book/src/build/other.md

{{#title Other build systems — Rust ♡ C++}}
# Some other build system

You will need to achieve at least these three things:

- Produce the CXX-generated C++ bindings code.
- Compile the generated C++ code.
- Link the resulting objects together with your other C++ and Rust objects.

*Not all build systems are created equal. If you're hoping to use a build system
from the '90s, especially if you're hoping to overlaying the limitations of 2 or
more build systems (like automake+cargo) and expect to solve them
simultaneously, then be mindful that your expectations are set accordingly and
seek sympathy from those who have imposed the same approach on themselves.*

### Producing the generated code

CXX's Rust code generation automatically happens when the `#[cxx::bridge]`
procedural macro is expanded during the normal Rust compilation process, so no
special build steps are required there.

But the C++ side of the bindings needs to be generated. Your options are:

- Use the `cxxbridge` command, which is a standalone command line interface to
  the CXX C++ code generator. Wire up your build system to compile and invoke
  this tool.

  ```console
  $  cxxbridge src/bridge.rs --header > path/to/bridge.rs.h
  $  cxxbridge src/bridge.rs > path/to/bridge.rs.cc
  ```

  It's packaged as the `cxxbridge-cmd` crate on crates.io or can be built from
  the *gen/cmd/* directory of the CXX GitHub repo.

- Or, build your own code generator frontend on top of the [cxx-gen] crate. This
  is currently unofficial and unsupported.

[cxx-gen]: https://docs.rs/cxx-gen

### Compiling C++

However you like. We can provide no guidance.

### Linking the C++ and Rust together

When linking a binary which contains mixed Rust and C++ code, you will have to
choose between using the Rust toolchain (`rustc`) or the C++ toolchain which you
may already have extensively tuned.

The generated C++ code and the Rust code generated by the procedural macro both
depend on each other. Simple examples may only require one or the other, but in
general your linking will need to handle both directions. For some linkers, such
as llvm-ld, this is not a problem at all. For others, such as GNU ld, flags like
`--start-lib`/`--end-lib` may help.

Rust does not generate simple standalone `.o` files, so you can't just throw the
Rust-generated code into your existing C++ toolchain linker. Instead you need to
choose one of these options:

* Use `rustc` as the final linker. Pass any non-Rust libraries using `-L
  <directory>` and `-l<library>` rustc arguments, and/or `#[link]` directives in
  your Rust code. If you need to link against C/C++ `.o` files you can use
  `-Clink-arg=file.o`.

* Use your C++ linker. In this case, you first need to use `rustc` and/or
  `cargo` to generate a _single_ Rust `staticlib` target and pass that into your
  foreign linker invocation.

  * If you need to link multiple Rust subsystems, you will need to generate a
    _single_ `staticlib` perhaps using lots of `extern crate` statements to
    include multiple Rust `rlib`s.  Multiple Rust `staticlib` files are likely
    to conflict.

Passing Rust `rlib`s directly into your non-Rust linker is not supported (but
apparently sometimes works).

See the [Rust reference's *Linkage*][linkage] page for some general information
here.

[linkage]: https://doc.rust-lang.org/reference/linkage.html

The following open rust-lang issues might hold more recent guidance or
inspiration: [rust-lang/rust#73632], [rust-lang/rust#73295].

[rust-lang/rust#73632]: https://github.com/rust-lang/rust/issues/73632
[rust-lang/rust#73295]: https://github.com/rust-lang/rust/issues/73295