[alexhutcheson]

> Rust is technically better than C++ in most possible ways — in some, significantly better — and worse in almost no way.

It's much worse in one way: Interoperation with existing C++ code. Sure, that's not a fair criteria - C++ is designed in a way that makes it almost impossible for other languages to use C++ libraries without a heavyweight wrapper like SWIG. However, even though it's not fair, it's still really important. If you have a project like LLVM with millions of lines of existing C++ code, adding expensive or complicated interop boundaries between different components within your system is not an acceptable price to pay.

[pcwalton]

Have you tried cxx? C++ interoperability in Rust has come a long way.

[alexhutcheson]

cxx is a huge improvement over manually writing bindings in C (with the inherent limitations imposed by C’s lack of expressiveness). At the same time, you still have to write a cxx::bridge to specify every boundary between Rust and C++ code, and the clients of on either side need to code against some generated code, not a simple .rs or .h file. It’s a huge improvement, but it’s still a lot more developer effort and boilerplate to use a C++ class from a Rust file than it would be to use that same C++ class from another C++ file.

cxx also can only handle a subset of the interfaces expressive in C++. If you haven’t written the interface with the specific goal of making it usable via cxx, it’s pretty likely you’ll have issues wrapping it with cxx.