rust: add cargo2nix integration for Rust project imports

This adds support for importing Rust projects using cargo2nix, enabling granular
builds and better caching. Users can now import Rust projects and their
dependencies as Nix packages using the new `languages.rust.import` option.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: domenkozar

src/modules/languages/rust.nix

@@ -10,6 +10,13 @@ let
     follows = [ "nixpkgs" ];
   };
 
+  cargo2nix = config.lib.getInput {
+    name = "cargo2nix";
+    url = "github:cargo2nix/cargo2nix";
+    attribute = "languages.rust.cargo2nixInput";
+    follows = [ "nixpkgs" ];
+  };
+
   # https://github.com/nix-community/fenix/blob/cdfd7bf3e3edaf9e3f6d1e397d3ee601e513613c/lib/combine.nix
   combine = name: paths:
     pkgs.symlinkJoin {
@@ -122,6 +129,81 @@ in
       defaultText = lib.literalExpression "nixpkgs";
       description = "Rust component packages. May optionally define additional components, for example `miri`.";
     };
+
+    import = lib.mkOption {
+      type = lib.types.attrsOf (lib.types.submodule ({ name, config, ... }: {
+        options = {
+          root = lib.mkOption {
+            type = lib.types.path;
+            description = "Path to the directory containing Cargo.toml";
+          };
+
+          workspaceMember = lib.mkOption {
+            type = lib.types.nullOr lib.types.str;
+            default = null;
+            description = "Workspace member to build. If null, builds the entire workspace";
+          };
+
+          package = lib.mkOption {
+            type = lib.types.package;
+            readOnly = true;
+            description = "The built package or workspace";
+            default =
+              let
+                cargoNixPath = config.root + "/Cargo.nix";
+                cargoTomlPath = config.root + "/Cargo.toml";
+
+                # Check if Cargo.nix exists
+                cargoNixExists = builtins.pathExists cargoNixPath;
+
+                # Check if Cargo.toml exists
+                cargoTomlExists = builtins.pathExists cargoTomlPath;
+
+                # Use the same rust version as configured
+                rustVersion = if cfg.channel != "nixpkgs" then cfg.version else "latest";
+                rustChannel = cfg.channel;
+
+                rustPkgs = pkgs.rustBuilder.makePackageSet {
+                  rustVersion = if rustChannel == "nixpkgs" then "latest" else rustVersion;
+                  rustChannel = if rustChannel == "nixpkgs" then "stable" else rustChannel;
+                  packageFun = import cargoNixPath;
+                };
+              in
+              assert lib.assertMsg cargoTomlExists
+                "Cargo.toml not found at ${toString cargoTomlPath}. Please ensure the 'root' path points to a directory containing Cargo.toml.";
+              if !cargoNixExists then
+                throw "Cargo.nix not found at ${toString cargoNixPath}. Please run 'devenv tasks run cargo2nix:${name}' to generate it."
+              else if config.workspaceMember != null then
+                rustPkgs.workspace.${config.workspaceMember} { }
+              else
+                rustPkgs.workspace;
+          };
+        };
+      }));
+      default = { };
+      description = ''
+        Import Rust projects using cargo2nix for granular builds.
+        
+        Example:
+        ```nix
+        languages.rust.import = {
+          myProject = {
+            root = ./.; # Directory containing Cargo.toml
+          };
+          
+          anotherProject = {
+            root = ./other;
+            workspaceMember = "my-crate";
+          };
+        };
+        
+        # Then use the packages:
+        packages = [ 
+          config.languages.rust.import.myProject.package
+        ];
+        ```
+      '';
+    };
   };
 
   config = lib.mkIf cfg.enable (lib.mkMerge [
@@ -218,5 +300,23 @@ in
         ];
       }
     ))
+
+    (lib.mkIf (cfg.import != { }) {
+      # Apply cargo2nix overlay if not already applied
+      overlays = [ cargo2nix.overlays.default ];
+
+      # Create cargo2nix tasks for each import
+      tasks = lib.mapAttrs'
+        (name: importCfg:
+          lib.nameValuePair "cargo2nix:${name}" {
+            exec = ''
+              echo "Generating Cargo.nix for ${name}..."
+              ${cargo2nix.packages.${pkgs.system}.default}/bin/cargo2nix ${toString importCfg.root}
+            '';
+            description = "Generate Cargo.nix for ${name} project";
+          }
+        )
+        cfg.import;
+    })
   ]);
 }

