Avoid IFD as much as possible

build.nix

@@ -17,8 +17,8 @@ src:
 , buildInputs ? []
 , nativeBuildInputs ? []
 , builtDependencies ? []
-, cargolockPath ? null
-, cargotomlPath ? null
+, cargolock ? null
+, cargotoml ? null
 , release ? true
 , stdenv
 , lib
@@ -43,13 +43,11 @@ with
 
 with rec
   {
-    drv = stdenv.mkDerivation
+    drv = stdenv.mkDerivation (
       { inherit
           src
           doCheck
           nativeBuildInputs
-          cargolockPath
-          cargotomlPath
           cratePaths
           name
           version;
@@ -88,24 +86,24 @@ with rec
           ''
             runHook preConfigure
 
-            if [ -n "$cargolockPath" ]
+            if [ -n "$cargolock" ]
             then
               echo "Setting Cargo.lock"
               if [ -f "Cargo.lock" ]
               then
                 echo "WARNING: replacing existing Cargo.lock"
               fi
-              cp --no-preserve mode "$cargolockPath" Cargo.lock
+              echo "$cargolock" > Cargo.lock
             fi
 
-            if [ -n "$cargotomlPath" ]
+            if [ -n "$cargotoml" ]
             then
               echo "Setting Cargo.toml"
               if [ -f "Cargo.toml" ]
               then
                 echo "WARNING: replacing existing Cargo.toml"
               fi
-              cp "$cargotomlPath" Cargo.toml
+              echo "$cargotoml" > Cargo.toml
             fi
 
             mkdir -p target
@@ -209,7 +207,13 @@ with rec
 
             runHook postInstall
           '';
-      };
+      } //
+      lib.optionalAttrs (! isNull cargolock )
+        { cargolock = builtinz.toTOML cargolock; } //
+      lib.optionalAttrs (! isNull cargotoml )
+        { cargotoml = builtinz.toTOML cargotoml; }
+      )
+      ;
 
     # XXX: the actual crate format is not documented but in practice is a
     # gzipped tar; we simply unpack it and introduce a ".cargo-checksum.json"

builtins/default.nix

@@ -5,10 +5,10 @@
 , remarshal
 }:
 
+rec
 {
-    writeTOML = attrs:
-        let toTOML = import ./to-toml.nix { inherit lib; }; in
-        writeText "write-toml" (toTOML attrs);
+    toTOML = import ./to-toml.nix { inherit lib; };
+    writeTOML = attrs: writeText "write-toml" (toTOML attrs);
 
     readTOML = usePure: f:
       if usePure then

default.nix

@@ -45,18 +45,16 @@ with rec
   {
       commonAttrs = src: attrs: rec
         { usePureFromTOML = attrs.usePureFromTOML or true;
-          cargolockPath = attrs.cargolockPath or null;
-          cargotomlPath = attrs.cargotomlPath or null;
-          cargolock =
-            if isNull cargolockPath then
+          cargolock = attrs.cargolock or null;
+          cargotoml = attrs.cargotoml or null;
+          cargolock' =
+            if isNull cargolock then
               builtinz.readTOML usePureFromTOML "${src}/Cargo.lock"
-            else
-              builtinz.readTOML usePureFromTOML cargolockPath;
+            else cargolock;
           rootCargotoml =
-            if isNull cargotomlPath then
+            if isNull cargotoml then
               builtinz.readTOML usePureFromTOML "${src}/Cargo.toml"
-            else
-              builtinz.readTOML usePureFromTOML cargotomlPath;
+            else cargotoml;
 
           # All the Cargo.tomls, including the top-level one
           cargotomls =
@@ -81,7 +79,7 @@ with rec
 
             if isNull workspaceMembers then "."
             else lib.concatStringsSep "\n" workspaceMembers;
-          crateDependencies = libb.mkVersions cargolock;
+          crateDependencies = libb.mkVersions cargolock';
           targetInstructions =
             if builtins.hasAttr "targets" attrs then
               lib.concatMapStringsSep " " (target: "-p ${target}") attrs.targets
@@ -164,12 +162,10 @@ with rec
                 { cargoBuild = "source ${buildDepsScript}";
                   doCheck = false;
                   copyBuildArtifacts = true;
-                  cargolockPath = builtinz.writeTOML cargolock;
-                  cargotomlPath = builtinz.writeTOML
-                    (
+                  cargolock = cargolock';
+                  cargotoml =
                     { package = { name = "dummy"; version = "0.0.0"; }; } //
                         { dependencies = directDependencies; }
-                    )
                     ;
                 name =
                 if lib.length cargotomls == 0 then