Add `Value::NULL` constant

src/serde/mod.rs

@@ -1,7 +1,6 @@
 //! (De)Serialization support using serde.
 
 use std::os::raw::c_void;
-use std::ptr;
 
 use serde::{Deserialize, Serialize};
 
@@ -9,7 +8,6 @@ use crate::error::Result;
 use crate::lua::Lua;
 use crate::private::Sealed;
 use crate::table::Table;
-use crate::types::LightUserData;
 use crate::util::check_stack;
 use crate::value::Value;
 
@@ -200,7 +198,7 @@ pub trait LuaSerdeExt<'lua>: Sealed {
 
 impl<'lua> LuaSerdeExt<'lua> for Lua {
     fn null(&'lua self) -> Value<'lua> {
-        Value::LightUserData(LightUserData(ptr::null_mut()))
+        Value::NULL
     }
 
     fn array_metatable(&'lua self) -> Table<'lua> {

src/value.rs

@@ -63,6 +63,12 @@ pub enum Value<'lua> {
 pub use self::Value::Nil;
 
 impl<'lua> Value<'lua> {
+    /// A special value (lightuserdata) to represent null value.
+    ///
+    /// It can be used in Lua tables without downsides of `nil`.
+    pub const NULL: Value<'static> = Value::LightUserData(LightUserData(ptr::null_mut()));
+
+    /// Returns type name of this value.
     pub const fn type_name(&self) -> &'static str {
         match *self {
             Value::Nil => "nil",

tests/value.rs

@@ -28,6 +28,7 @@ fn test_value_eq() -> Result<()> {
     "#,
     )
     .exec()?;
+    globals.set("null", Value::NULL)?;
 
     let table1: Value = globals.get("table1")?;
     let table2: Value = globals.get("table2")?;
@@ -41,6 +42,7 @@ fn test_value_eq() -> Result<()> {
     let func3: Value = globals.get("func3")?;
     let thread1: Value = globals.get("thread1")?;
     let thread2: Value = globals.get("thread2")?;
+    let null: Value = globals.get("null")?;
 
     assert!(table1 != table2);
     assert!(table1.equals(&table2)?);
@@ -54,6 +56,7 @@ fn test_value_eq() -> Result<()> {
     assert!(!func1.equals(&func3)?);
     assert!(thread1 == thread2);
     assert!(thread1.equals(&thread2)?);
+    assert!(null == Value::NULL);
 
     assert!(!table1.to_pointer().is_null());
     assert!(!ptr::eq(table1.to_pointer(), table2.to_pointer()));