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",
@@ -169,17 +169,18 @@ SourceExprCommand::SourceExprCommand(bool supportReadOnlyMode)
  .labels = {"expr"},
  .handler = {&expr}
  });
+}
 
- if (supportReadOnlyMode) {
- 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},
- });
- }
+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 = {&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)
- : SourceExprCommand(supportReadOnlyMode)
+InstallableCommand::InstallableCommand()
+ : 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",