chore: builtin features

Use and reserve `std` and `core` features as
always-on builtin features.

src/atom.toml

@@ -3,6 +3,10 @@ name = "atom"
 version = "0.3.0"
 description = "A purpose built, unopinionated, and performant module system for Nix code."
 
+[atom.meta]
+# special attribute for the core
+__root_features__ = ["core"]
+
+# all features of atom.toml are builtin features
 [features]
-std = []
-default = ["std"]
+core = ["std"]

src/atom/compose.nix

@@ -6,9 +6,6 @@ in
   config,
   extern ? { },
   features ? [ ],
-  # internal features of the composer function
-  stdFeatures ? src.stdToml.features.default or [ ],
-  composeFeatures ? src.atomToml.features.default,
   # enable testing code paths
   __internal__test ? false,
   __isStd__ ? false,
@@ -17,21 +14,11 @@ dir':
 let
   dir = src.prepDir dir';
 
-  std = src.readStd {
-    features = stdFeatures;
-    inherit __internal__test;
-  } ../std.toml;
-
-  composeFeatures' = src.features.parse src.atomToml.features composeFeatures;
-  stdFeatures' = src.features.parse src.stdToml.features stdFeatures;
+  std = src.fromManifest { inherit features __internal__test; } ../std.toml;
 
   __atom = config // {
     features = config.features or { } // {
-      parsed = {
-        atom = features;
-        compose = composeFeatures';
-        std = stdFeatures';
-      };
+      __parsed = features;
     };
   };
 
@@ -65,7 +52,7 @@ let
           scope'' = src.set.inject scope' [
             preOpt
             {
-              _if = !__isStd__ && l.elem "std" composeFeatures';
+              _if = !__isStd__ && l.elem "std" features;
               inherit std;
             }
             {
@@ -145,7 +132,7 @@ let
         }
       )
       {
-        _if = __isStd__ && l.elem "pkg_lib" __atom.features.parsed.atom;
+        _if = __isStd__ && l.elem "pkg_lib" __atom.features.__parsed;
         inherit (extern) lib;
       }
       {

src/atom/fromManifest.nix

@@ -15,14 +15,13 @@ let
 
   features' =
     let
-      featSet = config.features or { };
-      featIn = if features == null then featSet.default or [ ] else features;
+      featSet = mod.features.addBuiltins (config.features or { });
+      featIn = mod.features.addRootFeatures (if features == null then featSet.default or [ ] else features);
     in
     mod.features.parse featSet featIn;
 
   backend = config.backend or { };
   nix = backend.nix or { };
-  compose = config.compose or { };
 
   root = atom.path or name;
   extern =
@@ -57,21 +56,9 @@ let
 
   meta = atom.meta or { };
 
-  composeFeatures = compose.features or { };
 in
 (mod.compose) {
   inherit extern __internal__test config;
   features = features';
-  composeFeatures =
-    let
-      feat = composeFeatures.atom or mod.atomToml.features.default;
-    in
-    mod.features.parse mod.atomToml.features feat;
-  stdFeatures =
-    let
-      feat = composeFeatures.std or mod.stdToml.features.default;
-    in
-    mod.features.parse mod.stdToml.features feat;
-
   __isStd__ = meta.__is_std__ or false;
 } (dirOf path + "/${root}")

src/atom/mod.nix

@@ -4,7 +4,6 @@
 # to keep the core impelementation clean
 let
   l = builtins;
-  fromManifest = import ./fromManifest.nix;
   fix = import ../std/fix.nix;
   when = scopedImport { std = builtins; } ../std/set/when.nix;
   filterMap = scopedImport { std = builtins; } ../std/set/filterMap.nix;
@@ -58,8 +57,6 @@ rec {
     || (type == "directory" && !l.pathExists "${path}/mod.nix")
   );
 
-  readStd = opts: fromManifest { inherit (opts) __internal__test features; };
-
   modIsValid =
     mod: dir:
     l.isAttrs mod
@@ -77,6 +74,9 @@ rec {
   features.parse =
     featureSet: l.foldl' (xs: x: if l.elem x xs then xs else [ x ] ++ featureSet.${x} ++ xs) [ ];
 
+  features.addBuiltins = featureSet: atomToml.features // stdToml.features // featureSet;
+  features.addRootFeatures = features: features ++ atomToml.meta.__root_features__;
+
   # It is crucial that the directory is a path literal, not a string
   # since the implicit copy to the /nix/store, which provides isolation,
   # only happens for path literals.

src/dev.toml

@@ -6,8 +6,7 @@ name = "nixpkgs"
 import = true
 args = [{}]
 
-[compose.features]
-atom = ["std"]
+[features]
 std = ["pkg_lib"]
 
 [backend.nix]

src/std.toml

@@ -21,6 +21,6 @@ fetcher = "npins"
 # for the Nix std library
 __is_std__ = true
 
+# all features of std.toml are builtin features
 [features]
-pkg_lib = ["lib"]
-default = []
+std = [] # can have lib

test/std-import/no-std.toml

@@ -2,5 +2,5 @@
 name = "test"
 path = "import"
 
-[compose.features]
-atom = [] # defualt is ["std"]
+[features]
+core = [] # defualt is ["std"]

test/std-import/pkglib.toml

@@ -2,5 +2,5 @@
 name = "test"
 path = "import"
 
-[compose.features]
+[features]
 std = ["pkg_lib"]