Accept any error in Function::wrap/wrap_mut/wrap_async

Previously wrapped functions were required to return `mlua::Result`.
Now it's possible to wrap functions returning any errors as long as
they implement `std::error::Error`.

Existing code remains compatible with `mlua::Result` as this type
is not converted to an external error.

src/error.rs

@@ -345,7 +345,11 @@ impl Error {
     /// Wraps an external error object.
     #[inline]
     pub fn external<T: Into<Box<DynStdError>>>(err: T) -> Self {
-        Error::ExternalError(err.into().into())
+        let boxed = err.into();
+        match boxed.downcast::<Self>() {
+            Ok(err) => *err,
+            Err(boxed) => Error::ExternalError(boxed.into()),
+        }
     }
 
     /// Attempts to downcast the external error object to a concrete type by reference.

src/function.rs

@@ -81,9 +81,10 @@
 
 use std::cell::RefCell;
 use std::os::raw::{c_int, c_void};
+use std::result::Result as StdResult;
 use std::{mem, ptr, slice};
 
-use crate::error::{Error, Result};
+use crate::error::{Error, ExternalError, ExternalResult, Result};
 use crate::state::Lua;
 use crate::table::Table;
 use crate::traits::{FromLuaMulti, IntoLua, IntoLuaMulti};
@@ -635,30 +636,32 @@ impl Function {
     /// Wraps a Rust function or closure, returning an opaque type that implements [`IntoLua`]
     /// trait.
     #[inline]
-    pub fn wrap<F, A, R>(func: F) -> impl IntoLua
+    pub fn wrap<F, A, R, E>(func: F) -> impl IntoLua
     where
-        F: LuaNativeFn<A, Output = Result<R>> + MaybeSend + 'static,
+        F: LuaNativeFn<A, Output = StdResult<R, E>> + MaybeSend + 'static,
         A: FromLuaMulti,
         R: IntoLuaMulti,
+        E: ExternalError,
     {
         WrappedFunction(Box::new(move |lua, nargs| unsafe {
             let args = A::from_stack_args(nargs, 1, None, lua)?;
-            func.call(args)?.push_into_stack_multi(lua)
+            func.call(args).into_lua_err()?.push_into_stack_multi(lua)
         }))
     }
 
     /// Wraps a Rust mutable closure, returning an opaque type that implements [`IntoLua`] trait.
-    pub fn wrap_mut<F, A, R>(func: F) -> impl IntoLua
+    pub fn wrap_mut<F, A, R, E>(func: F) -> impl IntoLua
     where
-        F: LuaNativeFnMut<A, Output = Result<R>> + MaybeSend + 'static,
+        F: LuaNativeFnMut<A, Output = StdResult<R, E>> + MaybeSend + 'static,
         A: FromLuaMulti,
         R: IntoLuaMulti,
+        E: ExternalError,
     {
         let func = RefCell::new(func);
         WrappedFunction(Box::new(move |lua, nargs| unsafe {
             let mut func = func.try_borrow_mut().map_err(|_| Error::RecursiveMutCallback)?;
             let args = A::from_stack_args(nargs, 1, None, lua)?;
-            func.call(args)?.push_into_stack_multi(lua)
+            func.call(args).into_lua_err()?.push_into_stack_multi(lua)
         }))
     }
 
@@ -671,6 +674,7 @@ impl Function {
     pub fn wrap_raw<F, A>(func: F) -> impl IntoLua
     where
         F: LuaNativeFn<A> + MaybeSend + 'static,
+        F::Output: IntoLuaMulti,
         A: FromLuaMulti,
     {
         WrappedFunction(Box::new(move |lua, nargs| unsafe {
@@ -687,6 +691,7 @@ impl Function {
     pub fn wrap_raw_mut<F, A>(func: F) -> impl IntoLua
     where
         F: LuaNativeFnMut<A> + MaybeSend + 'static,
+        F::Output: IntoLuaMulti,
         A: FromLuaMulti,
     {
         let func = RefCell::new(func);
@@ -701,11 +706,12 @@ impl Function {
     /// trait.
     #[cfg(feature = "async")]
     #[cfg_attr(docsrs, doc(cfg(feature = "async")))]
-    pub fn wrap_async<F, A, R>(func: F) -> impl IntoLua
+    pub fn wrap_async<F, A, R, E>(func: F) -> impl IntoLua
     where
-        F: LuaNativeAsyncFn<A, Output = Result<R>> + MaybeSend + 'static,
+        F: LuaNativeAsyncFn<A, Output = StdResult<R, E>> + MaybeSend + 'static,
         A: FromLuaMulti,
         R: IntoLuaMulti,
+        E: ExternalError,
     {
         WrappedAsyncFunction(Box::new(move |rawlua, nargs| unsafe {
             let args = match A::from_stack_args(nargs, 1, None, rawlua) {
@@ -714,7 +720,7 @@ impl Function {
             };
             let lua = rawlua.lua();
             let fut = func.call(args);
-            Box::pin(async move { fut.await?.push_into_stack_multi(lua.raw_lua()) })
+            Box::pin(async move { fut.await.into_lua_err()?.push_into_stack_multi(lua.raw_lua()) })
         }))
     }
 
