nixos/top-level: improve replaceRuntimeDependencies

Instead of iterating over all replacements and applying them one by one,
use the newly introduced replaceDependencies function to apply them all
at once for replaceRuntimeDependencies. The advantages are twofold in
case there are multiple replacements:
* Performance is significantly improved, because there is only one pass
  over the closure to be made.
* Correctness is improved, because replaceDependencies also replaces
  dependencies of the replacements themselves if applicable.

Fixes: NixOS#4336

nixos/modules/system/activation/top-level.nix

@@ -68,9 +68,15 @@ let
     else showWarnings config.warnings baseSystem;
 
   # Replace runtime dependencies
-  system = foldr ({ oldDependency, newDependency }: drv:
-      pkgs.replaceDependency { inherit oldDependency newDependency drv; }
-    ) baseSystemAssertWarn config.system.replaceRuntimeDependencies;
+  system = let replacements = config.system.replaceRuntimeDependencies; in
+    if replacements == [] then
+      # Avoid IFD if possible, by sidestepping replaceDependencies if no replacements are specified.
+      baseSystemAssertWarn
+    else
+      pkgs.replaceDependencies {
+        drv = baseSystemAssertWarn;
+        inherit replacements;
+      };
 
   systemWithBuildDeps = system.overrideAttrs (o: {
     systemBuildClosure = pkgs.closureInfo { rootPaths = [ system.drvPath ]; };

pkgs/build-support/replace-dependencies.nix

@@ -34,7 +34,7 @@ let
   inherit (builtins) unsafeDiscardStringContext appendContext;
   inherit (lib)
     trace substring stringLength concatStringsSep mapAttrsToList listToAttrs
-    attrValues mapAttrs filter hasAttr attrNames all;
+    attrValues mapAttrs filter hasAttr all;
   inherit (lib.attrsets) mergeAttrsList;
 
   toContextlessString = x: unsafeDiscardStringContext (toString x);
@@ -114,19 +114,20 @@ let
     else
       true) replacements;
 
-  rewriteMemo = mapAttrs (drv: references:
-    let
-      rewrittenReferences =
-        filter (dep: dep != drv && toString rewriteMemo.${dep} != dep)
-        references;
-      rewrites = listToAttrs (map (reference: {
-        name = reference;
-        value = rewriteMemo.${reference};
-      }) rewrittenReferences);
-      # Mind the order of how the three attrsets are merged here.
-      # The order of precedence needs to be "explicitly specified replacements" > "rewrite exclusion (cutoffPackages)" > "rewrite".
-      # So the attrset merge order is the opposite.
-    in rewriteHashes storePathOrKnownDerivationMemo.${drv} rewrites)
+  rewriteMemo =
+    # Mind the order of how the three attrsets are merged here.
+    # The order of precedence needs to be "explicitly specified replacements" > "rewrite exclusion (cutoffPackages)" > "rewrite".
+    # So the attrset merge order is the opposite.
+    mapAttrs (drv: references:
+      let
+        rewrittenReferences =
+          filter (dep: dep != drv && toString rewriteMemo.${dep} != dep)
+          references;
+        rewrites = listToAttrs (map (reference: {
+          name = reference;
+          value = rewriteMemo.${reference};
+        }) rewrittenReferences);
+      in rewriteHashes storePathOrKnownDerivationMemo.${drv} rewrites)
     relevantReferences // listToAttrs (map (drv: {
       name = toContextlessString drv;
       value = storePathOrKnownDerivationMemo.${toContextlessString drv};