printAttrs: Force item before determining whether to print multi-line

src/libexpr/print.cc

@@ -299,6 +299,9 @@ private:
             output << ANSI_NORMAL;
     }
 
+    /**
+     * @note This may force items.
+     */
     bool shouldPrettyPrintAttrs(AttrVec & v)
     {
         if (!options.shouldPrettyPrint() || v.empty()) {
@@ -315,6 +318,9 @@ private:
             return true;
         }
 
+        // It is ok to force the item(s) here, because they will be printed anyway.
+        state.forceValue(*item, item->determinePos(noPos));
+
         // Pretty-print single-item attrsets only if they contain nested
         // structures.
         auto itemType = item->type();

tests/functional/repl/pretty-print-idempotent.expected

@@ -2,9 +2,7 @@ Nix <nix version>
 Type :? for help.
 Added <number omitted> variables.
 
-{
-  homepage = "https://example.com";
-}
+{ homepage = "https://example.com"; }
 
 { homepage = "https://example.com"; }