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.

tests/test-sources/modules/extra-files.nix

@@ -1,15 +1,231 @@
+let
+  checkExtraFiles =
+    pkgs:
+    {
+      name,
+      extraFilesDir,
+      expectedTargets,
+    }:
+    pkgs.runCommand name
+      {
+        inherit extraFilesDir expectedTargets;
+        __structuredAttrs = true;
+      }
+      ''
+        set -euo pipefail
+
+        for target in "''${!expectedTargets[@]}"; do
+          file="$extraFilesDir/$target"
+          expected="''${expectedTargets["$target"]}"
+
+          if [ ! -e "$file" ]; then
+            echo "Missing target: $file"
+            exit 1
+          fi
+
+          if [ -n "$expected" ] && ! grep -qF "$expected" "$file"; then
+            echo "Content mismatch: $file"
+            exit 1
+          fi
+        done
+
+        touch $out
+      '';
+in
 {
+  # Example
   query = {
-    extraFiles = {
-      "testing/test-case.nix".source = ./extra-files.nix;
-      "testing/123.txt".text = ''
-        One
-        Two
-        Three
-      '';
-      "queries/lua/injections.scm".text = ''
-        ;; extends
-      '';
+    extraFiles."queries/lua/injections.scm".text = ''
+      ;; extends
+    '';
+  };
+
+  empty = {
+    extraFiles = { };
+  };
+
+  happy-path =
+    { config, pkgs, ... }:
+    {
+      extraFiles = {
+        # Basic tests
+        "testing/test-case.nix".source = ./extra-files.nix;
+        "testing/123.txt".text = ''
+          One
+          Two
+          Three
+        '';
+
+        # Multiple files in the same directory
+        "foo/a".text = "A";
+        "foo/b".text = "B";
+        "foo/c".text = "C";
+
+        # Nested deep path
+        "a/b/c/d/e.txt".text = "deep";
+
+        # Directory source
+        "beta/dir".source = pkgs.emptyDirectory;
+
+        # Mixed: source file + hello source
+        "hello/source".source = pkgs.hello;
+      };
+      test.runNvim = false;
+      test.extraInputs = [
+        (checkExtraFiles pkgs {
+          name = "check-happy-path";
+          extraFilesDir = config.build.extraFiles;
+          expectedTargets = {
+            "testing/test-case.nix" = null;
+            "testing/123.txt" = "One\nTwo\nThree";
+            "foo/a" = "A";
+            "foo/b" = "B";
+            "foo/c" = "C";
+            "a/b/c/d/e.txt" = "deep";
+            "beta/dir" = null;
+            "hello/source" = null;
+          };
+        })
+      ];
     };
+
+  # Special content edge cases
+  special-files =
+    { config, pkgs, ... }:
+    {
+      extraFiles = {
+        "empty.txt".text = "";
+        "whitespace.txt".text = "   \n\t  ";
+        "файл.txt".text = "unicode";
+        "目录/文件.txt".text = "chinese";
+      };
+      test.runNvim = false;
+      test.extraInputs = [
+        (checkExtraFiles pkgs {
+          name = "check-special-files";
+          extraFilesDir = config.build.extraFiles;
+          expectedTargets = {
+            "empty.txt" = "";
+            "whitespace.txt" = "   ";
+            "файл.txt" = "unicode";
+            "目录/文件.txt" = "chinese";
+          };
+        })
+      ];
+    };
+
+  identical-duplicates =
+    { config, pkgs, ... }:
+    {
+      extraFiles.a = {
+        target = "hello.txt";
+        text = "hello";
+      };
+      extraFiles.b = {
+        target = "hello.txt";
+        text = "hello";
+      };
+      extraFiles.c = {
+        target = "empty.txt";
+        source = pkgs.emptyFile;
+      };
+      extraFiles.d = {
+        target = "empty.txt";
+        source = pkgs.emptyFile;
+      };
+      extraFiles.e = {
+        target = "emptyDir";
+        source = pkgs.emptyDirectory;
+      };
+      extraFiles.f = {
+        target = "emptyDir";
+        source = pkgs.emptyDirectory;
+      };
+      test.buildNixvim = false;
+      test.extraInputs = [
+        (checkExtraFiles pkgs {
+          name = "check-identical-duplicates";
+          extraFilesDir = config.build.extraFiles;
+          expectedTargets = {
+            "hello.txt" = "hello";
+            "empty.txt" = null;
+            "emptyDir" = null;
+          };
+        })
+      ];
+    };
+
+  duplicate-mismatch =
+    { config, pkgs, ... }:
+    {
+      extraFiles.a = {
+        target = "conflict.txt";
+        text = "abc";
+      };
+      extraFiles.b = {
+        target = "conflict.txt";
+        text = "xyz";
+      };
+      test.buildNixvim = false;
+      test.extraInputs = [
+        (pkgs.runCommand "expect-duplicate-failure"
+          { failure = pkgs.testers.testBuildFailure config.build.extraFiles; }
+          ''
+            exit_code=$(cat "$failure/testBuildFailure.exit")
+            (( exit_code == 1 )) || {
+              echo "Expected exit code 1"
+              exit 1
+            }
+            grep -q "target 'conflict.txt' defined multiple times with different sources:" "$failure/testBuildFailure.log"
+            drv_count=$(grep -c '/nix/store/.*-nvim-.' "$failure/testBuildFailure.log")
+            (( drv_count == 2 )) || {
+              echo "Expected 2 store path matches, got $drv_count"
+              exit 1
+            }
+            touch $out
+          ''
+        )
+      ];
+    };
+
+  prefix-conflicts = {
+    extraFiles = {
+      # 2-level conflict
+      "foo".text = "root";
+      "foo/bar.txt".text = "child";
+
+      # 3-level conflict
+      "baz".text = "parent";
+      "baz/qux".text = "child";
+      "baz/qux/quux.txt".text = "grandchild";
+
+      # 5-level conflict
+      "a".text = "1";
+      "a/b".text = "2";
+      "a/b/c".text = "3";
+      "a/b/c/d".text = "4";
+      "a/b/c/d/e.txt".text = "5";
+
+      # Non-conflicting paths
+      "abc".text = "solo";
+      "def/ghi.txt".text = "leaf";
+      "solo".text = "single";
+      "x/y.txt".text = "leaf";
+    };
+
+    test.assertions = expect: [
+      (expect "count" 1)
+      (expect "anyExact" ''
+        Nixvim (extraFiles): Conflicting target prefixes:
+          - a ↔ a/b
+          - a/b ↔ a/b/c
+          - a/b/c ↔ a/b/c/d
+          - a/b/c/d ↔ a/b/c/d/e.txt
+          - baz ↔ baz/qux
+          - baz/qux ↔ baz/qux/quux.txt
+          - foo ↔ foo/bar.txt'')
+    ];
+
+    test.buildNixvim = false;
   };
 }