repl: --option pure-eval true actually enables pure eval mode

To quote Eelco in #5867:

> Unfortunately we can't do
>
>     evalSettings.pureEval.setDefault(false);
>
> because then we have to do the same in main.cc (where
> pureEval is set to true), and that would allow pure-eval
> to be disabled globally from nix.conf.

Instead, a command should specify that it should be impure by
default. Then, `evalSettings.pureEval` will be set to `false;` unless
it's overridden by e.g. a CLI flag.

In that case it's IMHO OK to be (theoretically) able to override
`pure-eval` via `nix.conf` because it doesn't have an effect on commands
where `forceImpureByDefault` returns `false` (i.e. everything where pure
eval actually matters).

Closes #5867

src/libcmd/repl.cc

@@ -1039,6 +1039,11 @@ struct CmdRepl : StoreCommand, MixEvalArgs
         });
     }
 
+    bool forceImpureByDefault() override
+    {
+        return true;
+    }
+
     std::string description() override
     {
         return "start an interactive environment for evaluating Nix expressions";
@@ -1053,8 +1058,6 @@ struct CmdRepl : StoreCommand, MixEvalArgs
 
     void run(ref<Store> store) override
     {
-        evalSettings.pureEval = false;
-
         auto evalState = make_ref<EvalState>(searchPath, store);
 
         auto repl = std::make_unique<NixRepl>(evalState);

src/libutil/args.hh

@@ -25,6 +25,8 @@ public:
     /* Return a short one-line description of the command. */
     virtual std::string description() { return ""; }
 
+    virtual bool forceImpureByDefault() { return false; }
+
     /* Return documentation about this command, in Markdown format. */
     virtual std::string doc() { return ""; }
 

src/nix/main.cc

@@ -380,6 +380,9 @@ void mainWrapped(int argc, char * * argv)
         settings.ttlPositiveNarInfoCache = 0;
     }
 
+    if (args.command->second->forceImpureByDefault() && !evalSettings.pureEval.overridden) {
+        evalSettings.pureEval = false;
+    }
     args.command->second->prepare();
     args.command->second->run();
 }

tests/repl.sh

@@ -42,6 +42,11 @@ testRepl () {
     echo "$replOutput"
     echo "$replOutput" | grep -qs "while evaluating the file" \
       || fail "nix repl --show-trace doesn't show the trace"
+
+    nix repl "${nixArgs[@]}" --option pure-eval true 2>&1 <<< "builtins.currentSystem" \
+      | grep "attribute 'currentSystem' missing"
+    nix repl "${nixArgs[@]}" 2>&1 <<< "builtins.currentSystem" \
+      | grep "$(nix-instantiate --eval -E 'builtins.currentSystem')"
 }
 
 # Simple test, try building a drv