devcontainer: init

lib/mkUserEnvironment.nix

@@ -0,0 +1,78 @@
+{ writeText, lib }:
+# Build a user environment purely with nix.
+#
+# The original implementation is a mix of C++ and nix code.
+#
+# See https://github.com/nixos/nix/blob/f4b94958543138671bc3641fc126589a5cffb24b/src/nix-env/user-env.cc
+#
+# TODO:
+# * also add the drvPath if the keepDerivations nix settings is set
+# * support "disabled" mode that breaks nix-env?
+# * remove the use of writeText. builtins.toFile forbits the use of references
+#   to derivations, which makes it impossible to create exactly the same
+#   manifest file as `nix-env`.
+#
+# Arguments:
+# * derivations: a list of derivations
+{
+  # A list of derivations to install
+  derivations
+}:
+# Supporting code
+with builtins;
+let
+  # Escape Nix strings
+  stringEscape = str:
+    "\"" + (
+      replaceStrings
+        [ "\\" "\"" "\n" "\r" "\t" ]
+        [ "\\\\" "\\" "\\n" "\\r" "\\t" ]
+        str
+    )
+    + "\"";
+
+  # Like builtins.JSON but to output Nix code
+  toNix = value:
+    if isString value then stringEscape value
+    else if isInt value then toString value
+    else if isPath value then toString value
+    else if true == value then "true"
+    else if false == value then "false"
+    else if null == value then "null"
+    else if isAttrs value then
+      "{ " + concatStringsSep " " (lib.mapAttrsToList (k: v: "${k} = ${toNix v};") value) + " }"
+    else if isList value then
+      "[ ${ concatStringsSep " " (map toNix value) } ]"
+    else throw "type ${typeOf value} not supported";
+
+  # Generate a nix-env compatible manifest.nix file
+  genManifest = drv:
+    let
+      outputs =
+        drv.meta.outputsToInstall or
+          # install the first output
+          [ (head drv.outputs) ];
+
+      base = {
+        inherit (drv) meta name outPath system type;
+        out = { inherit (drv) outPath; };
+        inherit outputs;
+      };
+
+      toOut = name: {
+        outPath = drv.${name}.outPath;
+      };
+
+      outs = lib.genAttrs outputs toOut;
+    in
+      base // outs;
+
+  writeManifest = derivations:
+    writeText "env-manifest.nix" (
+      toNix (map genManifest derivations)
+    );
+in
+import <nix/buildenv.nix> {
+  inherit derivations;
+  manifest = writeManifest derivations;
+}