feat(readTree): Add special here argument.

It's convenient for depending on sub-tree members of the current file as
well as gives access to siblings.

Change-Id: I74234cec6566177d88d3bc8507fa3f6ec789adb8
Reviewed-on: https://cl.snix.dev/c/snix/+/30098
Reviewed-by: adis bladis <adisbladis@gmail.com>
Tested-by: besadii

nix/readTree/README.md

@@ -84,9 +84,15 @@ the tree as empty nodes (`{}`).
   imported files via `builtins.scopedImport`. This will forcefully
   override the given values in the import scope, use with care!
 
-The package headers in this repository follow the form `{ pkgs, ... }:` where
-`pkgs` is a fixed-point of the entire package tree (see the `default.nix` at the
-root of the depot).
+The package headers in this repository follow the form
+`{depot, pkgs, lib, here, ... }:` where:
+
+* `depot` is a fixed-point of the entire package tree (see the `default.nix`
+  at the root of the depot).
+* `pkgs` is the nixpkgs used in the repo, see `third_party/nixpkgs`
+* `lib` is essentially a shortcut to `pkgs.lib` exposed for convenience
+* `here` is a special argument that points to the current location in the
+  tree. Useful to avoid specifying dependencies from the very top of the `depot`
 
 In theory `readTree` can pass arguments of different shapes, but I have found
 this to be a good solution for the most part.

nix/readTree/default.nix

@@ -125,7 +125,7 @@ let
       self =
         if rootDir
         then { __readTree = [ ]; }
-        else importFile args scopedArgs initPath parts argsFilter;
+        else importFile (args // { here = result; }) scopedArgs initPath parts argsFilter;
 
       # Import subdirectories of the current one, unless any skip
       # instructions exist.
@@ -162,7 +162,7 @@ let
           let
             p = joinChild (c + ".nix");
             childParts = parts ++ [ c ];
-            imported = importFile args scopedArgs p childParts argsFilter;
+            imported = importFile (args // { here = result; }) scopedArgs p childParts argsFilter;
           in
           {
             name = c;
@@ -181,14 +181,16 @@ let
         else nixChildren ++ children
       );
 
+      result =
+        if isAttrs nodeValue
+        then merge nodeValue (allChildren // (marker parts allChildren))
+        else nodeValue;
+
     in
     if skipTree
     then { skip = true; }
     else {
-      ok =
-        if isAttrs nodeValue
-        then merge nodeValue (allChildren // (marker parts allChildren))
-        else nodeValue;
+      ok = result;
     };
 
   # Top-level implementation of readTree itself.

nix/readTree/tests/default.nix

@@ -134,6 +134,13 @@ let
     (assertEq "default.nix drv is not changed by readTree"
       tree-tl.default-nix.can-be-drv
       (import ./test-tree-traversal/default-nix/can-be-drv/default.nix { }))
+    (assertEq "`here` argument represents the attrset a given file is part of"
+      (builtins.removeAttrs tree-tl.here-arg [ "__readTree" "__readTreeChildren" "subdir" ])
+      {
+        attr1 = "foo";
+        attr2 = "foo";
+        attr3 = "sibl1";
+      })
   ];
 
   # these each call readTree themselves because the throws have to happen inside assertThrows

nix/readTree/tests/test-tree-traversal/here-arg/default.nix

@@ -0,0 +1,6 @@
+{ here, ... }: {
+  attr1 = "foo";
+  attr2 = here.attr1;
+
+  attr3 = here.subdir.sibl2;
+}

nix/readTree/tests/test-tree-traversal/here-arg/subdir/sibl1.nix

@@ -0,0 +1 @@
+{ ... }: "sibl1"

nix/readTree/tests/test-tree-traversal/here-arg/subdir/sibl2.nix

@@ -0,0 +1,2 @@
+{ here, ... }:
+here.sibl1

snix/default.nix

@@ -1,10 +1,10 @@
 # Nix helpers for projects under //snix
-{ pkgs, lib, depot, ... }:
+{ pkgs, lib, depot, here, ... }:
 
 let
   # Load the crate2nix crate tree.
   crates = pkgs.callPackage ./Cargo.nix {
-    defaultCrateOverrides = depot.snix.utils.defaultCrateOverridesForPkgs pkgs;
+    defaultCrateOverrides = here.utils.defaultCrateOverridesForPkgs pkgs;
   };
 
   # Cargo dependencies to be used with nixpkgs rustPlatform functions.
@@ -30,9 +30,9 @@ let
   protos = pkgs.symlinkJoin {
     name = "snix-all-protos";
     paths = [
-      depot.snix.build.protos.protos
-      depot.snix.castore.protos.protos
-      depot.snix.store.protos.protos
+      here.build.protos.protos
+      here.castore.protos.protos
+      here.store.protos.protos
     ];
   };
 
@@ -112,7 +112,7 @@ in
 
   crate2nix-check =
     let
-      crate2nix-check = depot.snix.utils.mkCrate2nixCheck ./Cargo.nix;
+      crate2nix-check = here.utils.mkCrate2nixCheck ./Cargo.nix;
     in
     crate2nix-check.command.overrideAttrs {
       meta.ci.extraSteps = {