Try #1032:

ci.nix

@@ -30,8 +30,9 @@
       ghc884 = true;
       ghc8104 = true;
     } // nixpkgs.lib.optionalAttrs (nixpkgsName == "unstable") {
-      ghc884 = true;
+      ghc884 = false;
       ghc8104 = true;
+      ghc901 = true;
       ghc810420210212 = false;
     });
   systems = nixpkgs: nixpkgs.lib.filterAttrs (_: v: builtins.elem v supportedSystems) {
@@ -58,9 +59,7 @@
     # native so ifdLevel 1 includes compiler needed in ifdLevel2 eval)
     inherit (lib.systems.examples) musl64 aarch64-multiplatform;
   };
-  isDisabled = d:
-    let meta = d.meta or {};
-    in meta.disabled or false;
+  isDisabled = d: d.meta.disabled or false;
 in
 dimension "Nixpkgs version" nixpkgsVersions (nixpkgsName: nixpkgs-pin:
   let pinnedNixpkgsSrc = sources.${nixpkgs-pin};

compiler/ghc/default.nix

@@ -8,6 +8,7 @@ let self =
 
 # build-tools
 , bootPkgs
+, buildPackages
 , autoconf, automake, coreutils, fetchurl, fetchpatch, perl, python3, m4, sphinx, numactl, elfutils
 , autoreconfHook
 , bash
@@ -100,6 +101,9 @@ let
     CrossCompilePrefix = ${targetPrefix}
   '' + lib.optionalString isCrossTarget ''
     Stage1Only = ${if targetPlatform.system == hostPlatform.system then "NO" else "YES"}
+  ''
+    # GHC 9.0.1 fails to compile for musl unless HADDOC_DOCS = NO
+    + lib.optionalString (isCrossTarget || (targetPlatform.isMusl && builtins.compareVersions ghc-version "9.0.1" >= 0)) ''
     HADDOCK_DOCS = NO
     BUILD_SPHINX_HTML = NO
     BUILD_SPHINX_PDF = NO
@@ -337,6 +341,11 @@ stdenv.mkDerivation (rec {
     inherit (ghc.meta) license platforms;
   };
 
+  # Needed for `haddock` to work on source that includes non ASCII chars
+  LANG = "en_US.UTF-8";
+  LC_ALL = "en_US.UTF-8";
+} // lib.optionalAttrs (stdenv.buildPlatform.libc == "glibc") {
+  LOCALE_ARCHIVE = "${buildPackages.glibcLocales}/lib/locale/locale-archive";
 } // lib.optionalAttrs targetPlatform.useAndroidPrebuilt {
   dontStrip = true;
   dontPatchELF = true;