Cross Compiling with Cargo Lambda ​

AWS Lambda functions run on Linux sandboxes. These sandboxes only include the bare minimum functionality for Rust binaries to work. It's important to understand that if your function depends on native libraries, like libpq for example, it's unlikely that it'll work on AWS Lambda out of the box.

Cargo Lambda compiles your code for Linux targets using several techniques, regardless of whether you work on a Linux, Windows, or macOS machine. Cargo Lambda also compiles for ARM64 and X86-64 architectures, regardless of your host's architecture.

Cross Compiling with the Zig toolchain ​

By default, Cargo Lambda uses the Zig toolchain to cross compile your code. This is the most convenient cross compilation mechanism because it comes built in, and it works for the majority of use cases. Any pure Rust Lambda function should compile correctly with this toolchain.

Why is Zig desirable here? ​

While Rust has really good cross compiling support and many targets are currently supported, any crate having C components that need to be compiled by the host can have many practical compiling issues (see "Known cross compilation issues" below for a small selection).

Zig CC, a subcommand of the Zig's CLI, provides an extremely convenient multi-target, multi-platform and multi-library way to cross compile almost any C project. To appreciate the amount of work this user friendliness takes to attain, please read the following blogpost by Zig's author: zig cc: a Powerful Drop-In Replacement for GCC/Lang.

At the time of writing, this 'native' effort is unmatched by the Rust community... that is, without the use of cross, as detailed in the next section.

Cross Compiling with Cross ​

Cargo Lambda also supports cross as the compiler. Cross compiles your code inside Linux containers. If you want to use it with Cargo Lambda, you'll have to install it manually in your system, as well as installing Docker. All builds with Cross happen inside Docker, so they are slower, but it preserves Cargo Lambda's optimizations and conventions.

Once you've installed the dependencies, you can use cross by setting the --compiler option when you build your function:

cargo lambda build --compiler cross --release

cargo lambda build --compiler cross --release

Cargo Lambda without cross compilation ​

If you work on Linux, you might not need any cross compiling toolchain. You can still take advantage of Cargo Lambda's optimizations and conventions, and build directly with Cargo. You can tell Cargo Lambda to not cross compile your code by settings the --compiler option to cargo when you build your function:

cargo lambda build --compiler cargo --release

cargo lambda build --compiler cargo --release

Known cross compilation issues ​

As mentioned earlier, AWS Lambda uses Linux sandboxes to run your functions. Those sandboxes use Amazon Linux 2023, or Amazon Linux 2, as the operating system. By default, sandboxes only include the necessary libraries for the OS to work. *-sys libraries are not guaranteed to work unless they are completely linked to your binary, or you provide the native dependencies in some other way.

This is a list of non-exhaustive problems that you might bump into if you try to build your Rust application to work on AWS Lambda:

• reqwest uses OpenSSL as TLS backend by default. If you want to use reqwest in your application, you can enable the native-tls-vendored or the rustls features to include the TLS backend in your application.

• ring and any other crates that depend on cc-rs have compile-time requirements. Look at their documentation to see those requirements depending on your platform.

• diesel uses native dependencies to connect to Postgres and MySQL. Use diesel-async instead to have a better integration with the Rust Runtime for Lambda. See the example in the runtime's repository.