Factor out `nix-defexpr` path computation

Avoid duplicated code, and also avoid "on the fly" path construction
(which makes it harder to keep track of which paths we use).

The factored out code doesn't create the Nix state dir anymore, but this
is fine because other in nix-env and nix-channel does:

- nix-channel: Line 158 in this commit

- nix-env: Line 1407 in this commit

2 files changed, 13 insertions, 1 deletions

diff --git a/src/libexpr/eval-settings.cc b/src/libexpr/eval-settings.cc
index 422aaf8d5..93b4a5289 100644
--- a/src/libexpr/eval-settings.cc
+++ b/src/libexpr/eval-settings.cc
@@ -63,7 +63,7 @@ Strings EvalSettings::getDefaultNixPath()
     };
 
     if (!evalSettings.restrictEval && !evalSettings.pureEval) {
-        add(settings.useXDGBaseDirectories ? getStateDir() + "/nix/defexpr/channels" : getHome() + "/.nix-defexpr/channels");
+        add(getNixDefExpr() + "/channels");
         add(rootChannelsDir() + "/nixpkgs", "nixpkgs");
         add(rootChannelsDir());
     }
@@ -92,4 +92,11 @@ EvalSettings evalSettings;
 
 static GlobalConfig::Register rEvalSettings(&evalSettings);
 
+Path getNixDefExpr()
+{
+    return settings.useXDGBaseDirectories
+        ? getStateDir() + "/nix/defexpr"
+        : getHome() + "/.nix-defexpr";
+}
+
 }
diff --git a/src/libexpr/eval-settings.hh b/src/libexpr/eval-settings.hh
index 043af6cab..e6666061a 100644
--- a/src/libexpr/eval-settings.hh
+++ b/src/libexpr/eval-settings.hh
@@ -95,4 +95,9 @@ struct EvalSettings : Config
 
 extern EvalSettings evalSettings;
 
+/**
+ * Conventionally part of the default nix path in impure mode.
+ */
+Path getNixDefExpr();
+
 }