ghc: enable static RTS

This change allows loading statically compiled libreries into a running
GHC, thus fixing the build of haskell packages that use TemplateHaskell.
See [1] for the details.

Fixes issue NixOS#61575, NixOS#124284.

[1]: https://www.tweag.io/blog/2020-09-30-bazel-static-haskell/

pkgs/development/compilers/ghc/9.0.2.nix

@@ -33,7 +33,7 @@
 
 , # Whether to build dynamic libs for the standard library (on the target
   # platform). Static libs are always built.
-  enableShared ? !stdenv.targetPlatform.isWindows && !stdenv.targetPlatform.useiOSPrebuilt
+  enableShared ? with stdenv.targetPlatform; !isWindows && !useiOSPrebuilt && !isStatic
 
 , # Whether to build terminfo.
   enableTerminfo ? !stdenv.targetPlatform.isWindows
@@ -105,9 +105,13 @@ let
     CrossCompilePrefix = ${targetPrefix}
   '' + lib.optionalString (!enableProfiledLibs) ''
     GhcLibWays = "v dyn"
-  '' + lib.optionalString enableRelocatedStaticLibs ''
-    GhcLibHcOpts += -fPIC
-    GhcRtsHcOpts += -fPIC
+  '' +
+  # -fexternal-dynamic-refs apparently (because it's not clear from the documentation)
+  # makes the GHC RTS able to load static libraries, which may be needed for TemplateHaskell.
+  # This solution was described in https://www.tweag.io/blog/2020-09-30-bazel-static-haskell
+  lib.optionalString enableRelocatedStaticLibs ''
+    GhcLibHcOpts += -fPIC -fexternal-dynamic-refs
+    GhcRtsHcOpts += -fPIC -fexternal-dynamic-refs
   '' + lib.optionalString targetPlatform.useAndroidPrebuilt ''
     EXTRA_CC_OPTS += -std=gnu99
   '';

pkgs/development/compilers/ghc/9.2.2.nix

@@ -34,7 +34,7 @@
 
 , # Whether to build dynamic libs for the standard library (on the target
   # platform). Static libs are always built.
-  enableShared ? !stdenv.targetPlatform.isWindows && !stdenv.targetPlatform.useiOSPrebuilt
+  enableShared ? with stdenv.targetPlatform; !isWindows && !useiOSPrebuilt && !isStatic
 
 , # Whether to build terminfo.
   enableTerminfo ? !stdenv.targetPlatform.isWindows
@@ -106,9 +106,13 @@ let
     CrossCompilePrefix = ${targetPrefix}
   '' + lib.optionalString (!enableProfiledLibs) ''
     GhcLibWays = "v dyn"
-  '' + lib.optionalString enableRelocatedStaticLibs ''
-    GhcLibHcOpts += -fPIC
-    GhcRtsHcOpts += -fPIC
+  '' +
+  # -fexternal-dynamic-refs apparently (because it's not clear from the documentation)
+  # makes the GHC RTS able to load static libraries, which may be needed for TemplateHaskell.
+  # This solution was described in https://www.tweag.io/blog/2020-09-30-bazel-static-haskell
+  lib.optionalString enableRelocatedStaticLibs ''
+    GhcLibHcOpts += -fPIC -fexternal-dynamic-refs
+    GhcRtsHcOpts += -fPIC -fexternal-dynamic-refs
   '' + lib.optionalString targetPlatform.useAndroidPrebuilt ''
     EXTRA_CC_OPTS += -std=gnu99
   '';