Add `repl-overlays`

Adds a `repl-overlays` option, which specifies files that can overlay
and modify the top-level bindings in `nix repl`. For example, with the
following contents in `~/.config/nix/repl.nix`:

    info: final: prev: let
      optionalAttrs = predicate: attrs:
        if predicate
        then attrs
        else {};
    in
      optionalAttrs (prev ? legacyPackages && prev.legacyPackages ? ${info.currentSystem})
      {
        pkgs = prev.legacyPackages.${info.currentSystem};
      }

We can run `nix repl` and use `pkgs` to refer to `legacyPackages.${currentSystem}`:

    $ nix repl --repl-overlays ~/.config/nix/repl.nix nixpkgs
    Lix 2.90.0
    Type :? for help.
    Loading installable 'flake:nixpkgs#'...
    Added 5 variables.
    Loading 'repl-overlays'...
    Added 6 variables.
    nix-repl> pkgs.bash
    «derivation /nix/store/g08b5vkwwh0j8ic9rkmd8mpj878rk62z-bash-5.2p26.drv»

Change-Id: Ic12e0f2f210b2f46e920c33088dfe1083f42391a

19 files changed, 105 insertions, 1 deletions

diff --git a/tests/functional/repl_characterization/data/extra_data/repl-overlay-fail.nix b/tests/functional/repl_characterization/data/extra_data/repl-overlay-fail.nix
new file mode 100644
index 000000000..426127916
--- /dev/null
+++ b/tests/functional/repl_characterization/data/extra_data/repl-overlay-fail.nix
@@ -0,0 +1 @@
+info: final: prev: builtins.abort "uh oh!"
diff --git a/tests/functional/repl_characterization/data/extra_data/repl-overlay-no-dotdotdot.nix b/tests/functional/repl_characterization/data/extra_data/repl-overlay-no-dotdotdot.nix
new file mode 100644
index 000000000..e242d644d
--- /dev/null
+++ b/tests/functional/repl_characterization/data/extra_data/repl-overlay-no-dotdotdot.nix
@@ -0,0 +1,6 @@
+let
+  puppy = "doggy";
+in
+  {currentSystem}: final: prev: {
+    inherit puppy;
+  }
diff --git a/tests/functional/repl_characterization/data/extra_data/repl-overlay-no-formals.nix b/tests/functional/repl_characterization/data/extra_data/repl-overlay-no-formals.nix
new file mode 100644
index 000000000..cf31550c0
--- /dev/null
+++ b/tests/functional/repl_characterization/data/extra_data/repl-overlay-no-formals.nix
@@ -0,0 +1 @@
+info: final: prev: {}
diff --git a/tests/functional/repl_characterization/data/extra_data/repl-overlay-packages-is-pkgs.nix b/tests/functional/repl_characterization/data/extra_data/repl-overlay-packages-is-pkgs.nix
new file mode 100644
index 000000000..57895a97f
--- /dev/null
+++ b/tests/functional/repl_characterization/data/extra_data/repl-overlay-packages-is-pkgs.nix
@@ -0,0 +1,4 @@
+info: final: prev:
+{
+  pkgs = final.packages.x86_64-linux;
+}
diff --git a/tests/functional/repl_characterization/data/extra_data/repl-overlays-compose-1.nix b/tests/functional/repl_characterization/data/extra_data/repl-overlays-compose-1.nix
new file mode 100644
index 000000000..055e9ea0f
--- /dev/null
+++ b/tests/functional/repl_characterization/data/extra_data/repl-overlays-compose-1.nix
@@ -0,0 +1,7 @@
+info: final: prev:
+{
+  var = prev.var + "b";
+
+  # We can access the final value of `var` here even though it isn't defined yet:
+  varUsingFinal = "final value is: " + final.newVar;
+}
diff --git a/tests/functional/repl_characterization/data/extra_data/repl-overlays-compose-2.nix b/tests/functional/repl_characterization/data/extra_data/repl-overlays-compose-2.nix
new file mode 100644
index 000000000..e88407311
--- /dev/null
+++ b/tests/functional/repl_characterization/data/extra_data/repl-overlays-compose-2.nix
@@ -0,0 +1,6 @@
+info: final: prev:
+{
+  var = prev.var + "c";
+
+  newVar = "puppy";
+}
