Implement automatically setting up a vendor dir from source

* If the source root contains a Cargo.lock file it will be used to
  create a vendor directory
* Otherwise a (descriptive) error will be thrown describing how to
  remediate
* The caller can, of course, manually specify their own `cargoVendorDir`
  which will skip the default vendoring

lib/buildWithCargo.nix

@@ -2,8 +2,10 @@
 , configureCargoCommonVarsHook
 , configureCargoVendoredDepsHook
 , copyCargoTargetToOutputHook
+, inheritCargoTargetHook
 , lib
 , stdenv
+, vendorCargoDeps
 }:
 
 { doCompressTarget ? true
@@ -12,27 +14,54 @@
 , outputs ? [ "out" ]
 , ...
 }@args:
-stdenv.mkDerivation (args // {
-  inherit
-    doCompressTarget
-    doCopyTarget;
+let
+  vendorFromCargoLockPath = path:
+    let
+      cargoLock = path + "/Cargo.lock";
+    in
+    if builtins.pathExists cargoLock
+    then vendorCargoDeps { inherit cargoLock; }
+    else
+      throw ''
+        unable to find Cargo.lock at ${path}. please ensure one of the following:
+        - a Cargo.lock exists at the root of the source directory of the derivation
+        - set `cargoVendorDir = vendorCargoDeps { cargoLock = ./some/path/to/Cargo.lock; }`
+        - set `cargoVendorDir = null` to skip vendoring altogether
+      '';
 
-  nativeBuildInputs = nativeBuildInputs ++ [
-    cargo
-    configureCargoCommonVarsHook
-    configureCargoVendoredDepsHook
-    copyCargoTargetToOutputHook
-  ];
+  defaultValues = {
+    inherit
+      doCompressTarget
+      doCopyTarget;
 
-  outputs = outputs ++ lib.optional doCopyTarget "target";
+    buildPhase = ''
+      runHook preBuild
+      cargo check --release
+      runHook postBuild
+    '';
 
-  buildPhase = ''
-    cargo check --release
-  '';
+    installPhase = ''
+      runHook preInstall
+      mkdir -p $out
+      runHook postInstall
+    '';
 
-  installPhase = ''
-    runHook preInstall
-    mkdir -p $out
-    runHook postInstall
-  '';
-})
+    cargoVendorDir =
+      if args ? src
+      then vendorFromCargoLockPath args.src
+      else null;
+  };
+
+  additions = {
+    outputs = outputs ++ lib.optional doCopyTarget "target";
+
+    nativeBuildInputs = nativeBuildInputs ++ [
+      cargo
+      configureCargoCommonVarsHook
+      configureCargoVendoredDepsHook
+      copyCargoTargetToOutputHook
+      inheritCargoTargetHook
+    ];
+  };
+in
+stdenv.mkDerivation (defaultValues // args // additions)

pkgs/copyCargoTargetToOutputHook.sh

@@ -1,20 +1,19 @@
 copyCargoTargetToOutput() {
   echo "Executing copyCargoTargetToOutput"
-  trap "echo Finished copyCargoTargetToOutput" RETURN
 
   if [ "1" != "${doCopyTarget-}" ]; then
     return
   fi
 
-  if [ "1" != "${doCompressTarget-}" ]; then
-    # Copy the cargo `target` directory to the `target` output
-    mv "${CARGO_TARGET_DIR:-target}" "${target}"
-    return
-  fi
-
   export SOURCE_DATE_EPOCH=1
   mkdir -p "${target}"
 
+  if [ "1" != "${doCompressTarget-}" ]; then
+    # Copy the cargo `target` directory to the `target` output
+    mv "${CARGO_TARGET_DIR:-target}" "${target}/target"
+    return
+  fi
+
   # See: https://reproducible-builds.org/docs/archives/
   tar --sort=name \
     --mtime="@${SOURCE_DATE_EPOCH}" \

pkgs/default.nix

@@ -1,4 +1,5 @@
 { makeSetupHook
+, rsync
 , zstd
 }:
 
@@ -20,4 +21,13 @@
         zstd = "${zstd}/bin/zstd";
       };
     } ./copyCargoTargetToOutputHook.sh;
+
+    inheritCargoTargetHook = makeSetupHook
+    {
+      name = "inheritCargoTargetHook";
+      substitutions = {
+        rsync = "${rsync}/bin/rsync";
+        zstd = "${zstd}/bin/zstd";
+      };
+    } ./inheritCargoTargetHook.sh;
 }

pkgs/inheritCargoTargetHook.sh

@@ -0,0 +1,30 @@
+inheritCargoTarget() {
+  echo "Executing inheritCargoTarget"
+
+  local cargoTarget="${CARGO_TARGET_DIR:-target}"
+  mkdir -p "${cargoTarget}"
+
+  if [ -f "${inheritCargoTarget}/target.tar.zst" ]; then
+    @zstd@ -d "${inheritCargoTarget}/target.tar.zst" --stdout | \
+      tar -x -C "${cargoTarget}" --strip-components=1
+  elif [ -d "${inheritCargoTarget}/target" ]; then
+    @rsync@ \
+      --recursive \
+      --links \
+      --executability \
+      --chmod=+w \
+      --no-perms \
+      --no-owner \
+      --no-group \
+      "${inheritCargoTarget}/target" "${cargoTarget}"
+  else
+    echo "${inheritCargoTarget} looks invalid, are you sure it is pointing to a ".target" output?"
+    false
+  fi
+}
+
+if [ -n "${inheritCargoTarget-}" ]; then
+  preBuildHooks+=(inheritCargoTarget)
+else
+  echo "inheritCargoTarget not set"
+fi