Better handling negative zeros to match Lua 5.3+ behavior

In Lua 5.3+ the function `lua_isinteger` returns "false" for -0.0 numbers.
In earlier Lua versions we should follow the same behavior to avoid losing the sign when converting to Integer.
Close #618

Author: khvzak

mlua-sys/src/lua51/compat.rs

@@ -186,7 +186,8 @@ pub unsafe fn lua_isinteger(L: *mut lua_State, idx: c_int) -> c_int {
     if lua_type(L, idx) == LUA_TNUMBER {
         let n = lua_tonumber(L, idx);
         let i = lua_tointeger(L, idx);
-        if (n - i as lua_Number).abs() < lua_Number::EPSILON {
+        // Lua 5.3+ returns "false" for `-0.0`
+        if (n - i as lua_Number).abs() < lua_Number::EPSILON && !(n == 0.0 && n.is_sign_negative()) {
             return 1;
         }
     }

mlua-sys/src/lua52/compat.rs

@@ -51,7 +51,8 @@ pub unsafe fn lua_isinteger(L: *mut lua_State, idx: c_int) -> c_int {
     if lua_type(L, idx) == LUA_TNUMBER {
         let n = lua_tonumber(L, idx);
         let i = lua_tointeger(L, idx);
-        if (n - i as lua_Number).abs() < lua_Number::EPSILON {
+        // Lua 5.3+ returns "false" for `-0.0`
+        if (n - i as lua_Number).abs() < lua_Number::EPSILON && !(n == 0.0 && n.is_sign_negative()) {
             return 1;
         }
     }

mlua-sys/src/luau/compat.rs

@@ -120,7 +120,8 @@ pub unsafe fn lua_isinteger(L: *mut lua_State, idx: c_int) -> c_int {
     if lua_type(L, idx) == LUA_TNUMBER {
         let n = lua_tonumber(L, idx);
         let i = lua_tointeger(L, idx);
-        if (n - i as lua_Number).abs() < lua_Number::EPSILON {
+        // Lua 5.3+ returns "false" for `-0.0`
+        if (n - i as lua_Number).abs() < lua_Number::EPSILON && !(n == 0.0 && n.is_sign_negative()) {
             return 1;
         }
     }

src/state/raw.rs

@@ -726,9 +726,13 @@ impl RawLua {
             ffi::LUA_TNUMBER => {
                 use crate::types::Number;
 
+                fn same(n: Number, i: Integer) -> bool {
+                    (n - (i as Number)).abs() < Number::EPSILON && !(n == 0.0 && n.is_sign_negative())
+                }
+
                 let n = ffi::lua_tonumber(state, idx);
                 match num_traits::cast(n) {
-                    Some(i) if (n - (i as Number)).abs() < Number::EPSILON => Value::Integer(i),
+                    Some(i) if same(n, i) => Value::Integer(i),
                     _ => Value::Number(n),
                 }
             }

tests/conversion.rs

@@ -372,6 +372,19 @@ fn test_float_from_lua() -> Result<()> {
     // Should fallback to default conversion
     assert_eq!(f.call::<f32>("42.0")?, 42.0);
 
+    // Negative zero
+    let negative_zero = lua.load("return -0.0").eval::<f64>()?;
+    assert_eq!(negative_zero, 0.0);
+    assert!(negative_zero.is_sign_negative());
+
+    // In Lua <5.3 all numbers are floats
+    #[cfg(not(any(feature = "lua54", feature = "lua53")))]
+    {
+        let negative_zero = lua.load("return -0").eval::<f64>()?;
+        assert_eq!(negative_zero, 0.0);
+        assert!(negative_zero.is_sign_negative());
+    }
+
     Ok(())
 }