Swift

The Swift compiler is provided by the swift package:

# Compile and link a simple executable.
nix-shell -p swift --run 'swiftc -' <<< 'print("Hello world!")'
# Run it!
./main

The swift package also provides the swift command, with some caveats:

Swift Package Manager (SwiftPM) is packaged separately as swiftpm. If you need functionality like swift build, swift run, swift test, you must also add the swiftpm package to your closure.
On Darwin, the swift repl command requires an Xcode installation. This is because it uses the system LLDB debugserver, which has special entitlements.

Module search paths

Like other toolchains in Nixpkgs, the Swift compiler executables are wrapped to help Swift find your application's dependencies in the Nix store. These wrappers scan the buildInputs of your package derivation for specific directories where Swift modules are placed by convention, and automatically add those directories to the Swift compiler search paths.

Swift follows different conventions depending on the platform. The wrappers look for the following directories:

On Darwin platforms: lib/swift/macosx (If not targeting macOS, replace macosx with the Xcode platform name.)
On other platforms: lib/swift/linux/x86_64 (Where linux and x86_64 are from lowercase uname -sm.)
For convenience, Nixpkgs also adds simply lib/swift to the search path. This can save a bit of work packaging Swift modules, because many Nix builds will produce output for just one target any way.

Core libraries

In addition to the standard library, the Swift toolchain contains some additional 'core libraries' that, on Apple platforms, are normally distributed as part of the OS or Xcode. These are packaged separately in Nixpkgs, and can be found (for use in buildInputs) as:

swiftPackages.Dispatch
swiftPackages.Foundation
swiftPackages.XCTest

Packaging with SwiftPM

Nixpkgs includes a small helper swiftpm2nix that can fetch your SwiftPM dependencies for you, when you need to write a Nix expression to package your application.

The first step is to run the generator:

cd /path/to/my/project
# Enter a Nix shell with the required tools.
nix-shell -p swift swiftpm swiftpm2nix
# First, make sure the workspace is up-to-date.
swift package resolve
# Now generate the Nix code.
swiftpm2nix

This produces some files in a directory nix, which will be part of your Nix expression. The next step is to write that expression:

{ stdenv, swift, swiftpm, swiftpm2nix, fetchFromGitHub }:

let
  # Pass the generated files to the helper.
  generated = swiftpm2nix.helpers ./nix;
in

stdenv.mkDerivation rec {
  pname = "myproject";
  version = "0.0.0";

  src = fetchFromGitHub {
    owner = "nixos";
    repo = pname;
    rev = version;
    hash = "sha256-AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA=";
  };

  # Including SwiftPM as a nativeBuildInput provides a buildPhase for you.
  # This by default performs a release build using SwiftPM, essentially:
  #   swift build -c release
  nativeBuildInputs = [ swift swiftpm ];

  # The helper provides a configure snippet that will prepare all dependencies
  # in the correct place, where SwiftPM expects them.
  configurePhase = generated.configure;

  installPhase = ''
    # This is a special function that invokes swiftpm to find the location
    # of the binaries it produced.
    binPath="$(swiftpmBinPath)"
    # Now perform any installation steps.
    mkdir -p $out/bin
    cp $binPath/myproject $out/bin/
  '';
}

Custom build flags

If you'd like to build a different configuration than release:

swiftpmBuildConfig = "debug";

It is also possible to provide additional flags to swift build:

swiftpmFlags = [ "--disable-dead-strip" ];

The default buildPhase already passes -j for parallel building.

If these two customization options are insufficient, simply provide your own buildPhase that invokes swift build.

Running tests

Including swiftpm in your nativeBuildInputs also provides a default checkPhase, but it must be enabled with:

doCheck = true;

This essentially runs: swift test -c release

Patching dependencies

In some cases, it may be necessary to patch a SwiftPM dependency. SwiftPM dependencies are located in .build/checkouts, but the swiftpm2nix helper provides these as symlinks to read-only /nix/store paths. In order to patch them, we need to make them writable.

A special function swiftpmMakeMutable is available to replace the symlink with a writable copy:

configurePhase = generated.configure ++ ''
  # Replace the dependency symlink with a writable copy.
  swiftpmMakeMutable swift-crypto
  # Now apply a patch.
  patch -p1 -d .build/checkouts/swift-crypto -i ${./some-fix.patch}
'';

Considerations for custom build tools

Linking the standard library

The swift package has a separate lib output containing just the Swift standard library, to prevent Swift applications needing a dependency on the full Swift compiler at run-time. Linking with the Nixpkgs Swift toolchain already ensures binaries correctly reference the lib output.

Sometimes, Swift is used only to compile part of a mixed codebase, and the link step is manual. Custom build tools often locate the standard library relative to the swift compiler executable, and while the result will work, when this path ends up in the binary, it will have the Swift compiler as an unintended dependency.

In this case, you should investigate how your build process discovers the standard library, and override the path. The correct path will be something like: "${swift.swift.lib}/${swift.swiftModuleSubdir}"

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