Support native bignum (including static builds).

[mklca]

Enable BIGNUM_BACKEND=native support for GHC to avoid statically linking GMP (this could violate the LGPLv3 licensing of GMP).

The second patch corrects a typo in #1430.

[mklca]

I've got a working statically linked x86_64-unknown-linux-musl servant project working with these changes.

To use this in a flake, I've created a shim overlay ghcCompilerSettings:

    # overlays =
    {
      # …
      ghcCompilerSettings = let
        inherit (nixpkgs.lib.attrsets) mapAttrs recursiveUpdate;
        inherit (nixpkgs.lib.strings) hasPrefix;

        ghcOverrides = compiler-nix-name: attrs: attrs // {
          enableRelocatedStaticLibs = !attrs.enableShared or false;
          enableShared = attrs.enableShared or true;
        } // (if hasPrefix "ghc" compiler-nix-name && !hasPrefix "ghc8" compiler-nix-name then {
          enableNativeBignum = true;
        } else {
          enableIntegerSimple = true;
        });

        overrideHaskellNixCompilers = baseOverrides: pkgs: recursiveUpdate pkgs {
          haskell-nix.compiler =
            mapAttrs
            (cn: cpkg: cpkg.override (ghcOverrides cn baseOverrides))
            (nixpkgs.lib.attrsets.filterAttrs (
              n: nixpkgs.lib.trivial.const (n == haskellConfig.compiler-nix-name))
              pkgs.haskell-nix.compiler);
        };

        crossOverrides = {
          musl64 = {
            enableShared = false;
          };
        };
      in final: prev: recursiveUpdate (overrideHaskellNixCompilers {} prev) {
        pkgsCross = mapAttrs (n: v: overrideHaskellNixCompilers (crossOverrides.${n} or {}) v) prev.pkgsCross;
      };
      # …
    }

and then set up nixpkgs like this:

    import nixpkgs {
      inherit system;

      overlays = [
        haskellNix.overlay
        self.overlays.ghcCompilerSettings
        self.overlays.default
      ];
    }

[hamishmack]

bors try

[iohk-bors]

try

Timed out.

[hamishmack]

Merged in #1569.