src/lib.rs

//! This library provides a **safe** mechanism for calling C++ code from Rust
//! and Rust code from C++, not subject to the many ways that things can go
//! wrong when using bindgen or cbindgen to generate unsafe C-style bindings.
//!
//! <br>
//!
//! *Compiler support: requires rustc 1.42+ (beta on January 30, stable on March
//! 12)*
//!
//! <br>
//!
//! # Overview
//!
//! The idea is that we define the signatures of both sides of our FFI boundary
//! embedded together in one Rust module (the next section shows an example).
//! From this, CXX receives a complete picture of the boundary to perform static
//! analyses against the types and function signatures to uphold both Rust's and
//! C++'s invariants and requirements.
//!
//! If everything checks out statically, then CXX uses a pair of code generators
//! to emit the relevant `extern "C"` signatures on both sides together with any
//! necessary static assertions for later in the build process to verify
//! correctness. On the Rust side this code generator is simply an attribute
//! procedural macro. On the C++ side it can be a small Cargo build script if
//! your build is managed by Cargo, or for other build systems like Bazel or
//! Buck we provide a command line tool which generates the header and source
//! file and should be easy to integrate.
//!
//! The resulting FFI bridge operates at zero or negligible overhead, i.e. no
//! copying, no serialization, no memory allocation, no runtime checks needed.
//!
//! The FFI signatures are able to use native types from whichever side they
//! please, such as Rust's `String` or C++'s `std::string`, Rust's `Box` or
//! C++'s `std::unique_ptr`, Rust's `Vec` or C++'s `std::vector`, etc in any
//! combination. CXX guarantees an ABI-compatible signature that both sides
//! understand, based on builtin bindings for key standard library types to
//! expose an idiomatic API on those types to the other language. For example
//! when manipulating a C++ string from Rust, its `len()` method becomes a call
//! of the `size()` member function defined by C++; when manipulation a Rust
//! string from C++, its `size()` member function calls Rust's `len()`.
//!
//! <br>
//!
//! # Example
//!
//! A runnable version of this example is provided under the *demo-rs* directory
//! of https://github.com/dtolnay/cxx (with the C++ side of the implementation
//! in the *demo-cxx* directory). To try it out, jump into demo-rs and run
//! `cargo run`.
//!
//! ```no_run
//! #[cxx::bridge]
//! mod ffi {
//!     // Any shared structs, whose fields will be visible to both languages.
//!     struct SharedThing {
//!         z: i32,
//!         y: Box<ThingR>,
//!         x: UniquePtr<ThingC>,
//!     }
//!
//!     extern "C" {
//!         // One or more headers with the matching C++ declarations. Our code
//!         // generators don't read it but it gets #include'd and used in static
//!         // assertions to ensure our picture of the FFI boundary is accurate.
//!         include!("demo-cxx/demo.h");
//!
//!         // Zero or more opaque types which both languages can pass around but
//!         // only C++ can see the fields.
//!         type ThingC;
//!
//!         // Functions implemented in C++.
//!         fn make_demo(appname: &str) -> UniquePtr<ThingC>;
//!         fn get_name(thing: &ThingC) -> &CxxString;
//!         fn do_thing(state: SharedThing);
//!     }
//!
//!     extern "Rust" {
//!         // Zero or more opaque types which both languages can pass around but
//!         // only Rust can see the fields.
//!         type ThingR;
//!
//!         // Functions implemented in Rust.
//!         fn print_r(r: &ThingR);
//!     }
//! }
//! #
//! # pub struct ThingR(usize);
//! #
//! # fn print_r(r: &ThingR) {
//! #     println!("called back with r={}", r.0);
//! # }
//! #
//! # fn main() {}
//! ```
//!
//! Now we simply provide C++ definitions of all the things in the `extern "C"`
//! block and Rust definitions of all the things in the `extern "Rust"` block,
//! and get to call back and forth safely.
//!
//! Here are links to the complete set of source files involved in the demo:
//!
//! - [demo-rs/src/main.rs](https://github.com/dtolnay/cxx/blob/master/demo-rs/src/main.rs)
//! - [demo-rs/build.rs](https://github.com/dtolnay/cxx/blob/master/demo-rs/build.rs)
//! - [demo-cxx/demo.h](https://github.com/dtolnay/cxx/blob/master/demo-cxx/demo.h)
//! - [demo-cxx/demo.cc](https://github.com/dtolnay/cxx/blob/master/demo-cxx/demo.cc)
//!
//! To look at the code generated in both languages for the example by the CXX
//! code generators:
//!
//! ```console
//!    # run Rust code generator and print to stdout
//!    # (requires https://github.com/dtolnay/cargo-expand)
//! $ cargo expand --manifest-path demo-rs/Cargo.toml
//!
//!    # run C++ code generator and print to stdout
//! $ cargo run --manifest-path cmd/Cargo.toml -- demo-rs/src/main.rs
//! ```
//!
//! <br>
//!
//! # Details