@@ -728,6 +734,7 @@ impl Function {
     pub fn wrap_raw_async<F, A>(func: F) -> impl IntoLua
     where
         F: LuaNativeAsyncFn<A> + MaybeSend + 'static,
+        F::Output: IntoLuaMulti,
         A: FromLuaMulti,
     {
         WrappedAsyncFunction(Box::new(move |rawlua, nargs| unsafe {
@@ -789,14 +796,14 @@ impl<R: FromLuaMulti> Future for AsyncCallFuture<R> {
 
 /// A trait for types that can be used as Lua functions.
 pub trait LuaNativeFn<A: FromLuaMulti> {
-    type Output: IntoLuaMulti;
+    type Output;
 
     fn call(&self, args: A) -> Self::Output;
 }
 
 /// A trait for types with mutable state that can be used as Lua functions.
 pub trait LuaNativeFnMut<A: FromLuaMulti> {
-    type Output: IntoLuaMulti;
+    type Output;
 
     fn call(&mut self, args: A) -> Self::Output;
 }
@@ -804,7 +811,7 @@ pub trait LuaNativeFnMut<A: FromLuaMulti> {
 /// A trait for types that returns a future and can be used as Lua functions.
 #[cfg(feature = "async")]
 pub trait LuaNativeAsyncFn<A: FromLuaMulti> {
-    type Output: IntoLuaMulti;
+    type Output;
 
     fn call(&self, args: A) -> impl Future<Output = Self::Output> + MaybeSend + 'static;
 }
@@ -815,7 +822,6 @@ macro_rules! impl_lua_native_fn {
         where
             FN: Fn($($A,)*) -> R + MaybeSend + 'static,
             ($($A,)*): FromLuaMulti,
-            R: IntoLuaMulti,
         {
             type Output = R;
 
@@ -830,7 +836,6 @@ macro_rules! impl_lua_native_fn {
         where
             FN: FnMut($($A,)*) -> R + MaybeSend + 'static,
             ($($A,)*): FromLuaMulti,
-            R: IntoLuaMulti,
         {
             type Output = R;
 
@@ -847,7 +852,6 @@ macro_rules! impl_lua_native_fn {
             FN: Fn($($A,)*) -> Fut + MaybeSend + 'static,
             ($($A,)*): FromLuaMulti,
             Fut: Future<Output = R> + MaybeSend + 'static,
-            R: IntoLuaMulti,
         {
             type Output = R;
 

tests/async.rs

@@ -41,7 +41,7 @@ async fn test_async_function_wrap() -> Result<()> {
 
     let f = Function::wrap_async(|s: String| async move {
         tokio::task::yield_now().await;
-        Ok(s)
+        Ok::<_, Error>(s)
     });
     lua.globals().set("f", f)?;
     let res: String = lua.load(r#"f("hello")"#).eval_async().await?;

tests/error.rs

@@ -77,6 +77,19 @@ fn test_error_chain() -> Result<()> {
     Ok(())
 }
 
+#[test]
+fn test_external_error() {
+    // `Error::external` should preserve `mlua::Error`
+    let runtime_err = Error::runtime("test error");
+    let converted = Error::external(runtime_err);
+    assert!(matches!(converted, Error::RuntimeError(ref msg) if msg == "test error"));
+
+    // Other errors should become `ExternalError`
+    let converted = Error::external(io::Error::other("other error"));
+    assert!(matches!(converted, Error::ExternalError(_)));
+    assert!(converted.downcast_ref::<io::Error>().is_some());
+}
+
 #[cfg(feature = "anyhow")]
 #[test]
 fn test_error_anyhow() -> Result<()> {

tests/function.rs

@@ -1,3 +1,6 @@
+use std::fmt;
+use std::result::Result as StdResult;
+
 use mlua::{Error, Function, Lua, LuaString, Result, Table, Variadic};
 
 #[test]
@@ -343,7 +346,7 @@ fn test_function_deep_clone() -> Result<()> {
 fn test_function_wrap() -> Result<()> {
     let lua = Lua::new();
 
-    let f = Function::wrap(|s: LuaString, n| Ok(s.to_str().unwrap().repeat(n)));
+    let f = Function::wrap(|s: LuaString, n| Ok::<_, Error>(s.to_str().unwrap().repeat(n)));
     lua.globals().set("f", f)?;
     lua.load(r#"assert(f("hello", 2) == "hellohello")"#)
         .exec()
@@ -361,11 +364,40 @@ fn test_function_wrap() -> Result<()> {
     .exec()
     .unwrap();
 
