3 files changed, 38 insertions, 0 deletions
diff --git a/users/edef/depot-scan/default.nix b/users/edef/depot-scan/default.nix
new file mode 100644
index 000000000000..a9c0f382ff45
--- /dev/null
+++ b/users/edef/depot-scan/default.nix
@@ -0,0 +1,12 @@
+{ pkgs, ... }:
+
+pkgs.writeShellScriptBin "depot-scan" ''
+  set -euo pipefail
+
+  path="''${1:-$(git rev-parse --show-prefix)}"
+  path="''${path%%/}"
+  attr="''${path//\//.}"
+  root="$(git rev-parse --show-toplevel)"
+  echo "scanning //$path" >&2
+  nix-instantiate -E "import ${./wrap.nix} $root" -A "$attr" -vv 2> >(${pkgs.perl}/bin/perl ${./depot-scan.pl}) >&2
+''
diff --git a/users/edef/depot-scan/depot-scan.pl b/users/edef/depot-scan/depot-scan.pl
new file mode 100755
index 000000000000..8808e2eb0023
--- /dev/null
+++ b/users/edef/depot-scan/depot-scan.pl
@@ -0,0 +1,11 @@
+#! /usr/bin/env -S perl -ln
+use strict;
+
+if (/^evaluating file '(.*)'$/ or
+    /^copied source '(.*)' -> '.*'$/ or
+    /^trace: depot-scan '(.*)'$/) {
+    print $1;
+    next;
+}
+
+print STDERR unless /^instantiated '.*' -> '.*'$/;
diff --git a/users/edef/depot-scan/wrap.nix b/users/edef/depot-scan/wrap.nix
new file mode 100644
index 000000000000..dcb557a24b1e
--- /dev/null
+++ b/users/edef/depot-scan/wrap.nix
@@ -0,0 +1,15 @@
+# this wraps import to override readFile and readDir to trace the files it touches
+# technique inspired by lorri
+let
+
+  global = {
+    import = global.scopedImport {};
+    scopedImport = x: builtins.scopedImport (global // x);
+    builtins = builtins // {
+      inherit (global) import scopedImport;
+      readFile = path: builtins.trace "depot-scan '${toString path}'" (builtins.readFile path);
+      readDir  = path: builtins.trace "depot-scan '${toString path}'" (builtins.readDir  path);
+    };
+  };
+
+in global.import