//!
//! As seen in the example, the language of the FFI boundary involves 3 kinds of
//! items:
//!
//! - **Shared structs** &mdash; their fields are made visible to both
//!   languages. The definition written within cxx::bridge is the single source
//!   of truth.
//!
//! - **Opaque types** &mdash; their fields are secret from the other language.
//!   These cannot be passed across the FFI by value but only behind an
//!   indirection, such as a reference `&`, a Rust `Box`, or a `UniquePtr`. Can
//!   be a type alias for an arbitrarily complicated generic language-specific
//!   type depending on your use case.
//!
//! - **Functions** &mdash; implemented in either language, callable from the
//!   other language.
//!
//! Within the `extern "C"` part of the CXX bridge we list the types and
//! functions for which C++ is the source of truth, as well as the header(s)
//! that declare those APIs. In the future it's possible that this section could
//! be generated bindgen-style from the headers but for now we need the
//! signatures written out; static assertions will verify that they are
//! accurate.
//!
//! Within the `extern "Rust"` part, we list types and functions for which Rust
//! is the source of truth. These all implicitly refer to the `super` module,
//! the parent module of the CXX bridge. You can think of the two items listed
//! in the example above as being like `use super::ThingR` and `use
//! super::print_r` except re-exported to C++. The parent module will either
//! contain the definitions directly for simple things, or contain the relevant
//! `use` statements to bring them into scope from elsewhere.
//!
//! Your function implementations themselves, whether in C++ or Rust, *do not*
//! need to be defined as `extern "C"` ABI or no\_mangle. CXX will put in the
//! right shims where necessary to make it all work.
//!
//! <br>
//!
//! # Comparison vs bindgen and cbindgen
//!
//! Notice that with CXX there is repetition of all the function signatures:
//! they are typed out once where the implementation is defined (in C++ or Rust)
//! and again inside the cxx::bridge module, though compile-time assertions
//! guarantee these are kept in sync. This is different from [bindgen] and
//! [cbindgen] where function signatures are typed by a human once and the tool
//! consumes them in one language and emits them in the other language.
//!
//! [bindgen]: https://github.com/rust-lang/rust-bindgen
//! [cbindgen]: https://github.com/eqrion/cbindgen/
//!
//! This is because CXX fills a somewhat different role. It is a lower level
//! tool than bindgen or cbindgen in a sense; you can think of it as being a
//! replacement for the concept of `extern "C"` signatures as we know them,
//! rather than a replacement for a bindgen. It would be reasonable to build a
//! higher level bindgen-like tool on top of CXX which consumes a C++ header
//! and/or Rust module (and/or IDL like Thrift) as source of truth and generates
//! the cxx::bridge, eliminating the repetition while leveraging the static
//! analysis safety guarantees of CXX.
//!
//! But note in other ways CXX is higher level than the bindgens, with rich
//! support for common standard library types. Frequently with bindgen when we
//! are dealing with an idiomatic C++ API we would end up manually wrapping that
//! API in C-style raw pointer functions, applying bindgen to get unsafe raw
//! pointer Rust functions, and replicating the API again to expose those
//! idiomatically in Rust. That's a much worse form of repetition because it is
//! unsafe all the way through.
//!
//! By using a CXX bridge as the shared understanding between the languages,
//! rather than `extern "C"` C-style signatures as the shared understanding,
//! common FFI use cases become expressible using 100% safe code.
//!
//! It would also be reasonable to mix and match, using CXX bridge for the 95%
//! of your FFI that is straightforward and doing the remaining few oddball
//! signatures the old fashioned way with bindgen and cbindgen, if for some
//! reason CXX's static restrictions get in the way. Please file an issue if you
//! end up taking this approach so that we know what ways it would be worthwhile
//! to make the tool more expressive.
//!
//! <br>
//!
//! # Cargo-based setup
//!
//! For builds that are orchestrated by Cargo, you will use a build script that
//! runs CXX's C++ code generator and compiles the resulting C++ code along with
//! any other C++ code for your crate.
//!
//! The canonical build script is as follows. The indicated line returns a
//! [`cc::Build`] instance (from the usual widely used `cc` crate) on which you
//! can set up any additional source files and compiler flags as normal.
//!
//! [`cc::Build`]: https://docs.rs/cc/1.0/cc/struct.Build.html
//!
//! ```no_run
//! // build.rs
//!
//! fn main() {
//!     cxx::Build::new()
//!         .bridge("src/main.rs")  // returns a cc::Build
//!         .file("../demo-cxx/demo.cc")
