Using Rust code in R packages

The rextendr package provides two utility functions for developing R packages with Rust code using extendr:

rextendr::use_extendr(): create the scaffolding to use extendr, similar to usethis::use_cpp11().
rextendr::document(): compile Rust code and generate package documentation, similar to devtools::document().

One thing we want to emphasize here is that these functions are needed solely for package development. An R package using extendr Rust code doesn’t have to depend on or import the rextendr package, just like R packages don’t usually add the devtools package to Depends or Imports no matter how often the package developers use the functions provided by devtools.

Create a template package

Creating an R package with extendr is very easy with the rextendr package.

First, create an empty R package. You can do this with usethis::create_package(). Let’s pick myextendr as the package name.

usethis::create_package("path/to/myextendr")

Then, execute rextendr::use_extendr() inside the package directory to create the scaffolding to use extendr.

rextendr::use_extendr()
#> ✓ Creating src/rust/src.
#> ✓ Setting active project to 'path/to/myextendr'
#> ✓ Writing 'src/entrypoint.c'
#> ✓ Writing 'src/Makevars'
#> ✓ Writing 'src/Makevars.win'
#> ✓ Writing 'src/.gitignore'
#> ✓ Writing src/rust/Cargo.toml.
#> ✓ Writing 'src/rust/src/lib.rs'
#> ✓ Writing 'R/extendr-wrappers.R'
#> ✓ Finished configuring extendr for package myextendr.
#> • Please update the system requirement in DESCRIPTION file.
#> • Please run `rextendr::document()` for changes to take effect.

Now we are just one step away from calling Rust functions from R. As the message says, we need to run rextendr::document(). But, before moving forward, let’s look at the files added.

Package structure

The following files have been added by rextendr::use_extendr():

.
├── R
│   └── extendr-wrappers.R
...
└── src
    ├── Makevars
    ├── Makevars.win
    ├── entrypoint.c
    └── rust
        ├── Cargo.toml
        └── src
            └── lib.rs

R/extendr-wrappers.R: This file contains auto-generated R functions from Rust code. We don’t modify this file by hand.
src/Makevars, src/Makevars.win: These files hook cargo build at the installation of the R package. In most cases, we don’t edit these.
src/entrypoint.c: This file is needed to avoid the linker removing the static library. In 99.9% of cases, we don’t edit this (except for changing the crate name).
src/rust/: Rust code of a crate using extendr-api. This is where we mainly write code.

So, in short, what we should really look at is only these two files:

`src/rust/Cargo.toml`

[package]
name = 'myextendr'
version = '0.1.0'
edition = '2021'

[lib]
crate-type = [ 'staticlib' ]

[dependencies]
extendr-api = '*'

The crate name is the same name as the R package’s name by default. You can change this, but it might be a bit cumbersome to tweak other files accordingly, so we recommend leaving this.

You will probably want to specify a concrete extendr version, for example extendr-api = '0.2'. To try the development version of the extendr, you can modify the last line to read

extendr-api = { git = 'https://github.com/extendr/extendr' }

`src/rust/src/lib.rs`

use extendr_api::prelude::*;

/// Return string `"Hello world!"` to R.
/// @export
#[extendr]
fn hello_world() -> &'static str {
    "Hello world!"
}

// Macro to generate exports.
// This ensures exported functions are registered with R.
// See corresponding C code in `entrypoint.c`.
extendr_module! {
    mod myextendr;
    fn hello_world;
}

Let’s explain this file line by line.

The first line, containing the use statement, declares the commonly used extendr API functions to the Rust compiler.

use extendr_api::prelude::*;

Next, you may notice that / is repeated three times, while the usual Rust comments require only two slashes (i.e., //). This is one of Rust’s “doc comment” notation to generate the crate’s documentation. In extendr, these lines are copied to the auto-generated R code as roxygen comments. This is analogous to Rcpp/cpp11’s //'.

/// Return string `"Hello world!"` to R.
/// @export

The next line is the core of extendr’s mechanism. If the function is marked with this macro, the corresponding R function will be generated automatically. This is analogous to Rcpp’s [[Rcpp::export]] and cpp11’s [[cpp11::register]].

#[extendr]

The last 3 lines are the macro for generating exports, as the comment explains. If we implement another function than just hello_world, it needs to be listed here as well as marking it with #[extendr] macro.

// Macro to generate exports.
// This ensures exported functions are registered with R.
// See corresponding C code in `entrypoint.c`.
extendr_module! {
    mod myextendr;
    fn hello_world;
}

