modules/extraFiles: refactor to use symlinks and support directories

Instead of copying source files to the target, use a symlink.

This reduces nix store redundancy and enables using entire directories
as sources.

To support this, additional validation is done on file targets to
prevent unexpected conflicts.

modules/top-level/files/default.nix

@@ -56,6 +56,19 @@ in
   config =
     let
       extraFiles = lib.filter (file: file.enable) (lib.attrValues config.extraFiles);
+      targets = lib.pipe extraFiles [
+        (builtins.groupBy (entry: entry.target))
+        # Explicitly copy to store using "${}"
+        (lib.mapAttrs (_: map (entry: "${entry.finalSource}")))
+      ];
+      prefixConflicts = lib.optionals (targets != { }) (
+        let
+          names = lib.attrNames targets;
+          pairs = lib.zipLists names (lib.tail names);
+        in
+        lib.filter ({ fst, snd }: lib.hasPrefix "${fst}/" snd) pairs
+      );
+
       concatFilesOption = attr: lib.flatten (lib.mapAttrsToList (_: builtins.getAttr attr) config.files);
     in
     {
@@ -63,7 +76,15 @@ in
       extraPlugins = concatFilesOption "extraPlugins";
       extraPackages = concatFilesOption "extraPackages";
       warnings = concatFilesOption "warnings";
-      assertions = concatFilesOption "assertions";
+      assertions =
+        concatFilesOption "assertions"
+        ++ lib.nixvim.mkAssertions "extraFiles" {
+          assertion = prefixConflicts == [ ];
+          message = ''
+            Conflicting target prefixes:
+            ${lib.concatMapStringsSep "\n" ({ fst, snd }: "  - ${fst} ↔ ${snd}") prefixConflicts}
+          '';
+        };
 
       # Add files to extraFiles
       extraFiles = lib.mkDerivedConfig options.files (
@@ -76,28 +97,53 @@ in
       );
 
       # A directory with all the files in it
-      # Implementation based on NixOS's /etc module
-      build.extraFiles = pkgs.runCommandLocal "nvim-config" { passthru.vimPlugin = true; } ''
-        set -euo pipefail
+      # Implementation inspired by NixOS's /etc module
+      build.extraFiles =
+        pkgs.runCommandLocal "nvim-config"
+          {
+            __structuredAttrs = true;
+            nativeBuildInputs = [ pkgs.jq ];
+            inherit targets;
+            passthru.vimPlugin = true;
+          }
+          ''
+            set -euo pipefail
 
-        makeEntry() {
-          src="$1"
-          target="$2"
-          mkdir -p "$out/$(dirname "$target")"
-          cp "$src" "$out/$target"
-        }
+            # Ensure $out is created, in case there are no targets defined
+            mkdir -p "$out"
 
-        mkdir -p "$out"
-        ${lib.concatMapStringsSep "\n" (
-          { target, finalSource, ... }:
-          lib.escapeShellArgs [
-            "makeEntry"
-            # Force local source paths to be added to the store
-            "${finalSource}"
-            target
-          ]
-        ) extraFiles}
-      '';
+            # Extract targets into a dedicated file,
+            # to avoid reading all of attrs.json multiple times
+            jq .targets "$NIX_ATTRS_JSON_FILE" > targets.json
+
+            # Iterate target keys and read sources into a bash array
+            jq --raw-output 'keys[]' targets.json |
+            while IFS= read -r target; do
+              # read the list of source strings for this target into a Bash array
+              mapfile -t sources < <(
+                jq --raw-output --arg target "$target" '.[$target][]' targets.json
+              )
+
+              # If there are multiple sources, ensure all are identical
+              if (( ''${#sources[@]} > 1 )); then
+                base="''${sources[0]}"
+                for src in "''${sources[@]:1}"; do
+                  # Optimistically check store-path equality, then fallback to diff
+                  if [ "$src" != "$base" ] && ! diff -q "$base" "$src" >/dev/null
+                  then
+                    echo "error: target '$target' defined multiple times with different sources:" >&2
+                    printf '  %s\n' "''${sources[@]}" >&2
+                    exit 1
+                  fi
+                done
+              fi
+
+              # Install target symlink to the canonical source
+              dest="$out/$target"
+              mkdir -p "$(dirname "$dest")"
+              ln -s "''${sources[0]}" "$dest"
+            done
+          '';
 
       # Never combine user files with the rest of the plugins
       performance.combinePlugins.standalonePlugins = [ config.build.extraFiles ];