Merge pull request #567 from google/expose-string-make-unique

Expose string creation method formally

Author: adetaylor

engine/src/conversion/codegen_rs/mod.rs

@@ -70,8 +70,7 @@ enum Use {
     Custom(Box<Item>),
 }
 
-fn get_string_items(config: &IncludeCppConfig) -> Vec<Item> {
-    let makestring_name = make_ident(config.get_makestring_name());
+fn get_string_items() -> Vec<Item> {
     [
         Item::Trait(parse_quote! {
             pub trait ToCppString {
@@ -84,21 +83,21 @@ fn get_string_items(config: &IncludeCppConfig) -> Vec<Item> {
         Item::Impl(parse_quote! {
             impl ToCppString for &str {
                 fn into_cpp(self) -> cxx::UniquePtr<cxx::CxxString> {
-                    cxxbridge::#makestring_name(self)
+                    make_string(self)
                 }
             }
         }),
         Item::Impl(parse_quote! {
             impl ToCppString for String {
                 fn into_cpp(self) -> cxx::UniquePtr<cxx::CxxString> {
-                    cxxbridge::#makestring_name(&self)
+                    make_string(&self)
                 }
             }
         }),
         Item::Impl(parse_quote! {
             impl ToCppString for &String {
                 fn into_cpp(self) -> cxx::UniquePtr<cxx::CxxString> {
-                    cxxbridge::#makestring_name(self)
+                    make_string(self)
                 }
             }
         }),
@@ -402,10 +401,10 @@ impl<'a> RsCodeGenerator<'a> {
                         fn #make_string_name(str_: &str) -> UniquePtr<CxxString>;
                     ))),
                     bridge_items: Vec::new(),
-                    global_items: get_string_items(self.config),
+                    global_items: get_string_items(),
                     bindgen_mod_item: None,
                     impl_entry: None,
-                    materialization: Use::Unused,
+                    materialization: Use::UsedFromCxxBridgeWithAlias(make_ident("make_string")),
                 }
             }
             Api::ConcreteType { .. } => RsCodegenResult {

engine/src/integration_tests.rs

@@ -2913,6 +2913,19 @@ fn test_make_string() {
     run_test("", hdr, rs, &["Bob"], &[]);
 }
 
+#[test]
+fn test_string_make_unique() {
+    let hdr = indoc! {"
+        #include <string>
+        inline void take_string(const std::string*) {};
+    "};
+    let rs = quote! {
+        let s = ffi::make_string("");
+        unsafe { ffi::take_string(s.as_ref().unwrap()) };
+    };
+    run_test("", hdr, rs, &["take_string"], &[]);
+}
+
 #[test]
 fn test_string_constant() {
     let hdr = indoc! {"

src/lib.rs

@@ -225,7 +225,7 @@ use autocxx_engine::IncludeCppEngine;
 ///
 /// There's not a significant ergonomic problem from the use of [`cxx::UniquePtr`].
 /// The main negative of the automatic boxing into [`cxx::UniquePtr`] is performance:
-/// specifiaclly, the need to
+/// specifically, the need to
 /// allocate heap cells on the C++ side and move data into and out of them.
 /// You don't want to be doing this inside a tight loop (but if you're calling
 /// across the C++/Rust boundary in a tight loop, perhaps reconsider that boundary
@@ -330,14 +330,12 @@ use autocxx_engine::IncludeCppEngine;
 ///
 /// If you need to create a blank `UniquePtr<CxxString>` in Rust, such that
 /// (for example) you can pass its mutable reference or pointer into some
-/// pre-existing C++ API, there's currently no built in support for that.
-/// You should add an extra C++ API:
+/// pre-existing C++ API, call `ffi::make_string("")` which will return
+/// a blank `UniquePtr<CxxString>`.
 ///
-/// ```cpp
-/// std::string make_blank_string() { return std::string(); }
-/// ```
-///
-/// and then use [`generate`] to make bindings for that.
+/// Don't attempt to use [cxx::let_cpp_string] which will allocate the
+/// string on the stack, and is generally incompatible with the
+/// [cxx::UniquePtr]-based approaches we use here.
 ///
 /// ## Preprocessor symbols
 ///