Compile and use the package

Compile

Compiling Rust code into R functions is as easy as executing this one command:

rextendr::document()
#> ✓ Saving changes in the open files.
#> ℹ Generating extendr wrapper functions for package: myextendr.
#> ! No library found at src/myextendr.so, recompilation is required.
#> Re-compiling myextendr
#> ─  installing *source* package ‘myextendr’ ... (347ms)
#>    ** using staged installation
#>    ** libs
#>    rm -Rf myextendr.so ./rust/target/release/libmyextendr.a entrypoint.o
#>    gcc -std=gnu99 -I"/usr/share/R/include" -DNDEBUG      -fpic  -g -O2 -fdebug-prefix-map=/build/r-base-tbZjLv/r-base-4.1.0=. #> -fstack-protector-strong -Wformat -Werror=format-security -Wdate-time -D_FORTIFY_SOURCE=2 -g  -UNDEBUG -Wall -pedantic -g -O0 #> -fdiagnostics-color=always -c entrypoint.c -o entrypoint.o
#>    cargo build --lib --release --manifest-path=./rust/Cargo.toml
#>        Updating crates.io index
#>       Compiling proc-macro2 v1.0.27
#>       Compiling unicode-xid v0.2.2
#>       Compiling libR-sys v0.2.1
#>       Compiling syn v1.0.72
#>       Compiling extendr-engine v0.2.0
#>       Compiling lazy_static v1.4.0
#>       Compiling quote v1.0.9
#>       Compiling extendr-macros v0.2.0
#>       Compiling extendr-api v0.2.0
#>       Compiling myextendr v0.1.0 (path/to/myextendr/src/rust)
#>        Finished release [optimized] target(s) in 19.05s
#>    gcc -std=gnu99 -shared -L/usr/lib/R/lib -Wl,-Bsymbolic-functions -Wl,-z,relro -o myextendr.so entrypoint.o -L./rust/target/release #> -lmyextendr -L/usr/lib/R/lib -lR
#>    installing to /tmp/RtmpfMcL08/devtools_install_e2d6351b843c/00LOCK-myextendr/00new/myextendr/libs
#>    ** checking absolute paths in shared objects and dynamic libraries
#> ─  DONE (myextendr)
#> ✓ Writing 'R/extendr-wrappers.R'.
#> ℹ Updating myextendr documentation
#> ℹ Loading myextendr
#> Writing NAMESPACE
#> Writing NAMESPACE
#> Writing hello_world.Rd

You might wonder why compilation is triggered while the function name is just document(). Well, this is because the compilation is actually needed to generate documentation and R wrapper code from the Rust code. This is consistent with the behavior of devtools::document() for packages using C or C++.

By doing the above, the following files are updated or generated:

.
...
├── NAMESPACE                       ----------(4)
├── R
│   └── extendr-wrappers.R          ----------(3)
├── man
│   └── hello_world.Rd              ----------(4)
└── src
    ├── myextendr.so          ----------(2)
    └── rust
        └── target
            └── release
                ├── libmyextendr.a   ---(1)
                ...

src/rust/target/release/libmyextendr.a (the extension depends on the OS): This is the static library built from Rust code. This will be then used for compiling the shared object myextendr.so.
src/myextendr.so (the extension depends on the OS): This is the shared object that is actually called from R.
R/extendr-wrappers.R: The auto-generated R functions, including roxygen comments, go into this file. The roxygen comments are accordingly converted into Rd files and NAMESPACE.
man/, NAMESPACE: These are generated from roxygen comments.

Load and use

After running rextendr::document(), we can just load the package with devtools::load_all() (or alternatively install it and call with library()) and then call the function we have implemented in Rust.

devtools::load_all(".")

hello_world()
#> [1] "Hello world!"

Rust code vs generated R code

While we never edit the R wrapper code by hand, it might be good to know what R code is generated from the Rust code. Let’s look at R/extendr-wrappers.R:

# Generated by extendr: Do not edit by hand
#
# This file was created with the following call:
#   .Call("wrap__make_myextendr_wrappers", use_symbols = TRUE, package_name = "myextendr")

#' @docType package
#' @usage NULL
#' @useDynLib myextendr, .registration = TRUE
NULL

#' Return string `"Hello world!"` to R.
#' @export
hello_world <- function() .Call(wrap__hello_world)

Here, .Call("wrap__make_myextendr_wrappers", use_symbols = ... is a function call that was executed by rextendr::document().

A section of @docType package is needed to generate the useDynLib(myextendr, .registration = TRUE) entry in NAMESPACE.

The last section is for hello_world(). We can see the roxygen comments are copied to here. As the Rust function hello_world() has no arguments this R function also has no arguments. If the function had arguments, such as

fn add(x: i32, y: i32) -> i32 {
    x + y
}

then the generated function wrapper also would have arguments:

add <- function(x, y) .Call(wrap__add, x, y)

Implement a new Rust function

Now that we have roughly figured out how extendr works, let’s implement a new Rust function. The development flow would be:

Modify src/rust/src/lib.rs
Run rextendr::document()
Run devtools::load_all(".") and test the function

As an exercise, let’s add the add(i32, i32) function from the previous subsection.

1. Modify `src/rust/src/lib.rs`

Add the function with @export, so it will get exported from the generated R package. (Without this tag, the function would be available internally for package programming but not externally to users of the package.)

/// @export
#[extendr]
fn add(x: i32, y: i32) -> i32 {
    x + y
}

Don’t forget to add the function to extendr_module!:

extendr_module! {
    mod myextendr;
    fn hello_world;
    fn add;
}

2. Run `rextendr::document()`

Just run this command:

rextendr::document()

3. Run `devtools::load_all(".")` and test the function

Now you can load the package and call add():

devtools::load_all(".")

add(1L, 2L)
#> [1] 3