Excessive strictness involving // and fixpoints causing infrec

MWE:

{ pkgs ? import <nixpkgs> {}, lib ? pkgs.lib }:
let
  packages = self: 
  let 
    builtins_ = lib.mapAttrs self.build { };
in
  {
    build = pname: deps: derivation {
      name = "mwe-${pname}";
      inherit deps;
      builder = "/bin/sh";
      system = "x86_64-linux";
    };
    builtins_ = builtins_;
  } // builtins_;
in pkgs.lib.fix' packages

This will work in Nix, but not in Tvix, causes an infrec.

1. Root cause: // builtins_ seemingly? Is it related to partial attrset construction observation?

   raitobezarius at 2023-12-30T02·02+00

2. Nope, this is a bug in builtins.mapAttrs, namely it forcing f too early. This can be seen when replacing lib.mapAttrs (== builtins.mapAttrs) with a hand rolled version:

   { pkgs ? import <nixpkgs> {}, lib ? pkgs.lib }:
   
   let
     mapAttrs = f: attrs:
      let
       names = builtins.attrNames attrs;
       name = builtins.head names;
      in
      if builtins.length names == 0
      then { }
      else { ${name} = f name attrs.${name}; } // mapAttrs f (builtins.removeAttrs attrs [ name ]);
   
     packages = self:
     let
       builtins_ = mapAttrs self.build { };
     in
     {
       build = pname: deps: derivation {
         name = "mwe-${pname}";
         inherit deps;
         builder = "/bin/sh";
         system = "x86_64-linux";
       };
       builtins_ = builtins_;
     } // builtins_;
   in lib.fix' packages
   

   sterni at 2023-12-30T10·22+00

3. Ah I see, that makes sense, thank you for finding about it!

   raitobezarius at 2023-12-31T01·34+00