Merge branch 'optionalDrvAttr'

doc/default.nix

@@ -25,6 +25,7 @@ let
       { name = "gvariant"; description = "GVariant formatted string serialization functions"; }
       { name = "customisation"; description = "Functions to customise (derivation-related) functions, derivatons, or attribute sets"; }
       { name = "meta"; description = "functions for derivation metadata"; }
+      { name = "derivations"; description = "miscellaneous derivation-specific functions"; }
     ];
   };
 

lib/default.nix

@@ -116,7 +116,7 @@ let
     inherit (self.customisation) overrideDerivation makeOverridable
       callPackageWith callPackagesWith extendDerivation hydraJob
       makeScope makeScopeWithSplicing makeScopeWithSplicing';
-    inherit (self.derivations) lazyDerivation;
+    inherit (self.derivations) lazyDerivation optionalDrvAttr;
     inherit (self.meta) addMetaAttrs dontDistribute setName updateName
       appendToName mapDerivationAttrset setPrio lowPrio lowPrioSet hiPrio
       hiPrioSet getLicenseFromSpdxId getExe getExe';

lib/derivations.nix

@@ -98,4 +98,30 @@ in
       # `lazyDerivation` caller knew a shortcut, be taken from there.
       meta = args.meta or checked.meta;
     } // passthru;
+
+  /* Conditionally set a derivation attribute.
+
+     Because `mkDerivation` sets `__ignoreNulls = true`, a derivation
+     attribute set to `null` will not impact the derivation output hash.
+     Thus, this function passes through its `value` argument if the `cond`
+     is `true`, but returns `null` if not.
+
+     Type: optionalDrvAttr :: Bool -> a -> a | Null
+
+     Example:
+       (stdenv.mkDerivation {
+         name = "foo";
+         x = optionalDrvAttr true 1;
+         y = optionalDrvAttr false 1;
+       }).drvPath == (stdenv.mkDerivation {
+         name = "foo";
+         x = 1;
+       }).drvPath
+       => true
+  */
+  optionalDrvAttr =
+    # Condition
+    cond:
+    # Attribute value
+    value: if cond then value else null;
 }

lib/tests/misc.nix

@@ -1902,7 +1902,7 @@ runTests {
     expected = true;
   };
 
-  # lazyDerivation
+  # DERIVATIONS
 
   testLazyDerivationIsLazyInDerivationForAttrNames = {
     expr = attrNames (lazyDerivation {
@@ -1955,6 +1955,24 @@ runTests {
     expected = derivation;
   };
 
+  testOptionalDrvAttr = let
+    mkDerivation = args: derivation (args // {
+      builder = "builder";
+      system = "system";
+      __ignoreNulls = true;
+    });
+  in {
+    expr = (mkDerivation {
+      name = "foo";
+      x = optionalDrvAttr true 1;
+      y = optionalDrvAttr false 1;
+    }).drvPath;
+    expected = (mkDerivation {
+      name = "foo";
+      x = 1;
+    }).drvPath;
+  };
+
   testTypeDescriptionInt = {
     expr = (with types; int).description;
     expected = "signed integer";