Add offset option in cast method (#243)

crates/lune-std-ffi/src/c/type_info.rs

@@ -21,6 +21,8 @@ pub trait CTypeCast {
         into_ctype: &LuaAnyUserData,
         _from: &Ref<dyn FfiData>,
         _into: &Ref<dyn FfiData>,
+        _from_offset: isize,
+        _into_offset: isize,
     ) -> LuaResult<()> {
         // Show error if have no cast implement
         Err(Self::cast_failed_with(self, from_ctype, into_ctype))
@@ -114,17 +116,21 @@ where
         methods.add_function(
             "cast",
             |_,
-             (from_type, into_type, from, into): (
+             (from_type, into_type, from, into, from_offset, into_offset): (
                 LuaAnyUserData,
                 LuaAnyUserData,
                 LuaAnyUserData,
                 LuaAnyUserData,
+                Option<isize>,
+                Option<isize>,
             )| {
                 from_type.borrow::<Self>()?.cast(
                     &from_type,
                     &into_type,
                     &from.get_ffi_data()?,
                     &into.get_ffi_data()?,
+                    from_offset.unwrap_or(0),
+                    into_offset.unwrap_or(0),
                 )
             },
         );

crates/lune-std-ffi/src/c/types/mod.rs

@@ -25,7 +25,7 @@ pub mod u64;
 pub mod u8;
 pub mod usize;
 
-// create CType userdata and export
+// CType userdata export
 macro_rules! create_ctypes {
     ($lua:ident, $(( $name:expr, $rust_type:ty, $libffi_type:expr ),)* ) => {
         Ok(vec![$((
@@ -82,9 +82,9 @@ pub fn export_fixed_types(lua: &Lua) -> LuaResult<Vec<(&'static str, LuaAnyUserD
 
 // Implement type-casting for numeric ctypes
 macro_rules! define_cast_num {
-    ($from_rust_type:ident, $self:ident, $from_ctype:ident, $into_ctype:ident, $from:ident, $into:ident, $($into_rust_type:ty)*) => {
+    ($from_rust_type:ident, $self:ident, $from_ctype:ident, $into_ctype:ident, $from:ident, $into:ident, $fromOffset:ident, $intoOffset:ident, $($into_rust_type:ty)*) => {
         $( if $into_ctype.is::<CTypeInfo<$into_rust_type>>() {
-            num_cast::<$from_rust_type, $into_rust_type>($from, $into)
+            num_cast::<$from_rust_type, $into_rust_type>($from, $into, $fromOffset, $intoOffset)
         } else )* {
             Err($self.cast_failed_with($from_ctype, $into_ctype))
         }
@@ -113,9 +113,11 @@ where
         into_info: &LuaAnyUserData,
         from: &Ref<dyn FfiData>,
         into: &Ref<dyn FfiData>,
+        from_offset: isize,
+        into_offset: isize,
     ) -> LuaResult<()> {
         define_cast_num!(
-            From, self, from_info, into_info, from, into,
+            From, self, from_info, into_info, from, into, from_offset, into_offset,
             u8 u16 u32 u64 u128 i8 i16 i32 i64 i128 f32 f64 usize isize
         )
     }

crates/lune-std-ffi/src/ffi/cast.rs

@@ -6,13 +6,26 @@ use num::cast::AsPrimitive;
 use super::FfiData;
 
 #[inline]
-pub fn num_cast<From, Into>(from: &Ref<dyn FfiData>, into: &Ref<dyn FfiData>) -> LuaResult<()>
+pub fn num_cast<From, Into>(
+    from: &Ref<dyn FfiData>,
+    into: &Ref<dyn FfiData>,
+    from_offset: isize,
+    into_offset: isize,
+) -> LuaResult<()>
 where
     From: AsPrimitive<Into>,
     Into: 'static + Copy,
 {
-    let from_ptr = unsafe { from.get_inner_pointer().cast::<From>() };
+    let from_ptr = unsafe {
-    let into_ptr = unsafe { into.get_inner_pointer().cast::<Into>() };
+        from.get_inner_pointer()
+            .byte_offset(from_offset)
+            .cast::<From>()
+    };
+    let into_ptr = unsafe {
+        into.get_inner_pointer()
+            .byte_offset(into_offset)
+            .cast::<Into>()
+    };
 
     unsafe {
         *into_ptr = (*from_ptr).as_();