add automated usage mode to the repl

This is definitely not a stable thing, but it does feel slightly crimes
to put it as an experimental feature. Shrug, up for bikeshedding.

src/libcmd/repl-interacter.cc

@@ -1,4 +1,6 @@
 #include <cstdio>
+#include <iostream>
+#include <string>
 
 #ifdef USE_READLINE
 #include <readline/history.h>
@@ -183,4 +185,24 @@ ReadlineLikeInteracter::~ReadlineLikeInteracter()
     write_history(historyFile.c_str());
 }
 
+AutomationInteracter::Guard AutomationInteracter::init(detail::ReplCompleterMixin *)
+{
+    return Guard([] {});
+}
+
+// ASCII ENQ character
+constexpr const char * automationPrompt = "\x05";
+
+bool AutomationInteracter::getLine(std::string & input, ReplPromptType promptType)
+{
+    std::cout << std::unitbuf;
+    std::cout << automationPrompt;
+    if (!std::getline(std::cin, input)) {
+        // reset failure bits on EOF
+        std::cin.clear();
+        return false;
+    }
+    return true;
+}
+
 };

src/libcmd/repl-interacter.hh

@@ -45,4 +45,13 @@ public:
     virtual ~ReadlineLikeInteracter() override;
 };
 
+class AutomationInteracter : public virtual ReplInteracter
+{
+public:
+    AutomationInteracter() = default;
+    virtual Guard init(detail::ReplCompleterMixin * repl) override;
+    virtual bool getLine(std::string & input, ReplPromptType promptType) override;
+    virtual ~AutomationInteracter() override = default;
+};
+
 };

src/libcmd/repl.cc

@@ -122,14 +122,20 @@ std::string removeWhitespace(std::string s)
     return s;
 }
 
+static box_ptr<ReplInteracter> makeInteractor() {
+    if (experimentalFeatureSettings.isEnabled(Xp::ReplAutomation))
+        return make_box_ptr<AutomationInteracter>();
+    else
+        return make_box_ptr<ReadlineLikeInteracter>(getDataDir() + "/nix/repl-history");
+}
 
 NixRepl::NixRepl(const SearchPath & searchPath, nix::ref<Store> store, ref<EvalState> state,
             std::function<NixRepl::AnnotatedValues()> getValues)
     : AbstractNixRepl(state)
     , debugTraceIndex(0)
     , getValues(getValues)
     , staticEnv(new StaticEnv(nullptr, state->staticBaseEnv.get()))
-    , interacter(make_box_ptr<ReadlineLikeInteracter>(getDataDir() + "/nix/repl-history"))
+    , interacter(makeInteractor())
 {
 }
 

src/libutil/experimental-features.cc

@@ -22,7 +22,7 @@ struct ExperimentalFeatureDetails
  * feature, we either have no issue at all if few features are not added
  * at the end of the list, or a proper merge conflict if they are.
  */
-constexpr size_t numXpFeatures = 1 + static_cast<size_t>(Xp::VerifiedFetches);
+constexpr size_t numXpFeatures = 1 + static_cast<size_t>(Xp::ReplAutomation);
 
 constexpr std::array<ExperimentalFeatureDetails, numXpFeatures> xpFeatureDetails = {{
     {
@@ -275,6 +275,13 @@ constexpr std::array<ExperimentalFeatureDetails, numXpFeatures> xpFeatureDetails
             Enables verification of git commit signatures through the [`fetchGit`](@docroot@/language/builtins.md#builtins-fetchGit) built-in.
         )",
     },
+    {
+        .tag = Xp::ReplAutomation,
+        .name = "repl-automation",
+        .description = R"(
+            Makes the repl not use readline/editline, print ENQ (U+0005) when ready for a command, and take commands followed by newline.
+        )",
+    },
 }};
 
 static_assert(

src/libutil/experimental-features.hh

@@ -36,6 +36,7 @@ enum struct ExperimentalFeature
     ConfigurableImpureEnv,
     MountedSSHStore,
     VerifiedFetches,
+    ReplAutomation,
 };
 
 /**