diff --git a/pkgs/development/julia-modules/tests/.gitignore b/pkgs/development/julia-modules/tests/.gitignore
new file mode 100644
index 000000000000..07d2f69d765b
--- /dev/null
+++ b/pkgs/development/julia-modules/tests/.gitignore
@@ -0,0 +1,3 @@
+test_runs/
+.stack-work/
+*~
diff --git a/pkgs/development/julia-modules/tests/README.md b/pkgs/development/julia-modules/tests/README.md
new file mode 100644
index 000000000000..1be3487161e6
--- /dev/null
+++ b/pkgs/development/julia-modules/tests/README.md
@@ -0,0 +1,25 @@
+
+# Testing `julia.withPackages`
+
+This folder contains a test suite for ensuring that the top N most popular Julia packages (as measured by download count) work properly. The key parts are
+
+* `top-julia-packages.nix`: an impure derivation for fetching Julia download data and processing it into a file called `top-julia-packages.yaml`. This YAML file contains an array of objects with fields "name", "uuid", and "count", and is sorted in decreasing order of count.
+* `julia-top-n`: a small Haskell program which reads `top-julia-packages.yaml` and builds a `julia.withPackages` environment for each package, with a nice interactive display and configurable parallelism. It also tests whether evaluating `using <package-name>` works in the resulting environment.
+
+> **Warning:**
+> These tests should only be run on maintainer machines, not Hydra! `julia.withPackages` uses IFD, which is not allowed in Hydra.
+
+## Quick start
+
+``` shell
+# Test the top 100 Julia packages
+./run_tests.sh -n 100
+```
+
+## Options
+
+You can run `./run_tests.sh --help` to see additional options for the test harness. The main ones are
+
+* `-n`/`--top-n`: how many of the top packages to build (default: 100).
+* `-p`/`--parallelism`: how many builds to run at once (default: 10).
+* `-c`/`--count-file`: path to `top-julia-packages.yaml`.
diff --git a/pkgs/development/julia-modules/tests/julia-top-n/app/Main.hs b/pkgs/development/julia-modules/tests/julia-top-n/app/Main.hs
new file mode 100644
index 000000000000..ed9f8d405b12
--- /dev/null
+++ b/pkgs/development/julia-modules/tests/julia-top-n/app/Main.hs
@@ -0,0 +1,89 @@
+{-# LANGUAGE DataKinds #-}
+{-# LANGUAGE DeriveAnyClass #-}
+{-# LANGUAGE DeriveGeneric #-}
+{-# LANGUAGE FlexibleContexts #-}
+{-# LANGUAGE LambdaCase #-}
+{-# LANGUAGE OverloadedStrings #-}
+{-# LANGUAGE RecordWildCards #-}
+{-# LANGUAGE ScopedTypeVariables #-}
+{-# LANGUAGE ViewPatterns #-}
+
+module Main (main) where
+
+import Control.Exception
+import Control.Monad
+import Data.Aeson as A hiding (Options, defaultOptions)
+import qualified Data.Aeson.Key             as A
+import qualified Data.Aeson.KeyMap          as HM
+import qualified Data.ByteString.Lazy.Char8 as BL8
+import qualified Data.List as L
+import Data.Text as T
+import qualified Data.Vector as V
+import qualified Data.Yaml as Yaml
+import GHC.Generics
+import Options.Applicative
+import System.Exit
+import System.FilePath
+import Test.Sandwich hiding (info)
+import UnliftIO.MVar
+import UnliftIO.Process
+
+
+data Args = Args {
+  countFilePath :: FilePath
+  , topN :: Int
+  , parallelism :: Int
+  }
+
+argsParser :: Parser Args
+argsParser = Args
+  <$> strOption (long "count-file" <> short 'c' <> help "YAML file containing package names and counts")
+  <*> option auto (long "top-n" <> short 'n' <> help "How many of the top packages to build" <> showDefault <> value 100 <> metavar "INT")
+  <*> option auto (long "parallelism" <> short 'p' <> help "How many builds to run at once" <> showDefault <> value 10 <> metavar "INT")
+
+data NameAndCount = NameAndCount {
+  name :: Text
+  , count :: Int
+  , uuid :: Text
+  } deriving (Show, Eq, Generic, FromJSON)
+
+newtype JuliaPath = JuliaPath FilePath
+  deriving Show
+
+julia :: Label "julia" (MVar (Maybe JuliaPath))
+julia = Label
+
+main :: IO ()
+main = do
+  clo <- parseCommandLineArgs argsParser (return ())
+  let Args {..} = optUserOptions clo
+
+  namesAndCounts :: [NameAndCount] <- Yaml.decodeFileEither countFilePath >>= \case
+    Left err -> throwIO $ userError ("Couldn't decode names and counts YAML file: " <> show err)
+    Right x -> pure x
+
+  runSandwichWithCommandLineArgs' defaultOptions argsParser $
+    describe ("Building environments for top " <> show topN <> " Julia packages") $
+      parallelN parallelism $
+        forM_ (L.take topN namesAndCounts) $ \(NameAndCount {..}) ->
+          introduce' (defaultNodeOptions { nodeOptionsVisibilityThreshold = 0 }) (T.unpack name) julia (newMVar Nothing) (const $ return ()) $ do
+            it "Builds" $ do
+              let cp = proc "nix" ["build", "--impure", "--no-link", "--json", "--expr"
+                                  , "with import ../../../../. {}; julia.withPackages [\"" <> T.unpack name <> "\"]"
+                                  ]
+              output <- readCreateProcessWithLogging cp ""
+              juliaPath <- case A.eitherDecode (BL8.pack output) of
+                Right (A.Array ((V.!? 0) -> Just (A.Object (aesonLookup "outputs" -> Just (A.Object (aesonLookup "out" -> Just (A.String t))))))) -> pure (JuliaPath ((T.unpack t) </> "bin" </> "julia"))
+                x -> expectationFailure ("Couldn't parse output: " <> show x)
+
+              getContext julia >>= flip modifyMVar_ (const $ return (Just juliaPath))
+
+            it "Uses" $ do
+              getContext julia >>= readMVar >>= \case
+                Nothing -> expectationFailure "Build step failed."
+                Just (JuliaPath juliaPath) -> do
+                  let cp = proc juliaPath ["-e", "using " <> T.unpack name]
+                  createProcessWithLogging cp >>= waitForProcess >>= (`shouldBe` ExitSuccess)
+
+aesonLookup :: Text -> HM.KeyMap v -> Maybe v
+aesonLookup = HM.lookup . A.fromText
diff --git a/pkgs/development/julia-modules/tests/julia-top-n/default.nix b/pkgs/development/julia-modules/tests/julia-top-n/default.nix
new file mode 100644
index 000000000000..ab8ed948e1ea
--- /dev/null
+++ b/pkgs/development/julia-modules/tests/julia-top-n/default.nix
@@ -0,0 +1,16 @@
+{ mkDerivation, aeson, base, filepath, lib, optparse-applicative
+, sandwich, text, unliftio, yaml
+}:
+mkDerivation {
+  pname = "julia-top-n";
+  version = "0.1.0.0";
+  src = ./.;
+  isLibrary = false;
+  isExecutable = true;
+  executableHaskellDepends = [
+    aeson base filepath optparse-applicative sandwich text unliftio
+    yaml
+  ];
+  license = lib.licenses.bsd3;
+  mainProgram = "julia-top-n-exe";
+}
diff --git a/pkgs/development/julia-modules/tests/julia-top-n/julia-top-n.cabal b/pkgs/development/julia-modules/tests/julia-top-n/julia-top-n.cabal
new file mode 100644
index 000000000000..834adac33f16
--- /dev/null
+++ b/pkgs/development/julia-modules/tests/julia-top-n/julia-top-n.cabal
@@ -0,0 +1,34 @@
+cabal-version: 2.2
+
+-- This file has been generated from package.yaml by hpack version 0.36.0.
