From 71b130c581c81bf3bd2a3c777f7cc11d746327ae Mon Sep 17 00:00:00 2001
From: Silvan Mosberger <contact@infinisil.com>
Date: Thu, 2 Dec 2021 18:12:51 +0100
Subject: [PATCH] lib.attrsets: Introduce showAttrPath

---
 lib/attrsets.nix   | 16 +++++++++++++++-
 lib/default.nix    |  2 +-
 lib/tests/misc.nix | 22 ++++++++++++++++++++++
 3 files changed, 38 insertions(+), 2 deletions(-)

diff --git a/lib/attrsets.nix b/lib/attrsets.nix
index a88947b45858..7a3df9059c08 100644
--- a/lib/attrsets.nix
+++ b/lib/attrsets.nix
@@ -4,7 +4,7 @@
 let
   inherit (builtins) head tail length;
   inherit (lib.trivial) id;
-  inherit (lib.strings) concatStringsSep sanitizeDerivationName;
+  inherit (lib.strings) concatStringsSep concatMapStringsSep escapeNixIdentifier sanitizeDerivationName;
   inherit (lib.lists) foldr foldl' concatMap concatLists elemAt all;
 in
 
@@ -477,6 +477,20 @@ rec {
   overrideExisting = old: new:
     mapAttrs (name: value: new.${name} or value) old;
 
+  /* Turns a list of strings into a human-readable description of those
+    strings represented as an attribute path. The result of this function is
+    not intended to be machine-readable.
+
+    Example:
+      showAttrPath [ "foo" "10" "bar" ]
+      => "foo.\"10\".bar"
+      showAttrPath []
+      => "<root attribute path>"
+  */
+  showAttrPath = path:
+    if path == [] then "<root attribute path>"
+    else concatMapStringsSep "." escapeNixIdentifier path;
+
   /* Get a package output.
      If no output is found, fallback to `.out` and then to the default.
 
diff --git a/lib/default.nix b/lib/default.nix
index 3fead03a4636..6e5465bc003e 100644
--- a/lib/default.nix
+++ b/lib/default.nix
@@ -78,7 +78,7 @@ let
       mapAttrs' mapAttrsToList mapAttrsRecursive mapAttrsRecursiveCond
       genAttrs isDerivation toDerivation optionalAttrs
       zipAttrsWithNames zipAttrsWith zipAttrs recursiveUpdateUntil
-      recursiveUpdate matchAttrs overrideExisting getOutput getBin
+      recursiveUpdate matchAttrs overrideExisting showAttrPath getOutput getBin
       getLib getDev getMan chooseDevOutputs zipWithNames zip
       recurseIntoAttrs dontRecurseIntoAttrs cartesianProductOfSets;
     inherit (self.lists) singleton forEach foldr fold foldl foldl' imap0 imap1
diff --git a/lib/tests/misc.nix b/lib/tests/misc.nix
index 5fa95828df69..c4a34369f50f 100644
--- a/lib/tests/misc.nix
+++ b/lib/tests/misc.nix
@@ -761,4 +761,26 @@ runTests {
       { a = 3; b = 30; c = 300; }
     ];
   };
+
+  # The example from the showAttrPath documentation
+  testShowAttrPathExample = {
+    expr = showAttrPath [ "foo" "10" "bar" ];
+    expected = "foo.\"10\".bar";
+  };
+
+  testShowAttrPathEmpty = {
+    expr = showAttrPath [];
+    expected = "<root attribute path>";
+  };
+
+  testShowAttrPathVarious = {
+    expr = showAttrPath [
+      "."
+      "foo"
+      "2"
+      "a2-b"
+      "_bc'de"
+    ];
+    expected = "\".\".foo.\"2\".a2-b._bc'de";
+  };
 }