Merge pull request #4093 from matthewbauer/eval-system

Add eval-system option

(cherry picked from commit 071dbbee33af9f27338c3e53e4ea067dbfa14010)
Change-Id: Ia81358c8cfb60241da07a4d0e84b9ee62a18a53f

src/libexpr/eval-settings.cc

@@ -88,6 +88,12 @@ std::string EvalSettings::resolvePseudoUrl(std::string_view url)
         return std::string(url);
 }
 
+const std::string & EvalSettings::getCurrentSystem()
+{
+    const auto & evalSystem = currentSystem.get();
+    return evalSystem != "" ? evalSystem : settings.thisSystem.get();
+}
+
 EvalSettings evalSettings;
 
 static GlobalConfig::Register rEvalSettings(&evalSettings);

src/libexpr/eval-settings.hh

@@ -25,6 +25,26 @@ struct EvalSettings : Config
           [`builtins.nixPath`](@docroot@/language/builtin-constants.md#builtins-nixPath).
         )"};
 
+    Setting<std::string> currentSystem{
+        this, "", "eval-system",
+        R"(
+          This option defines
+          [`builtins.currentSystem`](@docroot@/language/builtin-constants.md#builtins-currentSystem)
+          in the Nix language if it is set as a non-empty string.
+          Otherwise, if it is defined as the empty string (the default), the value of the
+          [`system` ](#conf-system)
+          configuration setting is used instead.
+
+          Unlike `system`, this setting does not change what kind of derivations can be built locally.
+          This is useful for evaluating Nix code on one system to produce derivations to be built on another type of system.
+        )"};
+
+    /**
+     * Implements the `eval-system` vs `system` defaulting logic
+     * described for `eval-system`.
+     */
+    const std::string & getCurrentSystem();
+
     Setting<bool> restrictEval{
         this, false, "restrict-eval",
         R"(

src/libexpr/primops.cc

@@ -4288,13 +4288,16 @@ void EvalState::createBaseEnv()
         .impureOnly = true,
     });
 
-    if (!evalSettings.pureEval) {
-        v.mkString(settings.thisSystem.get());
-    }
+    if (!evalSettings.pureEval)
+        v.mkString(evalSettings.getCurrentSystem());
     addConstant("__currentSystem", v, {
         .type = nString,
         .doc = R"(
-          The value of the [`system` configuration option](@docroot@/command-ref/conf-file.md#conf-pure-eval).
+          The value of the
+          [`eval-system`](@docroot@/command-ref/conf-file.md#conf-eval-system)
+          or else
+          [`system`](@docroot@/command-ref/conf-file.md#conf-system)
+          configuration option.
 
           It can be used to set the `system` attribute for [`builtins.derivation`](@docroot@/language/derivations.md) such that the resulting derivation can be built on the same system that evaluates the Nix expression: