From 6ae8efaf5f5c0e8ee87932cd5f44c1215964b863 Mon Sep 17 00:00:00 2001
From: zimbatm <zimbatm@zimbatm.com>
Date: Sat, 4 Apr 2026 14:00:10 +0200
Subject: [PATCH] splice: replace listToAttrs+map with mapAttrs

listToAttrs forces every list element to extract `.name`, calling merge
eagerly for the entire union of attribute names (~60k). mapAttrs already
knows the keys; the lambda is only invoked when a key is selected,
letting the 99.7% unforced ones stay as untouched thunks.

NIX_SHOW_STATS delta on pkgsCross.aarch64-multiplatform.hello.drvPath:

  nrFunctionCalls:  567,512 -> 507,404   (-10.6%)
  nrThunks:         613,138 -> 552,356   (-9.9%)
  sets.number:      158,787 -> 98,339    (-38.1%)
  list.elements:    295,085 -> 174,189   (-41.0%)
  gc.totalBytes:      117.5M -> 111.7M   (-4.9%)

Repro:

  NIX_SHOW_STATS=1 nix-instantiate --eval \
    -E '(import ./. {}).pkgsCross.aarch64-multiplatform.hello.drvPath' \
    2>stats.json >/dev/null
  jq . stats.json
---
 pkgs/top-level/splice.nix | 69 +++++++++++++++++++--------------------
 1 file changed, 34 insertions(+), 35 deletions(-)

diff --git a/pkgs/top-level/splice.nix b/pkgs/top-level/splice.nix
index 2c6f060f23e1..5468d23c377d 100644
--- a/pkgs/top-level/splice.nix
+++ b/pkgs/top-level/splice.nix
@@ -31,43 +31,42 @@ let
         # The same pkgs sets one probably intends
         // inputs.buildHost
         // inputs.hostTarget;
-      merge = name: {
-        inherit name;
-        value =
-          let
-            defaultValue = mash.${name};
-            # `or {}` is for the non-derivation attsert splicing case, where `{}` is the identity.
-            value' = mapCrossIndex (x: x.${name} or { }) inputs;
+      # perf: mapAttrs defers merge calls until a key is selected, avoiding
+      # ~60k eager closures that listToAttrs+map would create.
+      merge =
+        name: defaultValue:
+        let
+          # `or {}` is for the non-derivation attsert splicing case, where `{}` is the identity.
+          value' = mapCrossIndex (x: x.${name} or { }) inputs;
 
-            augmentedValue = defaultValue // {
-              __spliced = lib.filterAttrs (k: v: inputs.${k} ? ${name}) value';
-            };
-            # Get the set of outputs of a derivation. If one derivation fails to
-            # evaluate we don't want to diverge the entire splice, so we fall back
-            # on {}
-            tryGetOutputs =
-              value0:
-              let
-                inherit (builtins.tryEval value0) success value;
-              in
-              getOutputs (lib.optionalAttrs success value);
-            getOutputs =
-              value: lib.genAttrs (value.outputs or (lib.optional (value ? out) "out")) (output: value.${output});
-          in
-          # The derivation along with its outputs, which we recur
-          # on to splice them together.
-          if lib.isDerivation defaultValue then
-            augmentedValue
-            // spliceReal (mapCrossIndex tryGetOutputs value' // { hostTarget = getOutputs value'.hostTarget; })
-          else if lib.isAttrs defaultValue then
-            spliceReal value'
-          else
-            # Don't be fancy about non-derivations. But we could have used used
-            # `__functor__` for functions instead.
-            defaultValue;
-      };
+          augmentedValue = defaultValue // {
+            __spliced = lib.filterAttrs (k: v: inputs.${k} ? ${name}) value';
+          };
+          # Get the set of outputs of a derivation. If one derivation fails to
+          # evaluate we don't want to diverge the entire splice, so we fall back
+          # on {}
+          tryGetOutputs =
+            value0:
+            let
+              inherit (builtins.tryEval value0) success value;
+            in
+            getOutputs (lib.optionalAttrs success value);
+          getOutputs =
+            value: lib.genAttrs (value.outputs or (lib.optional (value ? out) "out")) (output: value.${output});
+        in
+        # The derivation along with its outputs, which we recur
+        # on to splice them together.
+        if lib.isDerivation defaultValue then
+          augmentedValue
+          // spliceReal (mapCrossIndex tryGetOutputs value' // { hostTarget = getOutputs value'.hostTarget; })
+        else if lib.isAttrs defaultValue then
+          spliceReal value'
+        else
+          # Don't be fancy about non-derivations. But we could have used used
+          # `__functor__` for functions instead.
+          defaultValue;
     in
-    lib.listToAttrs (map merge (lib.attrNames mash));
+    builtins.mapAttrs merge mash;
 
   splicePackages =
     {