Add mkDummySrc

lib/default.nix

@@ -15,6 +15,7 @@ lib.makeScope newScope (self:
     buildWithCargo = callPackage ./buildWithCargo.nix { };
     cleanCargoToml = callPackage ./cleanCargoToml.nix { };
     downloadCargoPackage = callPackage ./downloadCargoPackage.nix { };
+    mkDummySrc = callPackage ./mkDummySrc.nix { };
     urlForCargoPackage = callPackage ./urlForCargoPackage.nix { };
     vendorCargoDeps = callPackage ./vendorCargoDeps.nix { };
     writeTOML = callPackage ./writeTOML.nix { };

lib/mkDummySrc.nix

@@ -0,0 +1,78 @@
+{ cleanCargoToml
+, lib
+, runCommand
+, writeText
+, writeTOML
+}:
+
+{ src ? null
+, cargoToml ? src + /Cargo.toml
+, cargoLock ? src + /Cargo.lock
+, cargoConfig ? src + /.cargo/config
+, cargoConfigToml ? src + /.cargo/config.toml
+}:
+let
+  inherit (builtins)
+    dirOf
+    concatStringsSep
+    hasAttr
+    pathExists;
+
+  dummyrs = writeText "dummy.rs" ''
+    #![allow(dead_code)]
+    pub fn main() {}
+  '';
+
+  trimmedCargoToml = cleanCargoToml {
+    inherit cargoToml;
+  };
+
+  p = trimmedCargoToml.package;
+  name = "${p.name}-${p.version}-dummy-src";
+
+  copyCargoConfig =
+    # the .toml extension is preferred, but the extension-less path takes precedence
+    # https://doc.rust-lang.org/cargo/reference/config.html
+    if pathExists cargoConfig
+    then "cp ${cargoConfig} $out/.cargo"
+    else if pathExists cargoConfigToml
+    then "cp ${cargoConfigToml} $out/.cargo"
+    else "";
+
+  cpDummy = path: ''
+    mkdir -p $out/${dirOf path}
+    cp -f ${dummyrs} $out/${path}
+  '';
+
+  safeStubLib =
+    if hasAttr "lib" trimmedCargoToml
+    then cpDummy (trimmedCargoToml.lib.path or "src/lib.rs")
+    else "";
+
+  safeStubList = attr: defaultPath:
+    let
+      targetList = trimmedCargoToml.${attr} or [ ];
+      paths = map (t: t.path or "${defaultPath}/${t.name}") targetList;
+      commands = map cpDummy paths;
+    in
+    concatStringsSep "\n" commands;
+in
+runCommand name { } ''
+  # Base configuration
+  mkdir -p $out/.cargo
+  ${copyCargoConfig}
+  cp ${writeTOML "Cargo.toml" trimmedCargoToml} $out/Cargo.toml
+  cp ${cargoLock} $out/Cargo.lock
+
+  # To build build-dependencies
+  ${cpDummy "build.rs"}
+  # To build regular and dev dependencies (cargo build + cargo test)
+  ${cpDummy "src/main.rs"}
+
+  # Stub all other targets in case they have particular feature combinations
+  ${safeStubLib}
+  ${safeStubList "bench" "benches"}
+  ${safeStubList "bin" "bin"}
+  ${safeStubList "example" "examples"}
+  ${safeStubList "test" "tests"}
+''