haskell.compiler.ghc*: assert cross is possible

pkgs/development/compilers/ghc/8.10.7.nix

@@ -77,6 +77,9 @@ assert !enableIntegerSimple -> gmp != null;
 # see note [HADDOCK_DOCS].
 assert (stdenv.buildPlatform != stdenv.hostPlatform || stdenv.targetPlatform != stdenv.hostPlatform) -> !enableHaddockProgram;
 
+# GHC does not support building when all 3 platforms are different.
+assert stdenv.buildPlatform == stdenv.hostPlatform || stdenv.hostPlatform == stdenv.targetPlatform;
+
 let
   inherit (stdenv) buildPlatform hostPlatform targetPlatform;
 

pkgs/development/compilers/ghc/common-hadrian.nix

@@ -239,6 +239,12 @@
 
 assert !enableNativeBignum -> gmp != null;
 
+# GHC does not support building when all 3 platforms are different.
+assert stdenv.buildPlatform == stdenv.hostPlatform || stdenv.hostPlatform == stdenv.targetPlatform;
+
+# It is currently impossible to cross-compile GHC with Hadrian.
+assert stdenv.buildPlatform == stdenv.hostPlatform;
+
 let
   inherit (stdenv) buildPlatform hostPlatform targetPlatform;
 
@@ -359,13 +365,6 @@ let
 
 in
 
-# C compiler, bintools and LLVM are used at build time, but will also leak into
-# the resulting GHC's settings file and used at runtime. This means that we are
-# currently only able to build GHC if hostPlatform == buildPlatform.
-assert !targetPlatform.isGhcjs -> targetCC == pkgsHostTarget.targetPackages.stdenv.cc;
-assert buildTargetLlvmPackages.llvm == llvmPackages.llvm;
-assert stdenv.targetPlatform.isDarwin -> buildTargetLlvmPackages.clang == llvmPackages.clang;
-
 stdenv.mkDerivation ({
   pname = "${targetPrefix}ghc${variantSuffix}";
   inherit version;

pkgs/development/compilers/ghc/common-make-native-bignum.nix

@@ -82,6 +82,9 @@ assert !enableNativeBignum -> gmp != null;
 # see note [HADDOCK_DOCS].
 assert (stdenv.buildPlatform != stdenv.hostPlatform || stdenv.targetPlatform != stdenv.hostPlatform) -> !enableHaddockProgram;
 
+# GHC does not support building when all 3 platforms are different.
+assert stdenv.buildPlatform == stdenv.hostPlatform || stdenv.hostPlatform == stdenv.targetPlatform;
+
 let
   inherit (stdenv) buildPlatform hostPlatform targetPlatform;