shellFor: provide build tools for all packages when exactDeps = false. Fix #1753. (#1769)

* shellFor: provide build tools for all packages when exactDeps = false

 When not using `exactDeps` cabal may try to build arbitrary dependencies
 so in this case we need to provide the build tools for all of hsPkgs.

* comp-builder: exclude hsc2hs from build-tools as it is never used,

 since it is provided by ghc env, which is first in PATH.
 This avoid possible collisions when using eg. devshells, which
 fuse all binaries into a single output directory.

Author: jbgi

builder/comp-builder.nix

@@ -239,9 +239,12 @@ let
   executableToolDepends =
     (lib.concatMap (c: if c.isHaskell or false
       then builtins.attrValues (c.components.exes or {})
-      else [c]) 
-      (builtins.filter (x: !(isNull x))
-      (map 
+      else [c])
+      (builtins.filter (x: !(isNull x)
+        # We always exclude hsc2hs from build-tools because it is unecessary as it is provided by ghc
+        # and hsc2hs from ghc is first in PATH so the one from build-tools is never used.
+        && x.identifier.name or "" != "hsc2hs")
+      (map
         (p: if builtins.isFunction p
           then p { inherit  (package.identifier) version; inherit revision; }
           else p) build-tools))) ++
@@ -328,7 +331,7 @@ let
     outputHashMode = "recursive";
     outputHashAlgo = "sha256";
   };
-                    
+
   drv = stdenv.mkDerivation (commonAttrs // contentAddressedAttrs // {
     pname = nameOnly;
     inherit (package.identifier) version;

builder/shell-for.nix

@@ -67,6 +67,8 @@ let
   selectedComponents =
     lib.filter isSelectedComponent  (lib.attrValues transitiveDependenciesComponents);
 
+  allHsPkgsComponents = lib.concatMap haskellLib.getAllComponents (builtins.attrValues hsPkgs);
+
   # Given a list of `depends`, removes those which are selected components
   removeSelectedInputs =
     lib.filter (input: !(isSelectedComponent input));
@@ -105,7 +107,10 @@ let
   systemInputs = removeSelectedInputs (uniqueWithName allSystemInputs);
 
   nativeBuildInputs = removeSelectedInputs
-    (lib.concatMap (c: c.executableToolDepends) selectedComponents);
+    (uniqueWithName (lib.concatMap (c: c.executableToolDepends)
+      # When not using `exactDeps` cabal may try to build arbitrary dependencies
+      # so in this case we need to provide the build tools for all of hsPkgs:
+      (if exactDeps then selectedComponents else allHsPkgsComponents)));
 
   # Set up a "dummy" component to use with ghcForComponent.
   component = {