diff --git a/tests/functional/repl_characterization/data/repl_overlays.nix b/tests/functional/repl_characterization/data/repl_overlays.nix
new file mode 100644
index 000000000..09f115f03
--- /dev/null
+++ b/tests/functional/repl_characterization/data/repl_overlays.nix
@@ -0,0 +1,3 @@
+{
+  packages.x86_64-linux.default = "my package";
+}
diff --git a/tests/functional/repl_characterization/data/repl_overlays.test b/tests/functional/repl_characterization/data/repl_overlays.test
new file mode 100644
index 000000000..56f1392e1
--- /dev/null
+++ b/tests/functional/repl_characterization/data/repl_overlays.test
@@ -0,0 +1,5 @@
+Check basic `repl-overlays` functionality.
+@args --repl-overlays
+@args ${PWD}/extra_data/repl-overlay-packages-is-pkgs.nix
+  nix-repl> pkgs
+  { default = "my package"; }
diff --git a/tests/functional/repl_characterization/data/repl_overlays_compose.nix b/tests/functional/repl_characterization/data/repl_overlays_compose.nix
new file mode 100644
index 000000000..aa6596dc7
--- /dev/null
+++ b/tests/functional/repl_characterization/data/repl_overlays_compose.nix
@@ -0,0 +1,3 @@
+{
+  var = "a";
+}
diff --git a/tests/functional/repl_characterization/data/repl_overlays_compose.test b/tests/functional/repl_characterization/data/repl_overlays_compose.test
new file mode 100644
index 000000000..b674a55be
--- /dev/null
+++ b/tests/functional/repl_characterization/data/repl_overlays_compose.test
@@ -0,0 +1,7 @@
+Check that multiple `repl-overlays` can compose together
+@args --repl-overlays
+@args "${PWD}/extra_data/repl-overlays-compose-1.nix ${PWD}/extra_data/repl-overlays-compose-2.nix"
+  nix-repl> var
+  "abc"
+  nix-repl> varUsingFinal
+  "final value is: puppy"
diff --git a/tests/functional/repl_characterization/data/repl_overlays_destructure_without_dotdotdot_errors.test b/tests/functional/repl_characterization/data/repl_overlays_destructure_without_dotdotdot_errors.test
new file mode 100644
index 000000000..daf1f27bd
--- /dev/null
+++ b/tests/functional/repl_characterization/data/repl_overlays_destructure_without_dotdotdot_errors.test
@@ -0,0 +1,10 @@
+`repl-overlays` that try to parse out the `info` argument without a `...` error.
+@args --repl-overlays
+@args ${PWD}/extra_data/repl-overlay-no-dotdotdot.nix
+@should-start false
+  error: Expected first argument of repl-overlays to have ... to allow future versions of Lix to add additional attributes to the argument
+         at $TEST_DATA/extra_data/repl-overlay-no-dotdotdot.nix:4:3:
+              3| in
+              4|   {currentSystem}: final: prev: {
+               |   ^
+              5|     inherit puppy;\n
diff --git a/tests/functional/repl_characterization/data/repl_overlays_destructure_without_formals_ok.test b/tests/functional/repl_characterization/data/repl_overlays_destructure_without_formals_ok.test
new file mode 100644
index 000000000..3aa89e434
--- /dev/null
+++ b/tests/functional/repl_characterization/data/repl_overlays_destructure_without_formals_ok.test
@@ -0,0 +1,5 @@
+`repl-overlays` that don't destructure the `info` argument are OK.
+@args --repl-overlays
+@args ${PWD}/extra_data/repl-overlay-no-formals.nix
+  nix-repl> 1
+  1
diff --git a/tests/functional/repl_characterization/data/repl_overlays_error.test b/tests/functional/repl_characterization/data/repl_overlays_error.test
new file mode 100644
index 000000000..2d8702df1
--- /dev/null
+++ b/tests/functional/repl_characterization/data/repl_overlays_error.test
@@ -0,0 +1,22 @@
+`repl-overlays` that fail to evaluate should error.
+@args --repl-overlays
+@args ${PWD}/extra_data/repl-overlay-fail.nix
+@should-start false
