refactor(nix): Move `mkLabel` from buildkite to readTree r/3774

This function is more generically useful than just for pipeline
construction.

A subsequent commit will use it inside of readTree itself.

Change-Id: I5eabd6f659726484667e060958865dddbc205762
Reviewed-on: https://cl.tvl.fyi/c/depot/+/5237
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>

3 files changed, 15 insertions, 10 deletions

diff --git a/nix/buildkite/default.nix b/nix/buildkite/default.nix
index d299d526241b..eb8061cf6b09 100644
--- a/nix/buildkite/default.nix
+++ b/nix/buildkite/default.nix
@@ -6,7 +6,7 @@
 #
 # The structure of the file that is being created is documented here:
 #   https://buildkite.com/docs/pipelines/defining-steps
-{ pkgs, ... }:
+{ depot, pkgs, ... }:
 
 let
   inherit (builtins)
@@ -29,6 +29,7 @@ let
     unsafeDiscardStringContext;
 
   inherit (pkgs) lib runCommandNoCC writeText;
+  inherit (depot.nix.readTree) mkLabel;
 in
 rec {
   # Creates a Nix expression that yields the target at the specified
@@ -46,13 +47,6 @@ rec {
     in
     if target ? __subtarget then subtargetExpr else targetExpr;
 
-  # Create a pipeline label from the target's tree location.
-  mkLabel = target:
-    let label = concatStringsSep "/" target.__readTree;
-    in if target ? __subtarget
-    then "${label}:${target.__subtarget}"
-    else label;
-
   # Determine whether to skip a target if it has not diverged from the
   # HEAD branch.
   shouldSkip = parentTargetMap: label: drvPath:
diff --git a/nix/readTree/default.nix b/nix/readTree/default.nix
index 22815a44c4ee..5d2e1fff6aa8 100644
--- a/nix/readTree/default.nix
+++ b/nix/readTree/default.nix
@@ -58,6 +58,13 @@ let
     __readTreeChildren = builtins.attrNames children;
   };
 
+  # Create a label from a target's tree location.
+  mkLabel = target:
+    let label = concatStringsSep "/" target.__readTree;
+    in if target ? __subtarget
+    then "${label}:${target.__subtarget}"
+    else label;
+
   # Merge two attribute sets, but place attributes in `passthru` via
   # `overrideAttrs` for derivation targets that support it.
   merge = a: b:
@@ -188,7 +195,7 @@ let
   isDerivation = x: isAttrs x && x ? type && x.type == "derivation";
 in
 {
-  inherit gather;
+  inherit gather mkLabel;
 
   __functor = _:
     { path
diff --git a/views/kit/default.nix b/views/kit/default.nix
index ba46b719e286..0548fb7bbb7b 100644
--- a/views/kit/default.nix
+++ b/views/kit/default.nix
@@ -15,9 +15,13 @@
 
 pkgs.lib.fix (self: {
   buildGo = import ./buildGo { inherit pkgs; };
-  buildkite = import ./buildkite { inherit pkgs; };
   readTree = import ./readTree { };
 
+  buildkite = import ./buildkite {
+    inherit pkgs;
+    depot.nix.readTree = self.readTree;
+  };
+
   besadii = import ./besadii {
     depot.nix.buildGo = self.buildGo;
   };