feat: add withConfig & buildConfig from nixpkgs

[MattSturgeon]

Inherit the withConfig and buildConfig wrapper functions added in NixOS/nixpkgs#390147

Fixes #558

If the current nixpkgs revision does not have these attrs, a sane error message is thrown when attempting to use them.

nix-repl> :lf .

nix-repl> packages.x86_64-linux.treefmt.withConfig
error:
       … while evaluating the attribute 'x86_64-linux.treefmt.withConfig'
       … while evaluating a branch condition

       error: Accessing `withConfig` requires a nixpkgs revision that has `treefmt.withConfig`.

I've tested this with and without nix flake lock --override-input nixpkgs 'github:nixos/nixpkgs/nixos-unstable-small'. With the newer nixpkgs revision the wrapper functions seem to work correctly.

The treefmt argument passed is overridden to refer to perSystem.self.treefmt.

I considered using lib.genAttrs to simplify the inheritence, however doing that with // caused the entire passthru to be indented by alejandra. I also considered using inherit (lib.genAttrs), however this didn't seem much of an improvement:

diff --git a/nix/packages/treefmt/default.nix b/nix/packages/treefmt/default.nix
index 4db1258..59032b7 100644
--- a/nix/packages/treefmt/default.nix
+++ b/nix/packages/treefmt/default.nix
@@ -84,8 +84,15 @@ in
         then attr.override {inherit treefmt;}
         else attr;
     in {
-      withConfig = inheritFromNixpkgs "withConfig";
-      buildConfig = inheritFromNixpkgs "buildConfig";
+      inherit
+        (lib.genAttrs [
+            "withConfig"
+            "buildConfig"
+          ]
+          inheritFromNixpkgs)
+        withConfig
+        buildConfig
+        ;
 
       no-vendor-hash = treefmt.overrideAttrs {
         vendorHash = "";

I added some calls to trace to verify that the treefmt package argument is being correctly overridden:

diff --git a/nix/packages/treefmt/default.nix b/nix/packages/treefmt/default.nix
index 4db1258..51d2b59 100644
--- a/nix/packages/treefmt/default.nix
+++ b/nix/packages/treefmt/default.nix
@@ -81,8 +81,8 @@ in
         attr = pkgs.treefmt.${name} or (throw error);
       in
         if attr.override.__functionArgs.treefmt or null != null
-        then attr.override {inherit treefmt;}
-        else attr;
+        then builtins.trace "overriding" attr.override {inherit treefmt;}
+        else builtins.trace "not overriding" attr;
     in {
       withConfig = inheritFromNixpkgs "withConfig";
       buildConfig = inheritFromNixpkgs "buildConfig";

We see that withConfig overrides its treefmt arg, while buildConfig does not depend on treefmt:

nix-repl> :lf .

nix-repl> packages.x86_64-linux.treefmt.withConfig
trace: overriding

nix-repl> packages.x86_64-linux.treefmt.buildConfig
trace: not overriding

[brianmcgee]

This reminds me, we should probably move away from alejandra and start using the official formatter 🤔