From a46a237fe7a9391a21b8251b3ff8ab4ceef2f461 Mon Sep 17 00:00:00 2001
From: David Tolnay <dtolnay@gmail.com>
Date: Fri, 6 Mar 2020 10:03:48 -0800
Subject: [PATCH] Make passing String by value to C++ const

---
 gen/write.rs        | 5 +++++
 macro/src/expand.rs | 6 ++++--
 2 files changed, 9 insertions(+), 2 deletions(-)

diff --git a/gen/write.rs b/gen/write.rs
index e6fb4e9a..33898ce5 100644
--- a/gen/write.rs
+++ b/gen/write.rs
@@ -169,6 +169,9 @@ fn write_cxx_function_shim(out: &mut OutFile, efn: &ExternFn, types: &Types) {
         if i > 0 {
             write!(out, ", ");
         }
+        if arg.ty == RustString {
+            write!(out, "const ");
+        }
         write_extern_arg(out, arg, types);
     }
     if indirect_return {
@@ -213,6 +216,8 @@ fn write_cxx_function_shim(out: &mut OutFile, efn: &ExternFn, types: &Types) {
         } else if let Type::UniquePtr(_) = &arg.ty {
             write_type(out, &arg.ty);
             write!(out, "({})", arg.ident);
+        } else if arg.ty == RustString {
+            write!(out, "::rust::String(::rust::unsafe_bitcopy, *{})", arg.ident);
         } else if types.needs_indirect_abi(&arg.ty) {
             write!(out, "::std::move(*{})", arg.ident);
         } else {
diff --git a/macro/src/expand.rs b/macro/src/expand.rs
index 1237892b..9ebb6235 100644
--- a/macro/src/expand.rs
+++ b/macro/src/expand.rs
@@ -126,7 +126,9 @@ fn expand_cxx_function_decl(namespace: &Namespace, efn: &ExternFn, types: &Types
     let args = efn.args.iter().map(|arg| {
         let ident = &arg.ident;
         let ty = expand_extern_type(&arg.ty);
-        if types.needs_indirect_abi(&arg.ty) {
+        if arg.ty == RustString {
+            quote!(#ident: *const #ty)
+        } else if types.needs_indirect_abi(&arg.ty) {
             quote!(#ident: *mut #ty)
         } else {
             quote!(#ident: #ty)
@@ -157,7 +159,7 @@ fn expand_cxx_function_shim(namespace: &Namespace, efn: &ExternFn, types: &Types
         let var = &arg.ident;
         match &arg.ty {
             Type::Ident(ident) if ident == RustString => {
-                quote!(#var.as_mut_ptr() as *mut ::cxx::private::RustString)
+                quote!(#var.as_mut_ptr() as *const ::cxx::private::RustString)
             }
             Type::RustBox(_) => quote!(::std::boxed::Box::into_raw(#var)),
             Type::UniquePtr(_) => quote!(::cxx::UniquePtr::into_raw(#var)),