When creating a expression filter string for a rule based renderer,

try to create a simple "field IN (...)" expression wherever possible

This avoids some issues with complex rule based renderers that have
many similar classes, where the generated expression was previously
an extremely long and complex "(field = ...) OR (field = ...) OR ..."
type expression (which some providers have issues with)

Fixes missing features in some circumstances in rule based renderers

Sponsored by SLYR

src/core/symbology/qgsrulebasedrenderer.cpp

@@ -453,31 +453,39 @@ bool QgsRuleBasedRenderer::Rule::startRender( QgsRenderContext &context, const Q
     {
       sf = QStringLiteral( "TRUE" );
     }
-    // If we have more than 50 rules (to stay on the safe side) make a binary tree or SQLITE will fail,
-    // see: https://github.com/qgis/QGIS/issues/27269
-    else if ( subfilters.count() > 50 )
+    else
     {
-      std::function<QString( const QStringList & )>bt = [ &bt ]( const QStringList & subf )
+      // test for a common case -- all subfilters can be combined into a single "field in (...)" expression
+      if ( QgsExpression::attemptReduceToInClause( subfilters, sf ) )
       {
-        if ( subf.count( ) == 1 )
-        {
-          return subf.at( 0 );
-        }
-        else if ( subf.count( ) == 2 )
-        {
-          return subf.join( QLatin1String( ") OR (" ) ).prepend( '(' ).append( ')' );
-        }
-        else
+        // success! we can use a simple "field IN (...)" list!
+      }
+      // If we have more than 50 rules (to stay on the safe side) make a binary tree or SQLITE will fail,
+      // see: https://github.com/qgis/QGIS/issues/27269
+      else if ( subfilters.count() > 50 )
+      {
+        std::function<QString( const QStringList & )>bt = [ &bt ]( const QStringList & subf )
         {
-          int midpos = static_cast<int>( subf.length() / 2 );
-          return QStringLiteral( "(%1) OR (%2)" ).arg( bt( subf.mid( 0, midpos ) ) ).arg( bt( subf.mid( midpos ) ) );
-        }
-      };
-      sf = bt( subfilters );
-    }
-    else
-    {
-      sf = subfilters.join( QLatin1String( ") OR (" ) ).prepend( '(' ).append( ')' );
+          if ( subf.count( ) == 1 )
+          {
+            return subf.at( 0 );
+          }
+          else if ( subf.count( ) == 2 )
+          {
+            return subf.join( QLatin1String( ") OR (" ) ).prepend( '(' ).append( ')' );
+          }
+          else
+          {
+            int midpos = static_cast<int>( subf.length() / 2 );
+            return QStringLiteral( "(%1) OR (%2)" ).arg( bt( subf.mid( 0, midpos ) ), bt( subf.mid( midpos ) ) );
+          }
+        };
+        sf = bt( subfilters );
+      }
+      else
+      {
+        sf = subfilters.join( QLatin1String( ") OR (" ) ).prepend( '(' ).append( ')' );
+      }
     }
   }