diff --git a/ofborg/src/nixenv.rs b/ofborg/src/nixenv.rs
index 0a05f90..91455c2 100644
--- a/ofborg/src/nixenv.rs
+++ b/ofborg/src/nixenv.rs
@@ -7,6 +7,7 @@ use serde_json;
 use std::fs;
 use std::fs::File;
 use std::io::BufReader;
+use std::io::Read;
 use std::io::Seek;
 use std::io::SeekFrom;
 use std::io::Write;
@@ -93,3 +94,24 @@ impl From<std::io::Error> for Error {
         Error::Io(e)
     }
 }
+
+impl Error {
+    pub fn display(self) -> String {
+        match self {
+            Error::Io(e) => format!("Failed during the setup of executing nix-env: {:?}", e),
+            Error::Fd(mut fd) => {
+                let mut buffer = Vec::new();
+                let read_result = fd.read_to_end(&mut buffer);
+                let bufstr = String::from_utf8_lossy(&buffer);
+
+                match read_result {
+                    Ok(_) => format!("nix-env failed:\n{}", bufstr),
+                    Err(e) => format!(
+                        "nix-env failed and loading the error result caused a new error {:?}\n\n{}",
+                        e, bufstr
+                    ),
+                }
+            }
+        }
+    }
+}
diff --git a/ofborg/src/outpathdiff.rs b/ofborg/src/outpathdiff.rs
index 95334a3..5bde638 100644
--- a/ofborg/src/outpathdiff.rs
+++ b/ofborg/src/outpathdiff.rs
@@ -1,6 +1,5 @@
-extern crate amqp;
-extern crate env_logger;
-
+use crate::nixenv::Error as NixEnvError;
+use crate::nixenv::HydraNixEnv;
 use crate::nixstats::EvaluationStats;
 use ofborg::nix;
 use serde_json;
@@ -15,7 +14,7 @@ use std::io::Write;
 use std::path::PathBuf;
 
 pub struct OutPathDiff {
-    calculator: OutPaths,
+    calculator: HydraNixEnv,
     pub original: Option<(PackageOutPaths, EvaluationStats)>,
     pub current: Option<(PackageOutPaths, EvaluationStats)>,
 }
@@ -23,43 +22,25 @@ pub struct OutPathDiff {
 impl OutPathDiff {
     pub fn new(nix: nix::Nix, path: PathBuf) -> OutPathDiff {
         OutPathDiff {
-            calculator: OutPaths::new(nix, path, false),
+            calculator: HydraNixEnv::new(nix, path, false),
             original: None,
             current: None,
         }
     }
 
-    pub fn find_before(&mut self) -> Result<bool, File> {
-        let x = self.run();
-        match x {
-            Ok(f) => {
-                self.original = Some(f);
-                Ok(true)
-            }
-            Err(e) => {
-                info!("Failed to find Before list");
-                Err(e)
-            }
-        }
+    pub fn find_before(&mut self) -> Result<(), NixEnvError> {
+        self.original = Some(self.run()?);
+        Ok(())
     }
 
-    pub fn find_after(&mut self) -> Result<bool, File> {
+    pub fn find_after(&mut self) -> Result<(), NixEnvError> {
         if self.original.is_none() {
             debug!("Before is None, not bothering with After");
-            return Ok(false);
+            return Ok(());
         }
 
-        let x = self.run();
-        match x {
-            Ok(f) => {
-                self.current = Some(f);
-                Ok(true)
-            }
-            Err(e) => {
-                info!("Failed to find After list");
-                Err(e)
-            }
-        }
+        self.current = Some(self.run()?);
+        Ok(())
     }
 
     pub fn package_diff(&self) -> Option<(Vec<PackageArch>, Vec<PackageArch>)> {
@@ -107,8 +88,8 @@ impl OutPathDiff {
         None
     }
 
-    fn run(&mut self) -> Result<(PackageOutPaths, EvaluationStats), File> {
-        self.calculator.find()
+    fn run(&mut self) -> Result<(PackageOutPaths, EvaluationStats), NixEnvError> {
+        self.calculator.execute()
     }
 }
 
diff --git a/ofborg/src/tasks/eval/nixpkgs.rs b/ofborg/src/tasks/eval/nixpkgs.rs
index 6afb934..d61fa1f 100644
--- a/ofborg/src/tasks/eval/nixpkgs.rs
+++ b/ofborg/src/tasks/eval/nixpkgs.rs
@@ -125,7 +125,7 @@ impl<'a> NixpkgsStrategy<'a> {
     fn check_outpaths_before(&mut self, dir: &Path) -> StepResult<()> {
         let mut rebuildsniff = OutPathDiff::new(self.nix.clone(), dir.to_path_buf());
 
-        if let Err(mut output) = rebuildsniff.find_before() {
+        if let Err(err) = rebuildsniff.find_before() {
             /*
             self.events
                 .notify(Event::TargetBranchFailsEvaluation(target_branch.clone()));
@@ -134,7 +134,7 @@ impl<'a> NixpkgsStrategy<'a> {
             Err(Error::FailWithGist(
                 String::from("The branch this PR will merge in to does not evaluate, and so this PR cannot be checked."),
                 String::from("Output path comparison"),
-                file_to_str(&mut output),
+                err.display(),
             ))
         } else {
             self.outpath_diff = Some(rebuildsniff);
@@ -144,11 +144,11 @@ impl<'a> NixpkgsStrategy<'a> {
 
     fn check_outpaths_after(&mut self) -> StepResult<()> {
         if let Some(ref mut rebuildsniff) = self.outpath_diff {
-            if let Err(mut output) = rebuildsniff.find_after() {
+            if let Err(mut err) = rebuildsniff.find_after() {
                 Err(Error::FailWithGist(
                     String::from("This PR breaks listing of package outputs after merging."),
                     String::from("Output path comparison"),
-                    file_to_str(&mut output),
+                    err.display(),
                 ))
             } else {
                 Ok(())