feat: add option --exitOnPanic (short version -e)

Lean does not exit on panic anymore.
The old behavior (`std::exit(1)`) produces a horrible debugging
experience for the elaborator since all trace messages are lost.
The new command line option restores the old behavior.

cc @Kha @dselsam

src/Init/Util.lean

@@ -25,7 +25,7 @@ f ()
 @[extern c inline "#4"]
 unsafe def unsafeCast {α : Type u} {β : Type v} [Inhabited β] (a : α) : β := arbitrary _
 
-@[neverExtract, extern c inline "lean_panic_fn(#3)"]
+@[neverExtract, extern "lean_panic_fn"]
 constant panic {α : Type u} [Inhabited α] (msg : String) : α := arbitrary _
 
 @[noinline] private def mkPanicMessage (modName : String) (line col : Nat) (msg : String) : String :=

src/runtime/lean.h

@@ -242,11 +242,13 @@ static inline bool lean_is_scalar(lean_object * o) { return ((size_t)(o) & 1) ==
 static inline lean_object * lean_box(size_t n) { return (lean_object*)(((size_t)(n) << 1) | 1); }
 static inline size_t lean_unbox(lean_object * o) { return (size_t)(o) >> 1; }
 
+void lean_set_exit_on_panic(bool flag);
+lean_object * lean_panic_fn(lean_object * default_val, lean_object * msg);
+
 __attribute__((noreturn)) void lean_panic(char const * msg);
 __attribute__((noreturn)) void lean_panic_out_of_memory();
 __attribute__((noreturn)) void lean_panic_unreachable();
 __attribute__((noreturn)) void lean_panic_rc_overflow();
-lean_object * lean_panic_fn(lean_object * msg);
 
 static inline size_t lean_align(size_t v, size_t a) {
     return (v / a)*a + a * (v % a != 0);

src/runtime/object.cpp

@@ -39,8 +39,20 @@ extern "C" void lean_panic_rc_overflow() {
     lean_panic("reference counter overflowed");
 }
 
-extern "C" object * lean_panic_fn(object * msg) {
-    lean_panic(lean_string_cstr(msg));
+bool g_exit_on_panic = false;
+
+extern "C" void lean_set_exit_on_panic(bool flag) {
+    g_exit_on_panic = flag;
+}
+
+extern "C" object * lean_panic_fn(object * default_val, object * msg) {
+    // TODO(Leo, Kha): add thread local buffer for interpreter.
+    std::cerr << lean_string_cstr(msg) << "\n";
+    if (g_exit_on_panic) {
+        std::exit(1);
+    }
+    lean_dec(msg);
+    return default_val;
 }
 
 extern "C" size_t lean_object_byte_size(lean_object * o) {

src/shell/lean.cpp

@@ -241,6 +241,7 @@ static struct option g_long_options[] = {
     {"timeout",      optional_argument, 0, 'T'},
     {"cpp",          optional_argument, 0, 'c'},
     {"c",            optional_argument, 0, 'C'},
+    {"exitOnPanic",  no_argument,       0, 'e'},
 #if defined(LEAN_JSON)
     {"json",         no_argument,       0, 'J'},
     {"server",       optional_argument, 0, 'S'},
@@ -257,7 +258,7 @@ static struct option g_long_options[] = {
 };
 
 static char const * g_opt_str =
-    "PdD:m::c:C:qgvht:012j:012rM:012T:012ap:"
+    "PdD:m::c:C:qgvht:012j:012rM:012T:012ap:e"
 #if defined(LEAN_MULTI_THREAD)
     "s:012"
 #endif
@@ -436,6 +437,9 @@ int main(int argc, char ** argv) {
         if (c == -1)
             break; // end of command line
         switch (c) {
+            case 'e':
+                lean_set_exit_on_panic(true);
+                break;
             case 'j':
                 // num_threads = static_cast<unsigned>(atoi(optarg));
                 break;