stdenv: remove assumption of exactly two cross stages

pkgs/stdenv/booter.nix

@@ -98,20 +98,26 @@ stageFuns: let
     thisStage =
       if args.__raw or false
       then args'
-      else allPackages ((builtins.removeAttrs args' ["selfBuild"]) // {
-        adjacentPackages = if args.selfBuild or true then null else rec {
-          pkgsBuildBuild = prevStage.buildPackages;
-          pkgsBuildHost = prevStage;
-          pkgsBuildTarget =
-            if args.stdenv.targetPlatform == args.stdenv.hostPlatform
-            then pkgsBuildHost
-            else assert args.stdenv.hostPlatform == args.stdenv.buildPlatform; thisStage;
-          pkgsHostHost =
-            if args.stdenv.hostPlatform == args.stdenv.targetPlatform
-            then thisStage
-            else assert args.stdenv.buildPlatform == args.stdenv.hostPlatform; pkgsBuildHost;
-          pkgsTargetTarget = nextStage;
-        };
+      else allPackages (args' // {
+        # Technically we only need to check `build!=target` since nixpkgs currently doesn't allow
+        # both `build!=host` and `host!=target` at the same time, but we check both in order to be
+        # future-proof.
+        actuallySplice = with thisStage.stdenv; buildPlatform != hostPlatform || hostPlatform  != targetPlatform;
+        adjacentPackages =
+          let
+            # search backwards from `stage` for the most recent stage containing packages which execute on `on` and target `for`
+            seek = stage: { on, for }@args:
+              if on  == stage.stdenv.hostPlatform &&
+                 for == stage.stdenv.targetPlatform
+              then stage
+              else seek stage.stdenv.__bootPackages or (throw "nixpkgs internal error: Bootstrapping stage with requested host and target platform not found.") args;
+            in rec {
+              pkgsBuildBuild   = seek thisStage { on = thisStage.stdenv.buildPlatform; for = thisStage.stdenv.buildPlatform;  };
+              pkgsBuildHost    = seek thisStage { on = thisStage.stdenv.buildPlatform; for = thisStage.stdenv.hostPlatform;   };
+              pkgsBuildTarget  = seek thisStage { on = thisStage.stdenv.buildPlatform; for = thisStage.stdenv.targetPlatform; };
+              pkgsHostHost     = seek thisStage { on = thisStage.stdenv.hostPlatform;  for = thisStage.stdenv.hostPlatform;   };
+              pkgsTargetTarget = nextStage;
+            };
       });
   in thisStage;
 

pkgs/stdenv/cross/default.nix

@@ -24,7 +24,6 @@ in lib.init bootStages ++ [
   # Build tool Packages
   (vanillaPackages: {
     inherit config overlays;
-    selfBuild = false;
     stdenv =
       assert vanillaPackages.stdenv.buildPlatform == localSystem;
       assert vanillaPackages.stdenv.hostPlatform == localSystem;
@@ -34,6 +33,27 @@ in lib.init bootStages ++ [
     allowCustomOverrides = true;
   })
 
+  # Prevent (some) unwanted assumptions about the number of stages
+  # from creeping in.  Previously, nixpkgs contained code which
+  # assumed that `pkgs.stdenv.__bootPackages == pkgs.stdenv.pkgsBuildHost`.
+  #
+  # We add this no-op stage here, between pkgsBuildHost and
+  # pkgsHostHost, in order to deliberately break any code which
+  # makes that assumption -- such code will likely fail since
+  # `pkgs.stdenv.__bootPackages.hostPlatform !=
+  # pkgs.stdenv.pkgsBuildHost.hostPlatform`, and will end up trying
+  # to run `hostPlatform` binaries during the build.
+  #
+  # We previously had two additional no-op stages: one before
+  # `pkgsBuildHost`, and one after `pkgsHostHost`.  However these
+  # appeared to increase eval-time CPU usage by 1%, which was deemed
+  # undesirable.  See https://github.com/NixOS/nixpkgs/pull/251299
+  #
+  (prevStage: {
+    inherit config overlays;
+    inherit (prevStage) stdenv;
+  })
+
   # Run Packages
   (buildPackages: let
     adaptStdenv =
@@ -43,7 +63,6 @@ in lib.init bootStages ++ [
   in {
     inherit config;
     overlays = overlays ++ crossOverlays;
-    selfBuild = false;
     stdenv = adaptStdenv (buildPackages.stdenv.override (old: rec {
       buildPlatform = localSystem;
       hostPlatform = crossSystem;

pkgs/top-level/stage.nix

@@ -45,6 +45,7 @@ in
   # avoid expensive splicing. `pkgsHostTarget` is skipped because it is always
   # defined as the current stage.
   adjacentPackages
+, actuallySplice
 
 , # The standard environment to use for building packages.
   stdenv
@@ -117,7 +118,7 @@ let
 
   stdenvBootstappingAndPlatforms = self: super: let
     withFallback = thisPkgs:
-      (if adjacentPackages == null then self else thisPkgs)
+      (if actuallySplice then thisPkgs else self)
       // { recurseForDerivations = false; };
   in {
     # Here are package sets of from related stages. They are all in the form
@@ -145,7 +146,7 @@ let
     inherit stdenv;
   };
 
-  splice = self: super: import ./splice.nix lib self (adjacentPackages != null);
+  splice = self: super: import ./splice.nix lib self actuallySplice;
 
   allPackages = self: super:
     let res = import ./all-packages.nix