Make --read-only a separate mixin

It is independent of SourceExprCommand, which is about parsing
installables, except for the fact that parsing installables is one of
the many things influenced by read-only mode.

src/libcmd/command.hh

@@ -94,9 +94,8 @@ struct SourceExprCommand : virtual Args, MixFlakeOptions
 {
     std::optional<Path> file;
     std::optional<std::string> expr;
-    bool readOnlyMode = false;
 
-    SourceExprCommand(bool supportReadOnlyMode = false);
+    SourceExprCommand();
 
     std::vector<std::shared_ptr<Installable>> parseInstallables(
         ref<Store> store, std::vector<std::string> ss);
@@ -111,6 +110,11 @@ struct SourceExprCommand : virtual Args, MixFlakeOptions
     void completeInstallable(std::string_view prefix);
 };
 
+struct MixReadOnlyOption : virtual Args
+{
+    MixReadOnlyOption();
+};
+
 /* A command that operates on a list of "installables", which can be
    store paths, attribute paths, Nix expressions, etc. */
 struct InstallablesCommand : virtual Args, SourceExprCommand
@@ -136,7 +140,7 @@ struct InstallableCommand : virtual Args, SourceExprCommand
 {
     std::shared_ptr<Installable> installable;
 
-    InstallableCommand(bool supportReadOnlyMode = false);
+    InstallableCommand();
 
     void prepare() override;
 

src/libcmd/installables.cc

@@ -147,7 +147,7 @@ void MixFlakeOptions::completionHook()
         completeFlakeInput(*prefix);
 }
 
-SourceExprCommand::SourceExprCommand(bool supportReadOnlyMode)
+SourceExprCommand::SourceExprCommand()
 {
     addFlag({
         .longName = "file",
@@ -169,18 +169,19 @@ SourceExprCommand::SourceExprCommand(bool supportReadOnlyMode)
         .labels = {"expr"},
         .handler = {&expr}
     });
+}
 
-    if (supportReadOnlyMode) {
+MixReadOnlyOption::MixReadOnlyOption()
+{
     addFlag({
         .longName = "read-only",
         .description =
             "Do not instantiate each evaluated derivation. "
             "This improves performance, but can cause errors when accessing "
             "store paths of derivations during evaluation.",
-            .handler = {&readOnlyMode, true},
+        .handler = {&settings.readOnlyMode, true},
     });
 }
-}
 
 Strings SourceExprCommand::getDefaultFlakeAttrPaths()
 {
@@ -426,10 +427,6 @@ std::vector<std::shared_ptr<Installable>> SourceExprCommand::parseInstallables(
 {
     std::vector<std::shared_ptr<Installable>> result;
 
-    if (readOnlyMode) {
-        settings.readOnlyMode = true;
-    }
-
     if (file || expr) {
         if (file && expr)
             throw UsageError("'--file' and '--expr' are exclusive");
@@ -724,8 +721,8 @@ std::vector<std::string> InstallablesCommand::getFlakesForCompletion()
     return _installables;
 }
 
-InstallableCommand::InstallableCommand(bool supportReadOnlyMode)
+InstallableCommand::InstallableCommand()
-    : SourceExprCommand(supportReadOnlyMode)
+    : SourceExprCommand()
 {
     expectArgs({
         .label = "installable",

src/nix/eval.cc

@@ -11,13 +11,13 @@
 
 using namespace nix;
 
-struct CmdEval : MixJSON, InstallableCommand
+struct CmdEval : MixJSON, InstallableCommand, MixReadOnlyOption
 {
     bool raw = false;
     std::optional<std::string> apply;
     std::optional<Path> writeTo;
 
-    CmdEval() : InstallableCommand(true /* supportReadOnlyMode */)
+    CmdEval() : InstallableCommand()
     {
         addFlag({
             .longName = "raw",