Add Value::to_pointer() function.

Closes #165 and #166.

src/value.rs

@@ -1,5 +1,6 @@
 use std::iter::{self, FromIterator};
-use std::{slice, str, vec};
+use std::os::raw::c_void;
+use std::{ptr, slice, str, vec};
 
 #[cfg(feature = "serialize")]
 use {
@@ -9,6 +10,7 @@ use {
 };
 
 use crate::error::{Error, Result};
+use crate::ffi;
 use crate::function::Function;
 use crate::lua::Lua;
 use crate::string::String;
@@ -93,6 +95,29 @@ impl<'lua> Value<'lua> {
             _ => Ok(self == other.as_ref()),
         }
     }
+
+    /// Converts the value to a generic C pointer.
+    ///
+    /// The value can be a userdata, a table, a thread, a string, or a function; otherwise it returns NULL.
+    /// Different objects will give different pointers.
+    /// There is no way to convert the pointer back to its original value.
+    ///
+    /// Typically this function is used only for hashing and debug information.
+    pub fn to_pointer(&self) -> *const c_void {
+        unsafe {
+            match self {
+                Value::LightUserData(ud) => ud.0,
+                Value::String(String(v))
+                | Value::Table(Table(v))
+                | Value::Function(Function(v))
+                | Value::Thread(Thread(v))
+                | Value::UserData(AnyUserData(v)) => v
+                    .lua
+                    .ref_thread_exec(|refthr| ffi::lua_topointer(refthr, v.index)),
+                _ => ptr::null(),
+            }
+        }
+    }
 }
 
 impl<'lua> PartialEq for Value<'lua> {

tests/value.rs

@@ -1,3 +1,5 @@
+use std::ptr;
+
 use mlua::{Lua, Result, Value};
 
 #[test]
@@ -41,17 +43,23 @@ fn test_value_eq() -> Result<()> {
     let thread2: Value = globals.get("thread2")?;
 
     assert!(table1 != table2);
-    assert!(table1.equals(table2)?);
+    assert!(table1.equals(&table2)?);
     assert!(string1 == string2);
-    assert!(string1.equals(string2)?);
+    assert!(string1.equals(&string2)?);
     assert!(num1 == num2);
     assert!(num1.equals(num2)?);
     assert!(num1 != num3);
     assert!(func1 == func2);
     assert!(func1 != func3);
-    assert!(!func1.equals(func3)?);
+    assert!(!func1.equals(&func3)?);
     assert!(thread1 == thread2);
-    assert!(thread1.equals(thread2)?);
+    assert!(thread1.equals(&thread2)?);
+
+    assert!(!table1.to_pointer().is_null());
+    assert!(!ptr::eq(table1.to_pointer(), table2.to_pointer()));
+    assert!(ptr::eq(string1.to_pointer(), string2.to_pointer()));
+    assert!(ptr::eq(func1.to_pointer(), func2.to_pointer()));
+    assert!(num1.to_pointer().is_null());
 
     Ok(())
 }