Impl generic From/IntoFfi for Option

crates/luaffi/Cargo.toml

@@ -7,6 +7,10 @@ authors.workspace = true
 homepage.workspace = true
 repository.workspace = true
 
+[features]
+option_ref_abi = []
+option_string_abi = []
+
 [dependencies]
 bstr = "1.12.0"
 luaffi_impl = { path = "../luaffi_impl" }

crates/luaffi/src/lib.rs

@@ -11,13 +11,13 @@ use std::{
     mem,
 };
 
-pub mod future;
-pub mod string;
-
 #[doc(hidden)]
 #[path = "./internal.rs"]
 pub mod __internal;
+pub mod future;
+pub mod option;
 pub mod result;
+pub mod string;
 
 // Dummy function to ensure that strings passed to Rust via wrapper objects will not be
 // garbage-collected until the end of the function (used in string.rs when string marshalling is
@@ -857,7 +857,9 @@ macro_rules! impl_ptr_intoabi {
 
 impl_ptr_intoabi!(*const T);
 impl_ptr_intoabi!(*mut T);
+#[cfg(feature = "option_ref_abi")] // disabled because it conflicts with the generic Option<T> impl
 impl_ptr_intoabi!(Option<&'static T>);
+#[cfg(feature = "option_ref_abi")]
 impl_ptr_intoabi!(Option<&'static mut T>);
 
 //

crates/luaffi/src/option.rs

@@ -0,0 +1,84 @@
+use crate::{
+    __internal::{disp, display},
+    Cdef, CdefBuilder, IntoFfi, KEEP_FN, Type, TypeBuilder, TypeType,
+};
+use std::{ffi::c_int, fmt::Display};
+
+#[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 ty() -> TypeType {
+        TypeType::Aggregate
+    }
+
+    fn cdecl(name: impl Display) -> impl Display {
+        display!("struct {} {name}", Self::name())
+    }
+
+    fn build(b: &mut TypeBuilder) {
+        b.cdef::<Self>();
+    }
+}
+
+unsafe impl<T: Type> Cdef for lua_option<T> {
+    fn build(b: &mut CdefBuilder) {
+        b.field::<c_int>("__tag");
+        (T::ty() != TypeType::Void).then(|| b.field::<T>("__value"));
+    }
+}
+
+unsafe impl<T: IntoFfi> IntoFfi for Option<T> {
+    type Into = lua_option<T::Into>;
+
+    fn convert(self) -> Self::Into {
+        match self {
+            Some(value) => lua_option::Some(T::convert(value)),
+            None => lua_option::None,
+        }
+    }
+
+    fn require_owned() -> bool {
+        // lua_option is only used to transmit information about whether we have a value or not and
+        // is forgotten immediately after use, so there is no need for an owned option
+        false
+    }
+
+    fn postlude(ret: &str) -> impl Display {
+        disp(move |f| {
+            write!(f, "if {ret}.__tag ~= 0 then ")?;
+            match T::Into::ty() {
+                TypeType::Void => write!(f, "{ret} = nil; ")?, // for void options, we don't have a __value
+                TypeType::Primitive => {
+                    // can always copy primitives to stack
+                    write!(f, "{ret} = {ret}.__value; {}", T::postlude(ret))?;
+                }
+                TypeType::Aggregate => {
+                    let ct = T::Into::name();
+                    if T::require_owned() {
+                        // inner value requires ownership; copy it into its own cdata and forget
+                        // option.
+                        write!(f, "{ret} = __new(__ct.{ct}, {ret}.__value); ")?;
+                        write!(f, "{}", T::postlude(ret))?;
+                    } else {
+                        // inner value is a "temporary" like an option itself and doesn't require
+                        // full ownership of itself. we just need to keep the option object alive
+                        // until its postlude completes.
+                        write!(f, "local {ret}_keep = {ret}; {ret} = {ret}.__value; ")?;
+                        write!(f, "do {}end; ", T::postlude(ret))?;
+                        write!(f, "__C.{KEEP_FN}({ret}_keep); ")?; // keep original option alive
+                    }
+                }
+            }
+            write!(f, "else {ret} = nil; end; ")
+        })
+    }
+}

crates/luaffi/src/result.rs

@@ -72,12 +72,12 @@ unsafe impl<T: IntoFfi, E: Display> IntoFfi for Result<T, E> {
                         write!(f, "{ret} = __new(__ct.{ct}, {ret}.__value); ")?;
                         write!(f, "{}", T::postlude(ret))?;
                     } else {
-                        // inner value is a "temporary" itself and doesn't require full ownership of
+                        // inner value is a "temporary" like a result itself and doesn't require
-                        // itself. we just need to keep the result object alive until its postlude
+                        // full ownership of itself. we just need to keep the result object alive
-                        // completes.
+                        // until its postlude completes.
-                        write!(f, "local __{ret} = {ret}; {ret} = {ret}.__value; ")?;
+                        write!(f, "local {ret}_keep = {ret}; {ret} = {ret}.__value; ")?;
                         write!(f, "do {}end; ", T::postlude(ret))?;
-                        write!(f, "__C.{KEEP_FN}(__{ret}); ")?; // keep original result alive
+                        write!(f, "__C.{KEEP_FN}({ret}_keep); ")?; // keep original result alive
                     }
                 }
             }

crates/luaffi/src/string.rs

@@ -4,7 +4,7 @@ use crate::{
 };
 use bstr::{BStr, BString};
 use luaffi_impl::{cdef, metatype};
-use std::{fmt::Display, mem::ManuallyDrop, ptr, slice};
+use std::{fmt::Display, mem::ManuallyDrop, slice};
 
 pub(crate) const IS_UTF8_FN: &str = "luaffi_is_utf8";
 pub(crate) const DROP_BUFFER_FN: &str = "luaffi_drop_buffer";
@@ -42,6 +42,7 @@ impl lua_buf {
         }
     }
 
+    #[cfg(feature = "option_string_ffi")]
     pub(crate) fn null() -> Self {
         Self {
             __ptr: ptr::null(),
@@ -71,6 +72,7 @@ impl lua_buffer {
         }
     }
 
+    #[cfg(feature = "option_string_ffi")]
     pub(crate) fn null() -> Self {
         Self {
             __ptr: ptr::null_mut(),
@@ -228,6 +230,13 @@ impl_into_via!(&'static str, &'static BStr);
 impl_into_via!(BString, Vec<u8>);
 impl_into_via!(String, BString);
 
+// `Option<String>: From/IntoFfi` isn't implemented because it conflicts with the generic
+// `Option<T>: From/IntoFfi` impl and rust doesn't have specialisation yet (and probably not anytime
+// soon). this is fine for now because we have specialisation for string-like parameters implemented
+// in the #[metatype] macro already, and string returns wrapped in `Option<T>` isn't much additional
+// overhead.
+#[cfg(feature = "option_string_abi")]
+mod impl_option_string {
     macro_rules! impl_optional_from {
         ($ty:ty) => {
             unsafe impl<'s> FromFfi for Option<$ty> {
@@ -281,3 +290,4 @@ impl_optional_into!(&'static str, lua_buf::null());
     impl_optional_into!(Vec<u8>, lua_buffer::null());
     impl_optional_into!(BString, lua_buffer::null());
     impl_optional_into!(String, lua_buffer::null());
+}