+--
+-- see: https://github.com/sol/hpack
+
+name:           julia-top-n
+version:        0.1.0.0
+author:         Tom McLaughlin
+maintainer:     tom@codedown.io
+license:        BSD-3-Clause
+build-type:     Simple
+
+executable julia-top-n-exe
+  main-is: Main.hs
+  other-modules:
+      Paths_julia_top_n
+  autogen-modules:
+      Paths_julia_top_n
+  hs-source-dirs:
+      app
+  ghc-options: -Wall -Wcompat -Widentities -Wincomplete-record-updates -Wincomplete-uni-patterns -Wmissing-export-lists -Wmissing-home-modules -Wpartial-fields -Wredundant-constraints -threaded -rtsopts -with-rtsopts=-N
+  build-depends:
+      aeson
+    , base >=4.7 && <5
+    , bytestring
+    , filepath
+    , optparse-applicative
+    , sandwich
+    , text
+    , unliftio
+    , vector
+    , yaml
+  default-language: Haskell2010
diff --git a/pkgs/development/julia-modules/tests/julia-top-n/package.yaml b/pkgs/development/julia-modules/tests/julia-top-n/package.yaml
new file mode 100644
index 000000000000..ffb9ab1d12ea
--- /dev/null
+++ b/pkgs/development/julia-modules/tests/julia-top-n/package.yaml
@@ -0,0 +1,37 @@
+name:                julia-top-n
+version:             0.1.0.0
+license:             BSD-3-Clause
+author:              "Tom McLaughlin"
+maintainer:          "tom@codedown.io"
+
+dependencies:
+- aeson
+- base >= 4.7 && < 5
+- bytestring
+- filepath
+- optparse-applicative
+- sandwich
+- text
+- unliftio
+- vector
+- yaml
+
+ghc-options:
+- -Wall
+- -Wcompat
+- -Widentities
+- -Wincomplete-record-updates
+- -Wincomplete-uni-patterns
+- -Wmissing-export-lists
+- -Wmissing-home-modules
+- -Wpartial-fields
+- -Wredundant-constraints
+
+executables:
+  julia-top-n-exe:
+    main:                Main.hs
+    source-dirs:         app
+    ghc-options:
+    - -threaded
+    - -rtsopts
+    - -with-rtsopts=-N
diff --git a/pkgs/development/julia-modules/tests/julia-top-n/stack.yaml b/pkgs/development/julia-modules/tests/julia-top-n/stack.yaml
new file mode 100644
index 000000000000..28bbc5a5f7ef
--- /dev/null
+++ b/pkgs/development/julia-modules/tests/julia-top-n/stack.yaml
@@ -0,0 +1,11 @@
+
+resolver:
+  url: https://raw.githubusercontent.com/commercialhaskell/stackage-snapshots/master/lts/22/4.yaml
+
+packages:
+- .
+
+nix:
+  pure: false
+  packages:
+  - zlib
diff --git a/pkgs/development/julia-modules/tests/julia-top-n/stack.yaml.lock b/pkgs/development/julia-modules/tests/julia-top-n/stack.yaml.lock
new file mode 100644
index 000000000000..3e782d80cc43
--- /dev/null
+++ b/pkgs/development/julia-modules/tests/julia-top-n/stack.yaml.lock
@@ -0,0 +1,13 @@
+# This file was autogenerated by Stack.
+# You should not edit this file by hand.
