lib: Introduce Cross Index concept

A Cross Index, short for Cross Platform Pair Index, is the essential
shape of a splice, without the invoking the more mind bending concept
of adding variations of for these 6 pairings to an existing thing so
that it can be switched out for something else.

So the purpose of a Cross Index is to contain the result of `f`\
(which may be reified in code, or just an abstract concept):
 - f "build" "build"
 - f "build" "host"
 - ...

Splicing on the other hand refers not just to these six variants, but
to the idea of tacking them onto one of the variants. (hostTarget,
I believe)

Cross Indexes are a necessity for making cross compilation work, but
splicing is more than necessary.

lib/customisation.nix

@@ -865,34 +865,37 @@ rec {
     };
 
   /**
-    Removes a prefix from the attribute names of a set of splices.
+    Removes a prefix from the attribute names of a cross index.
+
+    A cross index (short for "Cross Platform Pair Index") is a 6-field structure
+    organizing values by cross-compilation platform relationships.
 
     # Inputs
 
     `prefix`
-    : The prefix to remove from splice attribute names
+    : The prefix to remove from cross index attribute names
 
-    `splices`
-    : A set of splices with prefixed names
+    `crossIndex`
+    : A cross index with prefixed names
 
     # Type
 
     ```
-    renameSplicesFrom :: String -> AttrSet -> AttrSet
+    renameCrossIndexFrom :: String -> AttrSet -> AttrSet
     ```
 
     # Examples
 
     :::{.example}
-    ## `lib.customisation.renameSplicesFrom` usage example
+    ## `lib.customisation.renameCrossIndexFrom` usage example
 
     ```nix
-    renameSplicesFrom "pkgs" { pkgsBuildBuild = ...; pkgsBuildHost = ...; ... }
+    renameCrossIndexFrom "pkgs" { pkgsBuildBuild = ...; pkgsBuildHost = ...; ... }
     => { buildBuild = ...; buildHost = ...; ... }
     ```
     :::
   */
-  renameSplicesFrom = prefix: x: {
+  renameCrossIndexFrom = prefix: x: {
     buildBuild = x."${prefix}BuildBuild";
     buildHost = x."${prefix}BuildHost";
     buildTarget = x."${prefix}BuildTarget";
@@ -902,34 +905,37 @@ rec {
   };
 
   /**
-    Adds a prefix to the attribute names of a set of splices.
+    Adds a prefix to the attribute names of a cross index.
+
+    A cross index (short for "Cross Platform Pair Index") is a 6-field structure
+    organizing values by cross-compilation platform relationships.
 
     # Inputs
 
     `prefix`
-    : The prefix to add to splice attribute names
+    : The prefix to add to cross index attribute names
 
-    `splices`
-    : A set of splices to be prefixed
+    `crossIndex`
+    : A cross index to be prefixed
 
     # Type
 
     ```
-    renameSplicesTo :: String -> AttrSet -> AttrSet
+    renameCrossIndexTo :: String -> AttrSet -> AttrSet
     ```
 
     # Examples
 
     :::{.example}
-    ## `lib.customisation.renameSplicesTo` usage example
+    ## `lib.customisation.renameCrossIndexTo` usage example
 
     ```nix
-    renameSplicesTo "self" { buildBuild = ...; buildHost = ...; ... }
+    renameCrossIndexTo "self" { buildBuild = ...; buildHost = ...; ... }
     => { selfBuildBuild = ...; selfBuildHost = ...; ... }
     ```
     :::
   */
-  renameSplicesTo = prefix: x: {
+  renameCrossIndexTo = prefix: x: {
     "${prefix}BuildBuild" = x.buildBuild;
     "${prefix}BuildHost" = x.buildHost;
     "${prefix}BuildTarget" = x.buildTarget;
@@ -939,34 +945,42 @@ rec {
   };
 
   /**
-    Takes a function and applies it pointwise to each splice.
+    Takes a function and applies it pointwise to each field of a cross index.
+
+    A cross index (short for "Cross Platform Pair Index") is a 6-field structure
+    organizing values by cross-compilation platform relationships.
 
     # Inputs
 
     `f`
-    : Function to apply to each splice value
+    : Function to apply to each cross index value
 
-    `splices`
-    : A set of splices to transform
+    `crossIndex`
+    : A cross index to transform
 
     # Type
 
     ```
-    mapSplices :: (a -> b) -> AttrSet -> AttrSet
+    mapCrossIndex :: (a -> b) -> AttrSet -> AttrSet
     ```
 
     # Examples
 
     :::{.example}
-    ## `lib.customisation.mapSplices` usage example
+    ## `lib.customisation.mapCrossIndex` usage example
 
     ```nix
-    mapSplices (x: x * 10) { buildBuild = 1; buildHost = 2; ... }
+    mapCrossIndex (x: x * 10) { buildBuild = 1; buildHost = 2; ... }
     => { buildBuild = 10; buildHost = 20; ... }
     ```
+
+    ```nix
+    # Extract a package from package sets
+    mapCrossIndex (pkgs: pkgs.hello) crossIndexedPackageSets
+    ```
     :::
   */
-  mapSplices =
+  mapCrossIndex =
     f:
     {
       buildBuild,

lib/default.nix

@@ -398,9 +398,9 @@ let
         makeScopeWithSplicing
         makeScopeWithSplicing'
         extendMkDerivation
-        renameSplicesFrom
-        renameSplicesTo
-        mapSplices
+        renameCrossIndexFrom
+        renameCrossIndexTo
+        mapCrossIndex
         ;
       inherit (self.derivations) lazyDerivation optionalDrvAttr warnOnInstantiate;
       inherit (self.generators) mkLuaInline;

lib/tests/misc.nix

@@ -4432,10 +4432,10 @@ runTests {
     expected = "/non-existent/this/does/not/exist/for/real/please-dont-mess-with-your-local-fs/default.nix";
   };
 
-  # Tests for splicing utilities
+  # Tests for cross index utilities
 
-  testRenameSplicesFrom = {
-    expr = lib.renameSplicesFrom "pkgs" {
+  testRenameCrossIndexFrom = {
+    expr = lib.renameCrossIndexFrom "pkgs" {
       pkgsBuildBuild = "dummy-build-build";
       pkgsBuildHost = "dummy-build-host";
       pkgsBuildTarget = "dummy-build-target";
@@ -4453,8 +4453,8 @@ runTests {
     };
   };
 
-  testRenameSplicesTo = {
-    expr = lib.renameSplicesTo "self" {
+  testRenameCrossIndexTo = {
+    expr = lib.renameCrossIndexTo "self" {
       buildBuild = "dummy-build-build";
       buildHost = "dummy-build-host";
       buildTarget = "dummy-build-target";
@@ -4472,8 +4472,8 @@ runTests {
     };
   };
 
-  testMapSplices = {
-    expr = lib.mapSplices (x: x * 10) {
+  testMapCrossIndex = {
+    expr = lib.mapCrossIndex (x: x * 10) {
       buildBuild = 1;
       buildHost = 2;
       buildTarget = 3;
@@ -4491,8 +4491,8 @@ runTests {
     };
   };
 
-  testMapSplicesString = {
-    expr = lib.mapSplices (x: "prefix-${x}") {
+  testMapCrossIndexString = {
+    expr = lib.mapCrossIndex (x: "prefix-${x}") {
       buildBuild = "bb";
       buildHost = "bh";
       buildTarget = "bt";