Try #1450:

Author: iohk-bors[bot]

ci.nix

@@ -63,7 +63,7 @@
        || (system == "x86_64-darwin" && __elem compiler-nix-name ["ghc8107"]))) {
     inherit (lib.systems.examples) ghcjs;
   } // lib.optionalAttrs (system == "x86_64-linux" &&
-         nixpkgsName == "unstable" && (__elem compiler-nix-name ["ghc8107"])) {
+         nixpkgsName == "unstable" && (__elem compiler-nix-name ["ghc8107" "ghc902" "ghc922"])) {
     # Windows cross compilation is currently broken on macOS
     inherit (lib.systems.examples) mingwW64;
   } // lib.optionalAttrs (system == "x86_64-linux" && nixpkgsName == "unstable" && compiler-nix-name == "ghc8107") {

compiler/ghc/default.nix

@@ -384,13 +384,30 @@ stdenv.mkDerivation (rec {
   dontStrip = true;
   dontPatchELF = true;
   noAuditTmpdir = true;
-} // lib.optionalAttrs stdenv.buildPlatform.isDarwin {
+} // lib.optionalAttrs (stdenv.buildPlatform.isDarwin || stdenv.targetPlatform.isWindows) {
   # ghc install on macOS wants to run `xattr -r -c`
   # The macOS version fails because it wants python 2.
   # The nix version of xattr does not support those args.
   # Luckily setting the path to something that does not exist will skip the step.
-  preBuild = ''
+  preBuild = lib.optionalString stdenv.buildPlatform.isDarwin ''
     export XATTR=$(mktemp -d)/nothing
+  ''
+  # We need to point at a stand in `windows.h` header file so that the RTS headers can
+  # work on the hostPlatform.  We also need to work around case sensitve file system issues.
+  + lib.optionalString stdenv.targetPlatform.isWindows ''
+    export NIX_CFLAGS_COMPILE_${
+        # We want this only to apply to the non windows hostPlatform (the
+        # windows gcc cross compiler has a full `windows.h`).
+        # This matches the way `suffixSalt` is calculated in nixpkgs.
+        # See https://github.com/NixOS/nixpkgs/blob/8411006d6bcd7f6e6a8a1a80ce8fcdccdd16c6ab/pkgs/build-support/cc-wrapper/default.nix#L58
+        lib.replaceStrings ["-" "."] ["_" "_"] stdenv.hostPlatform.config
+      }+=" -I${../windows/include}"
+    if [[ -f libraries/base/include/winio_structs.h ]]; then
+      substituteInPlace libraries/base/include/winio_structs.h --replace Windows.h windows.h
+    fi
+    if [[ -f rts/win32/ThrIOManager.c ]]; then
+      substituteInPlace rts/win32/ThrIOManager.c --replace rts\\OSThreads.h rts/OSThreads.h
+    fi
   '';
 });
 in self

compiler/windows/include/synchapi.h

@@ -0,0 +1 @@
+#include <windows.h>

compiler/windows/include/windows.h

@@ -0,0 +1,18 @@
+// Just enoough windows.h stuff to make rts headers usable when building GHC cross compiler.
+// See compiler/ghc/default.nix for where this is used.
+#ifndef _WINDOWS_
+#define _WINDOWS_
+
+#define __stdcall
+
+typedef unsigned long DWORD;
+typedef void * PVOID;
+
+typedef struct _RTL_CONDITION_VARIABLE { PVOID Ptr; } RTL_CONDITION_VARIABLE;
+typedef RTL_CONDITION_VARIABLE CONDITION_VARIABLE;
+
+typedef struct _RTL_SRWLOCK { PVOID Ptr; } RTL_SRWLOCK;
+typedef RTL_SRWLOCK SRWLOCK;
+
+#endif
+