From dade6783f379e7307d1672238c7ba0a00f2c6635 Mon Sep 17 00:00:00 2001
From: Adrian Taylor <adetaylor@chromium.org>
Date: Mon, 28 Feb 2022 19:50:10 -0800
Subject: [PATCH] Fix moved-in-memory problem.

Partially fixes #833.
---
 .../codegen_rs/function_wrapper_rs.rs         |  6 ++++--
 src/value_param.rs                            | 21 ++++++++++++-------
 2 files changed, 18 insertions(+), 9 deletions(-)

diff --git a/engine/src/conversion/codegen_rs/function_wrapper_rs.rs b/engine/src/conversion/codegen_rs/function_wrapper_rs.rs
index 074501e8e..a098d00ea 100644
--- a/engine/src/conversion/codegen_rs/function_wrapper_rs.rs
+++ b/engine/src/conversion/codegen_rs/function_wrapper_rs.rs
@@ -109,7 +109,7 @@ impl TypeConversionPolicy {
                     panic!("Unexpected non-ident parameter name");
                 };
                 let space_var_name = make_ident(format!("{}_space", var_name));
-                let call = quote! { autocxx::ValueParamHandler::new(#var_name)  };
+                let call = quote! { #space_var_name.populate()  };
                 let call = if wrap_in_unsafe {
                     quote! {
                         unsafe {
@@ -121,7 +121,9 @@ impl TypeConversionPolicy {
                 };
                 (
                     Some(quote! {
-                        let mut #space_var_name = #call;
+                        let mut #space_var_name = autocxx::ValueParamHandler::new(#var_name);
+                        let #space_var_name = &mut #space_var_name;
+                        #call
                     }),
                     quote! {
                         #space_var_name.get_ptr()
diff --git a/src/value_param.rs b/src/value_param.rs
index a8319ded3..0b85ad5dd 100644
--- a/src/value_param.rs
+++ b/src/value_param.rs
@@ -162,11 +162,7 @@ impl<T, VP: ValueParam<T>> ValueParamHandler<T, VP> {
     /// this may be largely a no-op or it may involve storing a whole
     /// extra copy of the type.
     ///
-    /// # Safety
-    ///
-    /// Callers must guarantee that this type will not move
-    /// in memory.
-    pub unsafe fn new(param: VP) -> Self {
+    pub fn new(param: VP) -> Self {
         let mut this = Self {
             param,
             space: None,
@@ -174,12 +170,23 @@ impl<T, VP: ValueParam<T>> ValueParamHandler<T, VP> {
         };
         if this.param.needs_stack_space() {
             this.space = Some(MaybeUninit::uninit());
-            this.param
-                .populate_stack_space(Pin::new_unchecked(this.space.as_mut().unwrap()));
         }
         this
     }
 
+    /// Populate this stack space if needs be.
+    ///
+    /// # Safety
+    ///
+    /// Callers must guarantee that this type will not move
+    /// in memory between calls to [`populate`] and [`get_ptr`].
+    pub unsafe fn populate(&mut self) {
+        if self.param.needs_stack_space() {
+            self.param
+                .populate_stack_space(Pin::new_unchecked(self.space.as_mut().unwrap()));
+        }
+    }
+
     /// Return a pointer to the underlying value which can be passed to C++.
     /// Per the unsafety contract of `new`, the object must not have moved
     /// since it was created.