commit 0356d33acbcee50f41aa8e44157d64b08522bcce
author David Tolnay <dtolnay@gmail.com> Sat Oct 31 19:46:41 2020 -0700
committer David Tolnay <dtolnay@gmail.com> Sat Oct 31 20:33:39 2020 -0700
tree f201a38d06c211adc0aa14170eb844eb87e85f11
parent 54742b7ced559bac4eba72b45ae0c57905ae4215

Pass Str in PtrLen representation

MSVC is hesitant about passing private fields in an extern "C" signature.
Repro:

    struct Str1 {
      const char *ptr;
      size_t len;
    };

    struct Str2 {
    private:
      const char *ptr;
      size_t len;
    };

    extern "C" {
      Str1 str1();
      Str2 str2();
    }

Warning from MSVC v19.27:

    warning C4190: 'str2' has C-linkage specified, but returns UDT 'Str2' which is incompatible with C

include/cxx.h

diff --git a/include/cxx.h b/include/cxx.h
index de614d1..220f45f 100644
--- a/include/cxx.h
+++ b/include/cxx.h
@@ -79,6 +79,7 @@
   ~Str() noexcept = default;
 
 private:
+  friend impl<Str>;
   // Not necessarily ABI compatible with &str. Codegen will translate to
   // cxx::rust_str::RustStr which matches this layout.
   const char *ptr;