From 1c53d25f587f57dc45d3bd39964a039931756a57 Mon Sep 17 00:00:00 2001
From: Eman Resu <78693624+quatquatt@users.noreply.github.com>
Date: Sun, 21 Jun 2026 18:01:06 -0400
Subject: [PATCH 1/2] lib.attrsets.concatMapAttrs: rewrite for efficiency

---
 lib/attrsets.nix | 23 ++++++++++++++---------
 1 file changed, 14 insertions(+), 9 deletions(-)

diff --git a/lib/attrsets.nix b/lib/attrsets.nix
index 811225c542f1..ed2fa9770410 100644
--- a/lib/attrsets.nix
+++ b/lib/attrsets.nix
@@ -5,7 +5,6 @@
 
 let
   inherit (builtins) head length;
-  inherit (lib.trivial) mergeAttrs;
   inherit (lib.strings)
     concatStringsSep
     concatMapStringsSep
@@ -13,16 +12,18 @@ let
     sanitizeDerivationName
     ;
   inherit (lib.lists)
-    filter
-    foldr
-    foldl'
+    all
+    concatLists
     concatMap
     elemAt
-    all
-    partition
-    groupBy
-    take
+    filter
     foldl
+    foldl'
+    foldr
+    groupBy
+    partition
+    reverseList
+    take
     ;
 in
 
@@ -370,7 +371,11 @@ rec {
 
     :::
   */
-  concatMapAttrs = f: v: foldl' mergeAttrs { } (attrValues (mapAttrs f v));
+  concatMapAttrs =
+    f: v:
+    listToAttrs (
+      concatLists (reverseList (mapAttrsToList (name: value: attrsToList (f name value)) v))
+    );
 
   /**
     Update or set specific paths of an attribute set.

From f3e6fa555f71f2a4c87e51d457a1eeabcde459a8 Mon Sep 17 00:00:00 2001
From: Eman Resu <78693624+quatquatt@users.noreply.github.com>
Date: Sun, 21 Jun 2026 18:04:34 -0400
Subject: [PATCH 2/2] lib.attrsets.concatMapAttrs: add test for handling
 duplicates

---
 lib/tests/misc.nix | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/lib/tests/misc.nix b/lib/tests/misc.nix
index 021a7eca25b8..6c254cae232b 100644
--- a/lib/tests/misc.nix
+++ b/lib/tests/misc.nix
@@ -2164,6 +2164,21 @@ runTests {
     };
   };
 
+  testConcatMapAttrsDuplicates = {
+    expr =
+      concatMapAttrs
+        (name: value: {
+          final = value;
+        })
+        {
+          a = 1;
+          b = 2;
+        };
+    expected = {
+      final = 2;
+    };
+  };
+
   testFilterAttrs = {
     expr = filterAttrs (n: v: n != "a" && (v.hello or false) == true) {
       a.hello = true;