+    // Return external error
+    #[derive(Debug)]
+    struct MyError(String);
+    impl fmt::Display for MyError {
+        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+            write!(f, "MyError: {}", self.0)
+        }
+    }
+    impl std::error::Error for MyError {}
+
+    let fext = Function::wrap(|s: String| -> StdResult<String, MyError> {
+        if s == "bad" {
+            return Err(MyError("bad input".into()));
+        }
+        Ok(format!("ok: {s}"))
+    });
+    lua.globals().set("fext", fext)?;
+    lua.load(r#"assert(fext("hello") == "ok: hello")"#)
+        .exec()
+        .unwrap();
+    lua.load(
+        r#"
+        local ok, err = pcall(fext, "bad")
+        assert(not ok and tostring(err):find("MyError: bad input"))
+    "#,
+    )
+    .exec()
+    .unwrap();
+
     // Mutable callback
     let mut i = 0;
     let fmut = Function::wrap_mut(move || {
         i += 1;
-        Ok(i)
+        Ok::<_, Error>(i)
     });
     lua.globals().set("fmut", fmut)?;
     lua.load(r#"fmut(); fmut(); assert(fmut() == 3)"#).exec().unwrap();
@@ -385,7 +417,7 @@ fn test_function_wrap() -> Result<()> {
     // Check recursive mut callback error
     let fmut = Function::wrap_mut(|f: Function| match f.call::<()>(&f) {
         Err(Error::CallbackError { cause, .. }) => match cause.as_ref() {
-            Error::RecursiveMutCallback { .. } => Ok(()),
+            Error::RecursiveMutCallback { .. } => Ok::<_, Error>(()),
             other => panic!("incorrect result: {other:?}"),
         },
         other => panic!("incorrect result: {other:?}"),

tests/types.rs

@@ -1,6 +1,6 @@
 use std::os::raw::c_void;
 
-use mlua::{Function, LightUserData, Lua, LuaString, Number, Result, Thread};
+use mlua::{Error, Function, LightUserData, Lua, LuaString, Number, Result, Thread};
 
 #[test]
 fn test_lightuserdata() -> Result<()> {
@@ -30,7 +30,7 @@ fn test_boolean_type_metatable() -> Result<()> {
     let lua = Lua::new();
 
     let mt = lua.create_table()?;
-    mt.set("__add", Function::wrap(|a, b| Ok(a || b)))?;
+    mt.set("__add", Function::wrap(|a, b| Ok::<_, mlua::Error>(a || b)))?;
     assert_eq!(lua.type_metatable::<bool>(), None);
     lua.set_type_metatable::<bool>(Some(mt.clone()));
     assert_eq!(lua.type_metatable::<bool>().unwrap(), mt);
@@ -51,7 +51,7 @@ fn test_lightuserdata_type_metatable() -> Result<()> {
     mt.set(
         "__add",
         Function::wrap(|a: LightUserData, b: LightUserData| {
-            Ok(LightUserData((a.0 as usize + b.0 as usize) as *mut c_void))
+            Ok::<_, Error>(LightUserData((a.0 as usize + b.0 as usize) as *mut c_void))
         }),
     )?;
     lua.set_type_metatable::<LightUserData>(Some(mt.clone()));
@@ -79,7 +79,10 @@ fn test_number_type_metatable() -> Result<()> {
     let lua = Lua::new();
 
     let mt = lua.create_table()?;
-    mt.set("__call", Function::wrap(|n1: f64, n2: f64| Ok(n1 * n2)))?;
+    mt.set(
+        "__call",
+        Function::wrap(|n1: f64, n2: f64| Ok::<_, Error>(n1 * n2)),
+    )?;
     lua.set_type_metatable::<Number>(Some(mt.clone()));
     assert_eq!(lua.type_metatable::<Number>().unwrap(), mt);
 
@@ -96,7 +99,7 @@ fn test_string_type_metatable() -> Result<()> {
     let mt = lua.create_table()?;
     mt.set(
         "__add",
-        Function::wrap(|a: String, b: String| Ok(format!("{a}{b}"))),
+        Function::wrap(|a: String, b: String| Ok::<_, Error>(format!("{a}{b}"))),
     )?;
     lua.set_type_metatable::<LuaString>(Some(mt.clone()));
     assert_eq!(lua.type_metatable::<LuaString>().unwrap(), mt);
@@ -113,7 +116,7 @@ fn test_function_type_metatable() -> Result<()> {
     let mt = lua.create_table()?;
     mt.set(
         "__index",
-        Function::wrap(|_: Function, key: String| Ok(format!("function.{key}"))),
+        Function::wrap(|_: Function, key: String| Ok::<_, Error>(format!("function.{key}"))),
     )?;
     lua.set_type_metatable::<Function>(Some(mt.clone()));
     assert_eq!(lua.type_metatable::<Function>(), Some(mt));
@@ -132,7 +135,7 @@ fn test_thread_type_metatable() -> Result<()> {
     let mt = lua.create_table()?;
     mt.set(
         "__index",
-        Function::wrap(|_: Thread, key: String| Ok(format!("thread.{key}"))),
+        Function::wrap(|_: Thread, key: String| Ok::<_, Error>(format!("thread.{key}"))),
     )?;
     lua.set_type_metatable::<Thread>(Some(mt.clone()));
     assert_eq!(lua.type_metatable::<Thread>(), Some(mt));