'main_state' fix

Dont' confuse the state we're pushing the registry value for the main state to
with the main state itself, pop from the correct state.

src/tests.rs

@@ -882,3 +882,14 @@ fn string_views() {
     assert!(err.to_str().is_err());
     assert_eq!(err.as_bytes(), &b"but \xff isn't :("[..]);
 }
+
+#[test]
+fn coroutine_from_closure() {
+    let lua = Lua::new();
+    let thrd_main = lua.create_function(|lua, _| {
+        lua.pack(())
+    });
+    lua.globals().set("main", thrd_main).unwrap();
+    let thrd: Thread = lua.eval("coroutine.create(main)", None).unwrap();
+    thrd.resume::<_, ()>(()).unwrap();
+}

src/util.rs

@@ -478,9 +478,9 @@ pub unsafe extern "C" fn safe_xpcall(state: *mut ffi::lua_State) -> c_int {
 /// Does not call checkstack, uses 1 stack space
 pub unsafe fn main_state(state: *mut ffi::lua_State) -> *mut ffi::lua_State {
     ffi::lua_rawgeti(state, ffi::LUA_REGISTRYINDEX, ffi::LUA_RIDX_MAINTHREAD);
-    let state = ffi::lua_tothread(state, -1);
+    let main_state = ffi::lua_tothread(state, -1);
     ffi::lua_pop(state, 1);
-    state
+    main_state
 }
 
 pub struct WrappedError(pub Error);