add QgsFeatureRequest::setNoAttributes (#8326)

since it's more convenient than setSubsetOfAttributes( QgsAttributesList() )

python/core/auto_generated/qgsfeaturerequest.sip.in

@@ -513,6 +513,15 @@ Set a subset of attributes that will be fetched.
 An empty attributes list indicates that no attributes will be fetched.
 To revert a call to setSubsetOfAttributes and fetch all available attributes,
 the SubsetOfAttributes flag should be removed from the request.
+%End
+
+    QgsFeatureRequest &setNoAttributes();
+%Docstring
+Set that no attributes will be fetched.
+To revert a call to setNoAttributes and fetch all or some available attributes,
+the SubsetOfAttributes flag should be removed from the request.
+
+.. versionadded:: 3.4
 %End
 
     QgsAttributeList subsetOfAttributes() const;

src/core/qgsfeaturerequest.cpp

@@ -190,6 +190,11 @@ QgsFeatureRequest &QgsFeatureRequest::setSubsetOfAttributes( const QgsAttributeL
   return *this;
 }
 
+QgsFeatureRequest &QgsFeatureRequest::setNoAttributes()
+{
+  return setSubsetOfAttributes( QgsAttributeList() );
+}
+
 QgsFeatureRequest &QgsFeatureRequest::setSubsetOfAttributes( const QStringList &attrNames, const QgsFields &fields )
 {
   if ( attrNames.contains( QgsFeatureRequest::ALL_ATTRIBUTES ) )

src/core/qgsfeaturerequest.h

@@ -501,6 +501,14 @@ class CORE_EXPORT QgsFeatureRequest
      */
     QgsFeatureRequest &setSubsetOfAttributes( const QgsAttributeList &attrs );
 
+    /**
+     * Set that no attributes will be fetched.
+     * To revert a call to setNoAttributes and fetch all or some available attributes,
+     * the SubsetOfAttributes flag should be removed from the request.
+     * \since QGIS 3.4
+     */
+    QgsFeatureRequest &setNoAttributes();
+
     /**
      * Returns the subset of attributes which at least need to be fetched
      * \returns A list of attributes to be fetched