+  error:
+         … while calling the 'foldl'' builtin
+           at «string»:5:13:
+              4| functions:
+              5| let final = builtins.foldl'
+               |             ^
+              6|               (prev: function: prev // (function info final prev))
+
+         … in the right operand of the update (//) operator
+           at «string»:6:37:
+              5| let final = builtins.foldl'
+              6|               (prev: function: prev // (function info final prev))
+               |                                     ^
+              7|               initial
+
+         (stack trace truncated; use '--show-trace' to show the full trace)
+
+         error: evaluation aborted with the following error message: 'uh oh!'
diff --git a/tests/functional/repl_characterization/extra_data/repl-overlay-fail.nix b/tests/functional/repl_characterization/extra_data/repl-overlay-fail.nix
new file mode 100644
index 000000000..426127916
--- /dev/null
+++ b/tests/functional/repl_characterization/extra_data/repl-overlay-fail.nix
@@ -0,0 +1 @@
+info: final: prev: builtins.abort "uh oh!"
diff --git a/tests/functional/repl_characterization/extra_data/repl-overlay-no-dotdotdot.nix b/tests/functional/repl_characterization/extra_data/repl-overlay-no-dotdotdot.nix
new file mode 100644
index 000000000..1cced3324
--- /dev/null
+++ b/tests/functional/repl_characterization/extra_data/repl-overlay-no-dotdotdot.nix
@@ -0,0 +1 @@
+{currentSystem}: final: prev: {}
diff --git a/tests/functional/repl_characterization/extra_data/repl-overlay-packages-is-pkgs.nix b/tests/functional/repl_characterization/extra_data/repl-overlay-packages-is-pkgs.nix
new file mode 100644
index 000000000..57895a97f
--- /dev/null
+++ b/tests/functional/repl_characterization/extra_data/repl-overlay-packages-is-pkgs.nix
@@ -0,0 +1,4 @@
+info: final: prev:
+{
+  pkgs = final.packages.x86_64-linux;
+}
diff --git a/tests/functional/repl_characterization/extra_data/repl-overlays-compose-1.nix b/tests/functional/repl_characterization/extra_data/repl-overlays-compose-1.nix
new file mode 100644
index 000000000..055e9ea0f
--- /dev/null
+++ b/tests/functional/repl_characterization/extra_data/repl-overlays-compose-1.nix
@@ -0,0 +1,7 @@
+info: final: prev:
+{
+  var = prev.var + "b";
+
+  # We can access the final value of `var` here even though it isn't defined yet:
+  varUsingFinal = "final value is: " + final.newVar;
+}
diff --git a/tests/functional/repl_characterization/extra_data/repl-overlays-compose-2.nix b/tests/functional/repl_characterization/extra_data/repl-overlays-compose-2.nix
new file mode 100644
index 000000000..e88407311
--- /dev/null
+++ b/tests/functional/repl_characterization/extra_data/repl-overlays-compose-2.nix
@@ -0,0 +1,6 @@
+info: final: prev:
+{
+  var = prev.var + "c";
+
+  newVar = "puppy";
+}
diff --git a/tests/functional/repl_characterization/repl_characterization.cc b/tests/functional/repl_characterization/repl_characterization.cc
index ae4d4030f..f3c6065aa 100644
--- a/tests/functional/repl_characterization/repl_characterization.cc
+++ b/tests/functional/repl_characterization/repl_characterization.cc
@@ -184,7 +184,12 @@ REPL_TEST(no_nested_debuggers);
 REPL_TEST(regression_9917);
 REPL_TEST(regression_9918);
 REPL_TEST(regression_l145);
-REPL_TEST(stack_vars);
+REPL_TEST(repl_overlays);
+REPL_TEST(repl_overlays_compose);
+REPL_TEST(repl_overlays_destructure_without_dotdotdot_errors);
+REPL_TEST(repl_overlays_destructure_without_formals_ok);
+REPL_TEST(repl_overlays_error);
 REPL_TEST(repl_printing);
+REPL_TEST(stack_vars);
 
 }; // namespace nix