@@ -1512,8 +1512,12 @@ public:
         set_ctx(ctx);
         m_num_steps = 0;
         m_depth     = 0;
-        auto r = simplify(e);
-        return mk_pair(r.m_expr, get_proof(r));
+        try {
+            auto r = simplify(e);
+            return mk_pair(r.m_expr, get_proof(r));
+        } catch (stack_space_exception & ex) {
+            throw simplifier_stack_space_exception();
+        }
     }
 };
 
@@ -1560,6 +1564,13 @@ expr_pair simplify(expr const & e, ro_environment const & env, context const & c
     return simplify(e, env, ctx, opts, num_ns, rules.data(), monitor);
 }
 
+simplifier_stack_space_exception::simplifier_stack_space_exception():stack_space_exception("simplifier") {}
+char const * simplifier_stack_space_exception::what() const noexcept {
+    return "deep recursion was detected at 'simplifier', this is probably due to a non-terminating set of rewrite rules, you may use a simplifier_monitor object to track the simplifier behavior (see the simplifier manual); if your problem is very big, you may try to increase stack space in your system";
+}
+exception * simplifier_stack_space_exception::clone() const { return new simplifier_stack_space_exception(); }
+void simplifier_stack_space_exception::rethrow() const { throw *this; }
+
 DECL_UDATA(simplifier)
 DECL_UDATA(ro_simplifier)
 

src/library/simplifier/simplifier.h

@@ -105,6 +105,14 @@ public:
     virtual void failed_abstraction_eh(ro_simplifier const & s, expr const & e, failure_kind k) = 0;
 };
 
+class simplifier_stack_space_exception : public stack_space_exception {
+public:
+    simplifier_stack_space_exception();
+    virtual char const * what() const noexcept;
+    virtual exception * clone() const;
+    virtual void rethrow() const;
+};
+
 expr_pair simplify(expr const & e, ro_environment const & env, context const & ctx, options const & pts,
                    unsigned num_rs, rewrite_rule_set const * rs,
                    std::shared_ptr<simplifier_monitor> const & monitor = std::shared_ptr<simplifier_monitor>());

tests/lean/simp_loop.lean

@@ -0,0 +1,14 @@
+import tactic
+import macros
+
+variable f : Nat → Nat
+variable g : Nat → Nat
+
+axiom Ax1 (a : Nat) : f a = g a
+axiom Ax2 (a : Nat) : g a = f a
+
+add_rewrite Ax1 Ax2
+
+-- The following call to simp should produce a stack overflow
+theorem T : f 0 = 0
+:= by simp

tests/lean/simp_loop.lean.expected.out

@@ -0,0 +1,9 @@
+  Set: pp::colors
+  Set: pp::unicode
+  Imported 'tactic'
+  Imported 'macros'
+  Assumed: f
+  Assumed: g
+  Assumed: Ax1
+  Assumed: Ax2
+simp_loop.lean:13:1: error: deep recursion was detected at 'simplifier', this is probably due to a non-terminating set of rewrite rules, you may use a simplifier_monitor object to track the simplifier behavior (see the simplifier manual); if your problem is very big, you may try to increase stack space in your system