devshell: add pkg-config dependencies and set PKG_CONFIG_PATH. (#1725)

Author: jbgi

lib/project-overlays.nix

@@ -8,12 +8,23 @@
     devshell = let
     in {
       packages = final.shell.nativeBuildInputs
+      # Cannot add the whole final.shell.buildInputs list because many collide with each other when fused.
+      # So we only add what is really used (by pkg-config):
+      ++ map (p: p.dev or p) (lib.concatLists (lib.concatMap (p: p.components.library.pkgconfig or [] ++ p.components.setup.pkgconfig or [] ++ lib.concatMap (c: lib.concatMap (a: a.pkgconfig) (lib.attrValues c)) (lib.attrValues (removeAttrs p.components ["library" "setup"])))
+        (lib.attrValues final.pkg-set.config.packages)))
       # devshell does not use pkgs.mkShell / pkgs.stdenv.mkDerivation,
       # so we need to explicit required dependencies which
       # are provided implicitely by stdenv when using the normal shell:
       ++ (lib.filter lib.isDerivation final.shell.stdenv.defaultNativeBuildInputs)
       ++ lib.optional final.shell.stdenv.targetPlatform.isGnu final.pkgs.buildPackages.binutils;
-      env = lib.mapAttrsToList lib.nameValuePair ({
+      # We need to expose all the necessary env variables:
+      env = [
+        {
+          name = "PKG_CONFIG_PATH";
+          # devshell fuse every all `packages` into a single directory ($DEVSHELL_DIR), so we use it:
+          prefix = "$DEVSHELL_DIR/lib/pkgconfig";
+        }
+      ] ++ lib.mapAttrsToList lib.nameValuePair ({
         inherit (final.shell) NIX_GHC_LIBDIR;
       # CABAL_CONFIG is only set if the shell was built with exactDeps=true
       } // lib.optionalAttrs (final.shell ? CABAL_CONFIG) {