Fix infinite recursion in inputs

ChangeLog.md

@@ -1,4 +1,22 @@
 
+# 2022-12-17
+
+ - The old syntax `mkFlake { inherit self; }` is now strongly discouraged in
+ favor of:
+
+   ```nix
+   outputs = inputs@{ flake-parts, ... }:
+     flake-parts.lib.mkFlake { inherit inputs; } { /* module */ }
+   ```
+
+   This fixes an infinite recursion that occurs with the old syntax when
+   using the `inputs` module argument in `imports`.
+
+   If you're under the impression that this already worked, that's probably
+   because you were using `inputs` from the lexical scope (ie directly from
+   the flake outputs function arguments), rather than in a separate module file.
+
+
 # 2022-12-07
 
  - The `darwinModules` option has been removed. This was added in the early days

README.md

@@ -56,8 +56,8 @@ Otherwise, add the input,
 then slide `mkFlake` between your outputs function head and body,
 
 ```
-  outputs = { self, flake-parts, ... }:
-    flake-parts.lib.mkFlake { inherit self; } {
+  outputs = inputs@{ flake-parts, ... }:
+    flake-parts.lib.mkFlake { inherit inputs; } {
       flake = {
         # Put your original flake attributes here.
       };

dev/default.nix

@@ -1,13 +1,16 @@
 let
   flake = builtins.getFlake (toString ./.);
   fmc-lib = (builtins.getFlake (toString ../.)).lib;
+  args = {
+    inherit self;
+  } // flake.inputs;
   self = {
     inherit (flake) inputs;
     outPath = ../.; # used by pre-commit module, etc
     outputs = self.config.flake;
   } //
-  fmc-lib.evalFlakeModule
-    { inherit self; }
+  fmc-lib.mkFlake
+    { inputs = args; }
     ./flake-module.nix;
 in
 self.config.flake // { inherit (flake) inputs; }

lib.nix

@@ -7,7 +7,9 @@ let
     isAttrs
     isFunction
     showOption
+    throwIf
     types
+    warnIf
     ;
   inherit (lib.types)
     path
@@ -60,18 +62,55 @@ let
     }
   );
 
+  errorExample = ''
+    For example:
+
+        outputs = inputs@{ flake-parts, ... }:
+          flake-parts.lib.mkFlake { inherit inputs; } { /* module */ };
+
+    To avoid an infinite recursion, *DO NOT* pass `self.inputs` and
+    *DO NOT* pass `inherit (self) inputs`, but pass the output function
+    arguments as `inputs` like above.
+  '';
 
   flake-parts-lib = {
     evalFlakeModule =
-      { self
+      args@
+      { inputs ? self.inputs
       , specialArgs ? { }
-      }:
-      module:
 
-      lib.evalModules {
-        specialArgs = { inherit self flake-parts-lib; inherit (self) inputs; } // specialArgs;
-        modules = [ ./all-modules.nix module ];
-      };
+        # legacy
+      , self ? inputs.self or (throw ''
+          When invoking flake-parts, you must pass all the flake output arguments,
+          and not just `self.inputs`.
+
+          ${errorExample}
+        '')
+      }:
+      throwIf
+        (!args?self && !args?inputs) ''
+        When invoking flake-parts, you must pass in the flake output arguments.
+
+        ${errorExample}
+      ''
+        warnIf
+        (!args?inputs) ''
+        When invoking flake-parts, it is recommended to pass all the flake output
+        arguments in the `inputs` parameter. If you only pass `self`, it's not
+        possible to use the `inputs` module argument in the module `imports`.
+
+        Please pass the output function arguments. ${errorExample}
+      ''
+
+        (module:
+        lib.evalModules {
+          specialArgs = {
+            inherit self flake-parts-lib;
+            inputs = args.inputs or /* legacy, warned above */ self.inputs;
+          } // specialArgs;
+          modules = [ ./all-modules.nix module ];
+        }
+        );
 
     mkFlake = args: module:
       (flake-parts-lib.evalFlakeModule args module).config.flake;

template/default/flake.nix

@@ -5,8 +5,8 @@
     nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
   };
 
-  outputs = { self, flake-parts, ... }:
-    flake-parts.lib.mkFlake { inherit self; } {
+  outputs = inputs@{ flake-parts, ... }:
+    flake-parts.lib.mkFlake { inherit inputs; } {
       imports = [
         # To import a flake module
         # 1. Add foo to inputs

template/multi-module/flake.nix

@@ -5,8 +5,8 @@
     nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
   };
 
-  outputs = { self, flake-parts, ... }:
-    flake-parts.lib.mkFlake { inherit self; } {
+  outputs = inputs@{ flake-parts, ... }:
+    flake-parts.lib.mkFlake { inherit inputs; } {
       imports = [
         ./hello/flake-module.nix
       ];