repl: fix repl-overlays in pure eval mode

The reason this gets hit is because of the debugger in flakes. Otherwise
you never have a repl in pure mode anyway.

We evaluate the repl-overlay file in impure mode but this doesn't do
what one would initially expect.

Fixes: https://git.lix.systems/lix-project/lix/issues/777
Change-Id: I19b8ed2f5e9ce500b633b13301b42df69ab7deb3

doc/manual/rl-next/repl-overlays-pure.md

@@ -0,0 +1,20 @@
+---
+synopsis: "repl-overlays now work in the debugger for flakes"
+issues: [fj#777]
+cls: [3398]
+category: Fixes
+credits: [jade]
+---
+Due to a bug, it was previously not possible to use the debugger on flakes with repl-overlays, or with pure evaluation in general:
+
+```
+$ nix repl --pure-eval
+Lix 2.94.0-dev-pre20250617-87d99da
+Type :? for help.
+Loading 'repl-overlays'...
+error: access to absolute path '/Users/jade/.config/nix/repl.nix' is forbidden in pure eval mode (use '--impure' to override)
+```
+
+This is now fixed.
+The contents of the repl-overlays file itself (i.e. most typically the top level lambda in it) will be evaluated in impure mode.
+It may be necessary to use `builtins.seq` to force the impure operations to happen first if one wants to do impure operations inside a repl-overlays file in pure evaluation mode.

lix/libcmd/repl.cc

@@ -3,7 +3,6 @@
 #include <iostream>
 #include <cstdlib>
 #include <cstring>
-#include <climits>
 #include <string_view>
 
 #include "lix/libutil/box_ptr.hh"
@@ -1008,7 +1007,15 @@ Value * NixRepl::replOverlays()
     for (auto path : evalSettings.replOverlays.get()) {
         debug("Loading '%1%' path '%2%'...", "repl-overlays", path);
         SourcePath sourcePath((CanonPath(path)));
+
+        // XXX(jade): This is a somewhat unsatisfying solution to
+        // https://git.lix.systems/lix-project/lix/issues/777 which means that
+        // the top level item in the repl-overlays file (that is, the lambda)
+        // gets evaluated with pure eval off. This means that if you want to do
+        // impure eval stuff, you will have to force it with builtins.seq.
+        bool prevPureEval = evalSettings.pureEval.get();
         auto replInit = evalFile(sourcePath);
+        evalSettings.pureEval.setDefault(prevPureEval);
 
         if (!replInit->isLambda()) {
             evaluator.errors.make<TypeError>(

tests/functional/repl_characterization/data/extra_data/repl-overlay-trivial.nix

@@ -0,0 +1,3 @@
+info: final: prev: {
+  foo = 2;
+}

tests/functional/repl_characterization/data/repl_overlays_regression_l777.nix

@@ -0,0 +1,3 @@
+{
+  cat.meowy = true;
+}

tests/functional/repl_characterization/data/repl_overlays_regression_l777.test

@@ -0,0 +1,9 @@
+Checks that repl-overlays work correctly in pure eval mode
+@args --pure-eval
+@args --repl-overlays
+@args ${PWD}/extra_data/repl-overlay-trivial.nix
+
+  nix-repl> foo
+  2
+
+  nix-repl> :quit

tests/functional/repl_characterization/repl_characterization.cc

@@ -184,6 +184,7 @@ REPL_TEST(regression_l145);
 REPL_TEST(regression_l592);
 REPL_TEST(repl_input);
 REPL_TEST(repl_overlays);
+REPL_TEST(repl_overlays_regression_l777);
 REPL_TEST(repl_overlays_compose);
 REPL_TEST(repl_overlays_destructure_without_dotdotdot_errors);
 REPL_TEST(repl_overlays_destructure_without_formals_ok);