Use haskell.nix in for hadrian

compiler/ghc/default.nix

@@ -239,7 +239,7 @@ let
   # value for us.
   installStage1 = useHadrian && (haskell-nix.haskellLib.isCrossTarget || stdenv.targetPlatform.isMusl);
 
-  hadrian = buildPackages.haskell-nix.tool "ghc8107" "hadrian" {
+  hadrian = buildPackages.pinned-haskell-nix.tool "ghc8107" "hadrian" {
       compilerSelection = p: p.haskell.compiler;
       index-state = buildPackages.haskell-nix.internalHackageIndexState;
       # Verions of hadrian that comes with 9.6 depends on `time`

flake.nix

@@ -156,7 +156,15 @@
       let
         legacyPackages = (self.internal.compat { inherit system; }).pkgs;
         nix-tools-hydraJobs =
-          let cf = callFlake { pkgs = legacyPackages; inherit system; src = ./nix-tools; };
+          let cf = callFlake {
+            pkgs = legacyPackages;
+            inherit system;
+            src = ./nix-tools;
+            override-inputs = {
+              # Avoid downloading another `hackage.nix`.
+              inherit (inputs) hackage;
+            };
+          };
           in cf.defaultNix.hydraJobs;
       in rec {
         inherit legacyPackages;

nix-tools/flake.nix

@@ -7,8 +7,7 @@
       systems = [
         "x86_64-linux"
         "x86_64-darwin"
-        # TODO switch back on when ci.iog.io has builders for aarch64-linux
-        # "aarch64-linux"
+        "aarch64-linux"
         "aarch64-darwin"
       ];
 
@@ -48,6 +47,10 @@
 
       legacyPackages = forAllSystems (pkgs: pkgs);
 
+      lib = {
+        nix-tools = system: (haskellNix.legacyPackages.${system}.extend self.overlays.default).nix-tools;
+        haskell-nix = system: (haskellNix.legacyPackages.${system}.extend self.overlays.default).haskell-nix;
+      };
       project = forAllSystems (pkgs: pkgs.nix-tools.project);
 
       packages = forAllSystems (pkgs:

overlays/default.nix

@@ -43,16 +43,31 @@ let
             # add the missing exes attributes to the tarball derivation
             tarball // { exes = final.lib.genAttrs nix-tools-provided-exes (_: tarball); };
 
-        # Are we going to use the tarball?
-        use-tarball = final.stdenv.hostPlatform.isLinux && final.stdenv.hostPlatform.isx86_64;
+        # Version of nix-tools built with a pinned version of haskell.nix.
+        pinned-nix-tools-lib = (import (final.haskell-nix.sources.flake-compat) {
+            pkgs = final;
+            inherit (final) system;
+            src = ../nix-tools;
+            override-inputs = {
+              # Avoid downloading another `hackage.nix`.
+              inherit (final.haskell-nix.sources) hackage;
+            };
+          }).defaultNix.lib;
       in
       {
         haskell-nix =
           prev.haskell-nix // {
             inherit (nix-tools-pkgs) nix-tools nix-tools-set;
             # either nix-tools from its overlay or from the tarball.
-            nix-tools-unchecked = if use-tarball then static-nix-tools else nix-tools-pkgs.nix-tools;
+            nix-tools-unchecked =
+              # If possible use the static nix-tools tarball
+              if final.stdenv.hostPlatform.isLinux && final.stdenv.hostPlatform.isx86_64
+                then static-nix-tools
+                else pinned-nix-tools-lib.nix-tools final.system;
           };
+        # For use building hadrian.  This way updating anything that modifies the
+        # way hadrian is built will not cause a GHC rebuild.
+        pinned-haskell-nix = pinned-nix-tools-lib.haskell-nix final.system;
       });
 
     bootstrap = import ./bootstrap.nix;