Add ffi crate

crates/luaffi/src/option.rs

@@ -0,0 +1,80 @@
+use crate::{CDef, CDefBuilder, FromFfi, ToFfi, Type, TypeBuilder, display};
+use std::{ffi::c_int, fmt::Display, ptr};
+
+#[repr(C)]
+#[allow(non_camel_case_types)]
+pub enum lua_option<T> {
+    None,    // __tag = 0
+    Some(T), // __tag = 1
+}
+
+unsafe impl<T: Type> Type for lua_option<T> {
+    fn name() -> impl Display {
+        display!("option__{}", T::name())
+    }
+
+    fn cdecl(name: impl Display) -> impl Display {
+        display!("struct option__{} {name}", T::name())
+    }
+
+    fn build(b: &mut TypeBuilder) {
+        b.include::<T>().cdef::<Self>();
+    }
+}
+
+unsafe impl<T: Type> CDef for lua_option<T> {
+    fn build(b: &mut CDefBuilder) {
+        b.field::<c_int>("__tag").field::<T>("__value");
+    }
+}
+
+unsafe impl<T: FromFfi> FromFfi for Option<T> {
+    type From = *mut Self::FromValue; // pass by-ref
+    type FromValue = lua_option<T::FromValue>;
+
+    const ARG_KEEPALIVE: bool = T::ARG_KEEPALIVE;
+
+    fn prelude(arg: &str) -> impl Display {
+        let ct = Self::FromValue::name();
+        display!(
+            "if {arg} == nil then {arg} = {ct}(); else {}{arg} = {ct}(1, {arg}); end; ",
+            T::prelude(arg)
+        )
+    }
+
+    fn convert(from: Self::From) -> Self {
+        debug_assert!(
+            !from.is_null(),
+            "Option<T>::convert() called on a null lua_option<T>"
+        );
+
+        Self::convert_value(unsafe { ptr::replace(from, lua_option::None) })
+    }
+
+    fn convert_value(from: Self::FromValue) -> Self {
+        match from {
+            lua_option::Some(value) => Some(T::convert_value(value)),
+            lua_option::None => None,
+        }
+    }
+}
+
+unsafe impl<T: ToFfi> ToFfi for Option<T> {
+    type To = lua_option<T::To>;
+
+    fn convert(self) -> Self::To {
+        match self {
+            Some(value) => lua_option::Some(value.convert()),
+            None => lua_option::None,
+        }
+    }
+
+    fn postlude(ret: &str) -> impl Display {
+        // if we don't have a value, return nil. otherwise copy out the inner value immediately,
+        // forget the option cdata, then call postlude on the inner value.
+        display!(
+            "if {ret}.__tag == 0 then {ret} = nil; else {ret} = {ret}.__value; {}end; ",
+            T::postlude(ret)
+        )
+    }
+}