nix repl: Print which variables are just loaded

When we run `nix repl nixpkgs` we get "Added 6 variables". This is not
useful as it doesn't tell us which variables the flake has exported to
our global repl scope.

This patch prints the name of each variable that was just loaded. We
currently cap printing to 20 variables in order to avoid excessive
prints.

https://github.com/NixOS/nix/issues/11404

src/libcmd/repl.cc

@@ -813,6 +813,25 @@ void NixRepl::addAttrsToScope(Value & attrs)
     staticEnv->sort();
     staticEnv->deduplicate();
     notice("Added %1% variables.", attrs.attrs()->size());
+
+    const int max_print = 20;
+    int counter = 0;
+    std::ostringstream loaded;
+    for (auto & i : attrs.attrs()->lexicographicOrder(state->symbols)) {
+        if (counter >= max_print)
+            break;
+
+        if (counter > 0)
+            loaded << ", ";
+
+        printIdentifier(loaded, state->symbols[i->name]);
+        counter += 1;
+    }
+
+    notice("%1%", loaded.str());
+
+    if (attrs.attrs()->size() > max_print)
+        notice("... and %1% more", attrs.attrs()->size() - max_print);
 }
 
 

tests/functional/repl.sh

@@ -157,6 +157,32 @@ foo + baz
 ' "3" \
     ./flake ./flake\#bar --experimental-features 'flakes'
 
+testReplResponse $'
+:a { a = 1; b = 2; longerName = 3; "with spaces" = 4; }
+' 'Added 4 variables.
+a, b, longerName, "with spaces"
+'
+
+cat <<EOF > attribute-set.nix
+{
+    a = 1;
+    b = 2;
+    longerName = 3;
+    "with spaces" = 4;
+}
+EOF
+testReplResponse '
+:l ./attribute-set.nix
+' 'Added 4 variables.
+a, b, longerName, "with spaces"
+'
+
+testReplResponseNoRegex $'
+:a builtins.foldl\' (x: y: x // y) {} (map (x: { ${builtins.toString x} = x; }) (builtins.genList (x: x) 23))
+' 'Added 23 variables.
+"0", "1", "10", "11", "12", "13", "14", "15", "16", "17", "18", "19", "2", "20", "21", "22", "3", "4", "5", "6"
+... and 3 more'
+
 # Test the `:reload` mechansim with flakes:
 # - Eval `./flake#changingThing`
 # - Modify the flake
@@ -322,7 +348,8 @@ runRepl () {
       -e "s@$testDir@/path/to/tests/functional@g" \
       -e "s@$testDirNoUnderscores@/path/to/tests/functional@g" \
       -e "s@$nixVersion@<nix version>@g" \
-      -e "s@Added [0-9]* variables@Added <number omitted> variables@g" \
+      -e "/Added [0-9]* variables/{s@ [0-9]* @ <number omitted> @;n;d}" \
+      -e '/\.\.\. and [0-9]* more/d' \
     | grep -vF $'warning: you don\'t have Internet access; disabling some network-dependent features' \
     ;
 }