//!         .flag("-std=c++11")
//!         .compile("cxxbridge-demo");
//!
//!     println!("cargo:rerun-if-changed=src/main.rs");
//!     println!("cargo:rerun-if-changed=../demo-cxx/demo.h");
//!     println!("cargo:rerun-if-changed=../demo-cxx/demo.cc");
//! }
//! ```
//!
//! <br><br>
//!
//! # Non-Cargo setup
//!
//! For use in non-Cargo builds like Bazel or Buck, CXX provides an alternate
//! way of invoking the C++ code generator as a standalone command line tool.
//! The tool is packaged as the `cxxbridge-cmd` crate on crates.io or can be
//! built from the *cmd* directory of https://github.com/dtolnay/cxx.
//!
//! ```bash
//! $ cargo install cxxbridge-cmd
//!
//! $ cxxbridge src/main.rs --header > path/to/mybridge.h
//! $ cxxbridge src/main.rs > path/to/mybridge.cc
//! ```
//!
//! <br>
//!
//! # Safety
//!
//! Be aware that the design of this library is intentionally restrictive and
//! opinionated! It isn't a goal to be powerful enough to handle arbitrary
//! signatures in either language. Instead this project is about carving out a
//! reasonably expressive set of functionality about which we can make useful
//! safety guarantees today and maybe extend over time. You may find that it
//! takes some practice to use CXX bridge effectively as it won't work in all
//! the ways that you are used to.
//!
//! Some of the considerations that go into ensuring safety are:
//!
//! - By design, our paired code generators work together to control both sides
//!   of the FFI boundary. Ordinarily in Rust writing your own `extern "C"`
//!   blocks is unsafe because the Rust compiler has no way to know whether the
//!   signatures you've written actually match the signatures implemented in the
//!   other language. With CXX we achieve that visibility and know what's on the
//!   other side.
//!
//! - Our static analysis detects and prevents passing types by value that
//!   shouldn't be passed by value from C++ to Rust, for example because they
//!   may contain internal pointers that would be screwed up by Rust's move
//!   behavior.
//!
//! - To many people's surprise, it is possible to have a struct in Rust and a
//!   struct in C++ with exactly the same layout / fields / alignment /
//!   everything, and still not the same ABI when passed by value. This is a
//!   longstanding bindgen bug that leads to segfaults in absolutely
//!   correct-looking code ([rust-lang/rust-bindgen#778]). CXX knows about this
//!   and can insert the necessary zero-cost workaround transparently where
//!   needed, so go ahead and pass your structs by value without worries. This
//!   is made possible by owning both sides of the boundary rather than just
//!   one.
//!
//! - Template instantiations: for example in order to expose a UniquePtr\<T\>
//!   type in Rust backed by a real C++ unique\_ptr, we have a way of using a
//!   Rust trait to connect the behavior back to the template instantiations
//!   performed by the other language.
//!
//! [rust-lang/rust-bindgen#778]: https://github.com/rust-lang/rust-bindgen/issues/778
//!
//! <br>
//!
//! # Builtin types
//!
//! In addition to all the primitive types (i32 ⟷ int32_t), the following common
//! types may be used in the fields of shared structs and the arguments and
//! returns of functions.
//!
//! <table>
//! <tr><th>name in Rust</th><th>name in C++</th><th>restrictions</th></tr>
//! <tr><td>String</td><td>cxxbridge::RustString</td><td></td></tr>
//! <tr><td>&amp;str</td><td>cxxbridge::RustStr</td><td></td></tr>
//! <tr><td><a href="https://docs.rs/cxx/0.0/cxx/struct.CxxString.html">CxxString</a></td><td>std::string</td><td><sup><i>cannot be passed by value</i></sup></td></tr>
//! <tr><td>Box&lt;T&gt;</td><td>cxxbridge::RustBox&lt;T&gt;</td><td><sup><i>cannot hold opaque C++ type</i></sup></td></tr>
//! <tr><td><a href="https://docs.rs/cxx/0.0/cxx/struct.UniquePtr.html">UniquePtr&lt;T&gt;</a></td><td>std::unique_ptr&lt;T&gt;</td><td><sup><i>cannot hold opaque Rust type</i></sup></td></tr>
//! <tr><td></td><td></td><td></td></tr>
//! </table>
//!
//! The C++ API of the `cxxbridge` namespace is defined by the
//! *include/cxxbridge.h* file in https://github.com/dtolnay/cxx. You will need
//! to include this header in your C++ code when working with those types.
//!
//! The following types are intended to be supported "soon" but are just not
//! implemented yet. I don't expect any of these to be hard to make work but
//! it's a matter of designing a nice API for each in its non-native language.
//!
//! <table>
//! <tr><th>name in Rust</th><th>name in C++</th></tr>
//! <tr><td>&amp;[T]</td><td></td></tr>
//! <tr><td>Vec&lt;T&gt;</td><td></td></tr>
//! <tr><td>BTreeMap&lt;K, V&gt;</td><td></td></tr>
//! <tr><td>HashMap&lt;K, V&gt;</td><td></td></tr>
//! <tr><td></td><td>std::vector&lt;T&gt;</td></tr>
//! <tr><td></td><td>std::map&lt;K, V&gt;</td></tr>
//! <tr><td></td><td>std::unordered_map&lt;K, V&gt;</td></tr>
//! </table>