+# For more information, please see the documentation at:
+#   https://docs.haskellstack.org/en/stable/lock_files
+
+packages: []
+snapshots:
+- completed:
+    sha256: 8b211c5a6aad3787e023dfddaf7de7868968e4f240ecedf14ad1c5b2199046ca
+    size: 714097
+    url: https://raw.githubusercontent.com/commercialhaskell/stackage-snapshots/master/lts/22/4.yaml
+  original:
+    url: https://raw.githubusercontent.com/commercialhaskell/stackage-snapshots/master/lts/22/4.yaml
diff --git a/pkgs/development/julia-modules/tests/process_top_n.py b/pkgs/development/julia-modules/tests/process_top_n.py
new file mode 100755
index 000000000000..90de70ccec4d
--- /dev/null
+++ b/pkgs/development/julia-modules/tests/process_top_n.py
@@ -0,0 +1,33 @@
+#! /usr/bin/env nix-shell
+#! nix-shell -i python3 -p "python3.withPackages(ps: with ps; [ pyyaml toml ])"
+
+import csv
+from pathlib import Path
+import sys
+import toml
+import yaml
+
+requests_csv_path = Path(sys.argv[1])
+registry_path = Path(sys.argv[2])
+
+# Generate list of tuples (UUID, count)
+rows = []
+with open(requests_csv_path) as f:
+  reader = csv.reader(f)
+  for row in reader:
+    if row[2] == "user":
+      # Get UUID and request_count
+      rows.append((row[0], int(row[4])))
+rows.sort(key=(lambda x: x[1]), reverse=True)
+
+# Build a map from UUID -> name
+registry = toml.load(registry_path / "Registry.toml")
+uuid_to_name = {k: v["name"] for k, v in registry["packages"].items()}
+
+results = []
+for (uuid, count) in rows:
+  name = uuid_to_name.get(uuid)
+  if not name: continue
+  results.append({ "uuid": uuid, "name": uuid_to_name.get(uuid), "count": count })
+
+yaml.dump(results, sys.stdout, default_flow_style=False)
diff --git a/pkgs/development/julia-modules/tests/run_tests.sh b/pkgs/development/julia-modules/tests/run_tests.sh
new file mode 100755
index 000000000000..c7537c801d98
--- /dev/null
+++ b/pkgs/development/julia-modules/tests/run_tests.sh
@@ -0,0 +1,15 @@
+#! /usr/bin/env nix-shell
+#! nix-shell -i bash -p jq
+
+set -eo pipefail
+
+SCRIPTDIR="$( cd "$( dirname "${BASH_SOURCE[0]}" )" && pwd )"
+cd $SCRIPTDIR
+
+TOP_N_FILE=$(nix build --impure -f top-julia-packages.nix --no-link --json | jq -r '.[0].outputs.out')
+echo "Got top Julia packages: $TOP_N_FILE"
+
+TESTER_PROGRAM=$(nix build --impure --expr 'with import ../../../../. {}; haskellPackages.callPackage ./julia-top-n {}' --no-link --json | jq -r '.[0].outputs.out')/bin/julia-top-n-exe
+echo "Built tester program: $TESTER_PROGRAM"
+
+"$TESTER_PROGRAM" --tui -c "$TOP_N_FILE" $*
diff --git a/pkgs/development/julia-modules/tests/top-julia-packages.nix b/pkgs/development/julia-modules/tests/top-julia-packages.nix
new file mode 100644
index 000000000000..ca93f42875b3
--- /dev/null
+++ b/pkgs/development/julia-modules/tests/top-julia-packages.nix
@@ -0,0 +1,28 @@
+with import ../../../../. {};
+
+let
+  package-requests = stdenv.mkDerivation {
+    name = "julia-package-requests.csv";
+
+    __impure = true;
+
+    buildInputs = [cacert gzip wget];
+
+    buildCommand = ''
+      wget https://julialang-logs.s3.amazonaws.com/public_outputs/current/package_requests.csv.gz
+      gunzip package_requests.csv.gz
+      ls -lh
+      cp package_requests.csv $out
+    '';
+  };
+
+  registry = callPackage ../registry.nix {};
+
+in
+
+runCommand "top-julia-packages.yaml" {
+  __impure = true;
+  nativeBuildInputs = [(python3.withPackages (ps: with ps; [pyyaml toml]))];
+} ''
+  python ${./process_top_n.py} ${package-requests} ${registry} > $out
+''