tests/rust/Cargo.lock

@@ -0,0 +1,68 @@
+# This file was @generated by cargo2nix 0.12.0.
+# It is not intended to be manually edited.
+
+args@{ release ? true
+, rootFeatures ? [
+    "rust-test/default"
+  ]
+, rustPackages
+, buildRustPackages
+, hostPlatform
+, hostPlatformCpu ? null
+, hostPlatformFeatures ? [ ]
+, target ? null
+, codegenOpts ? null
+, profileOpts ? null
+, cargoUnstableFlags ? null
+, rustcLinkFlags ? null
+, rustcBuildFlags ? null
+, mkRustCrate
+, rustLib
+, lib
+, workspaceSrc
+, ignoreLockHash
+, cargoConfig ? { }
+,
+}:
+let
+  nixifiedLockHash = "ae8143e535fa63099a70b7be6c591a7c43cb93574a80b2d1ed2e8692a7b1e522";
+  workspaceSrc = if args.workspaceSrc == null then ./. else args.workspaceSrc;
+  currentLockHash = builtins.hashFile "sha256" (workspaceSrc + /Cargo.lock);
+  lockHashIgnored =
+    if ignoreLockHash
+    then builtins.trace "Ignoring lock hash" ignoreLockHash
+    else ignoreLockHash;
+in
+if !lockHashIgnored && (nixifiedLockHash != currentLockHash) then
+  throw ("Cargo.nix ${nixifiedLockHash} is out of sync with Cargo.lock ${currentLockHash}")
+else
+  let
+    inherit (rustLib) fetchCratesIo fetchCrateLocal fetchCrateGit fetchCrateAlternativeRegistry expandFeatures decideProfile genDrvsByProfile;
+    cargoConfig' = if cargoConfig != { } then cargoConfig else
+    if builtins.pathExists ./.cargo/config then lib.importTOML ./.cargo/config else
+    if builtins.pathExists ./.cargo/config.toml then lib.importTOML ./.cargo/config.toml else { };
+    profilesByName = { };
+    rootFeatures' = expandFeatures rootFeatures;
+    overridableMkRustCrate = f:
+      let
+        drvs = genDrvsByProfile profilesByName ({ profile, profileName }: mkRustCrate ({
+          inherit release profile hostPlatformCpu hostPlatformFeatures target profileOpts codegenOpts cargoUnstableFlags rustcLinkFlags rustcBuildFlags;
+          cargoConfig = cargoConfig';
+        } // (f profileName)));
+      in
+      { compileMode ? null, profileName ? decideProfile compileMode release }:
+      let drv = drvs.${profileName}; in if compileMode == null then drv else drv.override { inherit compileMode; };
+  in
+  {
+    cargo2nixVersion = "0.12.0";
+    workspace = {
+      rust-test = rustPackages.unknown.rust-test."0.1.0";
+    };
+    "unknown".rust-test."0.1.0" = overridableMkRustCrate (profileName: rec {
+      name = "rust-test";
+      version = "0.1.0";
+      registry = "unknown";
+      src = fetchCrateLocal workspaceSrc;
+    });
+
+  }

tests/rust/Cargo.nix

@@ -0,0 +1,8 @@
+[package]
+name = "rust-test"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]
+
+[workspace]

tests/rust/Cargo.toml

@@ -1,11 +1,45 @@
-{
+{ pkgs, config, ... }: {
   languages.rust.enable = true;
-  # TODO: what are we testing here? the mold feature?
   languages.rust.mold.enable = false;
+
+  # Test the cargo2nix import functionality
+  languages.rust.import = {
+    rustTest = {
+      root = ./.;
+      workspaceMember = "rust-test";
+    };
+  };
+
+  # Include the imported package in packages
+  packages = [
+    config.languages.rust.import.rustTest.package
+  ];
+
   enterTest = ''
+    # Test that RUSTFLAGS is not set when mold is disabled
     if [ -n "''${RUSTFLAGS+x}" ]; then
       echo "RUSTFLAGS is set, but it should not be"
       exit 1
     fi
+
+    # Test that cargo and rustc are available
+    cargo --version
+    rustc --version
+
+    # Test building and running the hello world app
+    cargo build
+    cargo run
+
+    # Test that the binary was created
+    if [ ! -f target/debug/rust-test ]; then
+      echo "Binary was not created"
+      exit 1
+    fi
+
+    # Run the binary directly
+    ./target/debug/rust-test
+
+    # Test that the imported binary from cargo2nix is available
+    rust-test
   '';
 }

tests/rust/devenv.nix

@@ -0,0 +1,8 @@
+inputs:
+  devenv:
+    url: path:../..?dir=src/modules
+  cargo2nix:
+    url: github:cargo2nix/cargo2nix
+    inputs:
+      nixpkgs:
+        follows: nixpkgs

tests/rust/devenv.yaml

@@ -0,0 +1,3 @@
+fn main() {
+    println!("Hello from devenv rust test!");
+}