#![deny(improper_ctypes)]
#![allow(
    clippy::large_enum_variant,
    clippy::missing_safety_doc,
    clippy::module_inception,
    clippy::new_without_default,
    clippy::or_fun_call,
    clippy::ptr_arg,
    clippy::toplevel_ref_arg,
    clippy::transmute_ptr_to_ptr,
    clippy::useless_let_if_seq
)]

mod cxx_string;
mod error;
mod gen;
mod opaque;
mod paths;
mod rust_str;
mod rust_string;
mod syntax;
mod unique_ptr;
mod unwind;

pub use crate::cxx_string::CxxString;
pub use crate::unique_ptr::UniquePtr;
pub use cxxbridge_macro::bridge;

// Not public API.
#[doc(hidden)]
pub mod private {
    pub use crate::opaque::Opaque;
    pub use crate::rust_str::RustStr;
    pub use crate::rust_string::RustString;
    pub use crate::unique_ptr::UniquePtrTarget;
    pub use crate::unwind::catch_unwind;
}

use crate::error::Result;
use std::fs;
use std::io::{self, Write};
use std::path::Path;
use std::process;

/// The CXX code generator for constructing and compiling C++ code.
///
/// This is intended to be used from Cargo build scripts to execute CXX's
/// C++ code generator, set up any additional compiler flags depending on
/// the use case, and make the C++ compiler invocation.
///
/// <br>
///
/// # Example
///
/// Example of a canonical Cargo build script that builds a CXX bridge:
///
/// ```no_run
/// // build.rs
///
/// fn main() {
///     cxx::Build::new()
///         .bridge("src/main.rs")
///         .file("../demo-cxx/demo.cc")
///         .flag("-std=c++11")
///         .compile("cxxbridge-demo");
///
///     println!("cargo:rerun-if-changed=src/main.rs");
///     println!("cargo:rerun-if-changed=../demo-cxx/demo.h");
///     println!("cargo:rerun-if-changed=../demo-cxx/demo.cc");
/// }
/// ```
///
/// A runnable working setup with this build script is shown in the
/// *demo-rs* and *demo-cxx* directories of
/// [https://github.com/dtolnay/cxx](https://github.com/dtolnay/cxx).
///
/// <br>
///
/// # Alternatives
///
/// For use in non-Cargo builds like Bazel or Buck, CXX provides an
/// alternate way of invoking the C++ code generator as a standalone command
/// line tool. The tool is packaged as the `cxxbridge-cmd` crate.
///
/// ```bash
/// $ cargo install cxxbridge-cmd  # or build it from the repo
///
/// $ cxxbridge src/main.rs --header > path/to/mybridge.h
/// $ cxxbridge src/main.rs > path/to/mybridge.cc
/// ```
#[must_use]
pub struct Build {
    _private: (),
}

impl Build {
    /// Begin with a [`cc::Build`] in its default configuration.
    pub fn new() -> Self {
        Build { _private: () }
    }

    /// This returns a [`cc::Build`] on which you should continue to set up
    /// any additional source files or compiler flags, and lastly call its
    /// [`compile`] method to execute the C++ build.
    ///
    /// [`compile`]: https://docs.rs/cc/1.0.49/cc/struct.Build.html#method.compile
    #[must_use]
    pub fn bridge(&self, rust_source_file: impl AsRef<Path>) -> cc::Build {
        match try_generate_bridge(rust_source_file.as_ref()) {
            Ok(build) => build,
            Err(err) => {
                let _ = writeln!(io::stderr(), "\n\ncxxbridge error: {}\n\n", err);
                process::exit(1);
            }
        }
    }
}

fn try_generate_bridge(rust_source_file: &Path) -> Result<cc::Build> {
    let header = gen::do_generate_header(rust_source_file);
    let header_path = paths::out_with_extension(rust_source_file, ".h")?;
    fs::create_dir_all(header_path.parent().unwrap())?;
    fs::write(&header_path, header)?;
    paths::symlink_header(&header_path, rust_source_file);

    let bridge = gen::do_generate_bridge(rust_source_file);
    let bridge_path = paths::out_with_extension(rust_source_file, ".cc")?;
    fs::write(&bridge_path, bridge)?;
    let mut build = paths::cc_build();
    build.file(&bridge_path);

    Ok(build)
}