Add test/shell.nix files for Emacs 26 & HEAD

test/shell.head.nix

@@ -0,0 +1,24 @@
+# Builds an sandbox for Doom Emacs with Emacs HEAD (27/28). Warning: it compiles
+# Emacs; this takes a while!
+#
+# To create a doom environment w/ Emacs 27/28 with the test config:
+#
+#   nix-shell shell.head.nix
+#
+# Or from a custom DOOMDIR:
+#
+#   nix-shell --argstr doomdir ~/.config/doom
+
+{ emacsdir ? "$(pwd)/../"
+, doomdir ? "$(pwd)"
+, doomlocaldir ? "$(pwd)/.local.nix.head" }:
+
+import ./shell.nix rec {
+  pkgs = import <nixpkgs> {
+    overlays = [
+      (import (builtins.fetchTarball https://github.com/nix-community/emacs-overlay/archive/master.tar.gz))
+    ];
+  };
+  emacs = pkgs.emacsGit;
+  inherit emacsdir doomdir doomlocaldir;
+}

test/shell.nix

@@ -0,0 +1,47 @@
+# Builds a sandbox for Doom Emacs with the latest stable Emacs (26.3). Use this
+# as a basis for module shell.nix's.
+#
+# Usage examples:
+#
+# To create a doom environment with the test config:
+#
+#   nix-shell
+#
+# With your own DOOMDIR:
+#
+#   nix-shell --argstr doomdir ~/.config/doom
+
+{ pkgs ? (import <nixpkgs> {})
+, emacs ? pkgs.emacs
+, emacsdir ? "$(pwd)/.."
+, doomdir ? "$(pwd)"
+, doomlocaldir ? "$(pwd)/.local.nix" }:
+
+pkgs.stdenv.mkDerivation {
+  name = "doom-emacs";
+  buildInputs = with pkgs; [
+    emacs
+    git
+    (ripgrep.override {withPCRE2 = true;})
+  ];
+  shellHook = ''
+    export EMACSDIR="$(readlink -f "${emacsdir}")/"
+    export DOOMDIR="$(readlink -f "${doomdir}")/"
+    export DOOMLOCALDIR="$(readlink -f "${doomlocaldir}")/"
+    export PATH="$EMACSDIR/bin:$PATH"
+    echo "EMACSDIR=$EMACSDIR"
+    echo "DOOMDIR=$DOOMDIR"
+    echo "DOOMLOCALDIR=$DOOMLOCALDIR"
+
+    # Copy your existing repos over to optimize on install times (but not the
+    # builds, because that may contain stale bytecode).
+    mkdir -p "$DOOMLOCALDIR/straight"
+    pushd "$DOOMLOCALDIR/straight" >/dev/null
+    if [[ -d "$EMACSDIR/.local/straight/repos" && ! -d ./repos ]]; then
+      cp -r "$EMACSDIR/.local/straight/repos" ./repos
+    fi
+    popd >/dev/null
+
+    doom sync
+  '';
+}