Optimize MultiValue allocations (recycle old container)

src/function.rs

@@ -6,7 +6,7 @@ use crate::error::{Error, Result};
 use crate::ffi;
 use crate::types::LuaRef;
 use crate::util::{assert_stack, check_stack, error_traceback, pop_error, StackGuard};
-use crate::value::{FromLuaMulti, MultiValue, ToLuaMulti};
+use crate::value::{FromLuaMulti, ToLuaMulti};
 
 #[cfg(feature = "async")]
 use {futures_core::future::LocalBoxFuture, futures_util::future};
@@ -59,7 +59,7 @@ impl<'lua> Function<'lua> {
     pub fn call<A: ToLuaMulti<'lua>, R: FromLuaMulti<'lua>>(&self, args: A) -> Result<R> {
         let lua = self.0.lua;
 
-        let args = args.to_lua_multi(lua)?;
+        let mut args = args.to_lua_multi(lua)?;
         let nargs = args.len() as c_int;
 
         let results = unsafe {
@@ -69,7 +69,7 @@ impl<'lua> Function<'lua> {
             ffi::lua_pushcfunction(lua.state, error_traceback);
             let stack_start = ffi::lua_gettop(lua.state);
             lua.push_ref(&self.0);
-            for arg in args {
+            for arg in args.drain_all() {
                 lua.push_value(arg)?;
             }
             let ret = ffi::lua_pcall(lua.state, nargs, ffi::LUA_MULTRET, stack_start);
@@ -77,7 +77,7 @@ impl<'lua> Function<'lua> {
                 return Err(pop_error(lua.state, ret));
             }
             let nresults = ffi::lua_gettop(lua.state) - stack_start;
-            let mut results = MultiValue::new();
+            let mut results = args; // Recycle MultiValue container
             assert_stack(lua.state, 2);
             for _ in 0..nresults {
                 results.push_front(lua.pop_value());

src/thread.rs

@@ -108,7 +108,7 @@ impl<'lua> Thread<'lua> {
         R: FromLuaMulti<'lua>,
     {
         let lua = self.0.lua;
-        let args = args.to_lua_multi(lua)?;
+        let mut args = args.to_lua_multi(lua)?;
         let nargs = args.len() as c_int;
         let results = unsafe {
             let _sg = StackGuard::new(lua.state);
@@ -123,7 +123,7 @@ impl<'lua> Thread<'lua> {
             }
 
             check_stack(thread_state, nargs)?;
-            for arg in args {
+            for arg in args.drain_all() {
                 lua.push_value(arg)?;
             }
             ffi::lua_xmove(lua.state, thread_state, nargs);
@@ -136,7 +136,7 @@ impl<'lua> Thread<'lua> {
                 return Err(pop_error(thread_state, ret));
             }
 
-            let mut results = MultiValue::new();
+            let mut results = args; // Recycle MultiValue container
             check_stack(lua.state, nresults + 2)?; // 2 is extra for `lua.pop_value()` below
             ffi::lua_xmove(thread_state, lua.state, nresults);
 

src/value.rs

@@ -241,6 +241,11 @@ impl<'lua> MultiValue<'lua> {
     pub fn iter(&self) -> iter::Rev<slice::Iter<Value<'lua>>> {
         self.0.iter().rev()
     }
+
+    #[inline]
+    pub(crate) fn drain_all(&mut self) -> iter::Rev<vec::Drain<Value<'lua>>> {
+        self.0.drain(..).rev()
+    }
 }
 
 /// Trait for types convertible to any number of Lua values.