fix: injection logic overhaul

Allow `_else` clauses in `when` checked sets, also exports `inject` via
the std library and uses it to fix the std import tests.

atom-nix/core/compose.nix

@@ -119,14 +119,18 @@ let
             __getEnv = errors.getEnv "";
             __getFlake = errors.import;
             get = extern;
-            std = if __isStd__ then atomScope else std;
           };
 
           scope'' = core.set.inject scope' [
             preOpt
             {
               _if = !__isStd__;
               atom = atomScope;
+              _else.std = atomScope;
+            }
+            {
+              _if = !__isStd__ && l.elem "std" coreFeatures';
+              inherit std;
             }
             {
               _if = __internal__test;

atom-nix/core/mod.nix

@@ -7,6 +7,12 @@ let
   importAtom = import ./importAtom.nix;
   fix = import ../std/fix.nix;
   when = scopedImport { std = builtins; } ../std/set/when.nix;
+  inject = scopedImport {
+    std = builtins;
+    mod = {
+      inherit when;
+    };
+  } ../std/set/inject.nix;
   filterMap = scopedImport { std = builtins; } ../std/set/filterMap.nix;
   make = scopedImport { std = builtins; } ../std/path/make.nix;
   parse = scopedImport { std = builtins; } ../std/file/parse.nix;
@@ -36,7 +42,7 @@ rec {
     inherit parse;
   };
   set = {
-    inherit when;
+    inherit when inject;
   };
 
   compose = import ./compose.nix;
@@ -82,8 +88,6 @@ rec {
              ${toString dir}/mod.nix
     '';
 
-  set.inject = l.foldl' (acc: x: acc // when x);
-
   pureBuiltinsForStd =
     std:
     filterMap (

atom-nix/std/set/inject.nix

@@ -0,0 +1 @@
+std.foldl' (acc: x: acc // mod.when x)

atom-nix/std/set/mod.nix

@@ -2,4 +2,5 @@
   When = mod.when;
   FilterMap = mod.filterMap;
   Merge = mod.merge;
+  Inject = mod.inject;
 }

atom-nix/std/set/when.nix

@@ -5,6 +5,7 @@
 
   ```nix
   when { _if = true; foo = "bar"; } => { foo = "bar"; }
+  when { _if = false; foo = "bar"; _else.foo = "buz"; } => { foo = "buz"; }
   when { _if = false; foo = "bar"; } => { }
   ```
 
@@ -20,6 +21,6 @@
 
   # Return Value
 
-  If the `_if` attribute of the input set is `true` or missing, returns the input set with the `_if` attribute removed. Otherwise, returns an empty attribute set.
+  If the `_if` attribute of the input set is `true` or missing, returns the input set with the `_if` attribute removed. Otherwise, returns the `_else` attribute or an empty attribute set if missing.
 */
-set: if set._if or true then std.removeAttrs set [ "_if" ] else { }
+set: if set._if or true then std.removeAttrs set [ "_if" ] else set._else or { }