lib/strings.escapeNixIdentifier: quote keywords

Previously `lib.strings.escapeNixIdentifier "assert"` would return `"assert"`; This is wrong, `assert` is not a valid identifier unless quoted.\

This also fixes `lib.generators.toPretty`: `toPretty {} { "assert" = false; }` would previously return an invalid expression.

lib/strings.nix

@@ -1432,9 +1432,27 @@ rec {
     :::
   */
   escapeNixIdentifier =
+    let
+      # see https://nix.dev/manual/nix/2.26/language/identifiers#keywords
+      nixKeywords = [
+        "assert"
+        "else"
+        "if"
+        "in"
+        "inherit"
+        "let"
+        "or"
+        "rec"
+        "then"
+        "with"
+      ];
+    in
     s:
     # Regex from https://github.com/NixOS/nix/blob/d048577909e383439c2549e849c5c2f2016c997e/src/libexpr/lexer.l#L91
-    if match "[a-zA-Z_][a-zA-Z0-9_'-]*" s != null then s else escapeNixString s;
+    if (match "[a-zA-Z_][a-zA-Z0-9_'-]*" s != null) && (!lib.elem s nixKeywords) then
+      s
+    else
+      escapeNixString s;
 
   /**
     Escapes a string `s` such that it is safe to include verbatim in an XML

lib/tests/misc.nix

@@ -865,6 +865,21 @@ runTests {
     expected = "'á'";
   };
 
+  testEscapeNixIdentifierNoQuote = {
+    expr = strings.escapeNixIdentifier "foo";
+    expected = ''foo'';
+  };
+
+  testEscapeNixIdentifierNumber = {
+    expr = strings.escapeNixIdentifier "1foo";
+    expected = ''"1foo"'';
+  };
+
+  testEscapeNixIdentifierKeyword = {
+    expr = strings.escapeNixIdentifier "assert";
+    expected = ''"assert"'';
+  };
+
   testSplitStringsDerivation = {
     expr = lib.dropEnd 1 (strings.splitString "/" dummyDerivation);
     expected = strings.splitString "/" builtins.storeDir;
@@ -2785,6 +2800,7 @@ runTests {
         ];
         emptylist = [ ];
         attrs = {
+          "assert" = false;
           foo = null;
           "foo b/ar" = "baz";
         };
@@ -2804,7 +2820,7 @@ runTests {
         functionArgs = "<function, args: {arg?, foo}>";
         list = "[ 3 4 ${function} [ false ] ]";
         emptylist = "[ ]";
-        attrs = "{ foo = null; \"foo b/ar\" = \"baz\"; }";
+        attrs = "{ \"assert\" = false; foo = null; \"foo b/ar\" = \"baz\"; }";
         emptyattrs = "{ }";
         drv = "<derivation ${deriv.name}>";
       };