diff --git a/python/core/auto_generated/qgspallabeling.sip.in b/python/core/auto_generated/qgspallabeling.sip.in
index 5ad4d53c0ed..423e30df377 100644
--- a/python/core/auto_generated/qgspallabeling.sip.in
+++ b/python/core/auto_generated/qgspallabeling.sip.in
@@ -386,9 +386,13 @@ Returns the QgsExpression for this label settings. May be None if isExpression i
 
     double zIndex;
 
-    void calculateLabelSize( const QFontMetricsF *fm, QString text, double &labelX, double &labelY, QgsFeature *f = 0, QgsRenderContext *context = 0 );
+    void calculateLabelSize( const QFontMetricsF *fm, const QString &text, double &labelX, double &labelY, const QgsFeature *f = 0, QgsRenderContext *context = 0 );
+%Docstring
+Calculates the space required to render the provided ``text`` in map units.
+Results will be written to ``labelX`` and ``labelY``.
+%End
 
-    void registerFeature( QgsFeature &f, QgsRenderContext &context  );
+    void registerFeature( const QgsFeature &f, QgsRenderContext &context  );
 
 %Docstring
 Register a feature for labeling.
@@ -457,7 +461,7 @@ Sets the label text formatting settings, e.g., font settings, buffer settings, e
 .. versionadded:: 3.0
 %End
 
-    QgsFeature *mCurFeat;
+    const QgsFeature *mCurFeat;
     QgsFields mCurFields;
     int fieldIndex;
     const QgsMapToPixel *xform;
@@ -469,7 +473,6 @@ Sets the label text formatting settings, e.g., font settings, buffer settings, e
     int mFeaturesToLabel;
     int mFeatsSendingToPal;
     int mFeatsRegPal;
-
 };
 
 class QgsLabelCandidate
diff --git a/src/core/dxf/qgsdxfpallabeling.cpp b/src/core/dxf/qgsdxfpallabeling.cpp
index d414b91d796..cf8d5d5d36e 100644
--- a/src/core/dxf/qgsdxfpallabeling.cpp
+++ b/src/core/dxf/qgsdxfpallabeling.cpp
@@ -34,7 +34,7 @@ void QgsDxfLabelProvider::drawLabel( QgsRenderContext &context, pal::LabelPositi
   mDxfExport->drawLabel( layerId(), context, label, mSettings );
 }
 
-void QgsDxfLabelProvider::registerDxfFeature( QgsFeature &feature, QgsRenderContext &context, const QString &dxfLayerName )
+void QgsDxfLabelProvider::registerDxfFeature( const QgsFeature &feature, QgsRenderContext &context, const QString &dxfLayerName )
 {
   registerFeature( feature, context );
   mDxfExport->registerDxfLayer( layerId(), feature.id(), dxfLayerName );
diff --git a/src/core/dxf/qgsdxfpallabeling.h b/src/core/dxf/qgsdxfpallabeling.h
index 6d2c9b22938..fa6df02a0a5 100644
--- a/src/core/dxf/qgsdxfpallabeling.h
+++ b/src/core/dxf/qgsdxfpallabeling.h
@@ -54,7 +54,7 @@ class QgsDxfLabelProvider : public QgsVectorLayerLabelProvider
      * \param context render context
      * \param dxfLayerName name of dxf layer
      */
-    void registerDxfFeature( QgsFeature &feature, QgsRenderContext &context, const QString &dxfLayerName );
+    void registerDxfFeature( const QgsFeature &feature, QgsRenderContext &context, const QString &dxfLayerName );
 
   protected:
     //! pointer to parent DXF export where this instance is used
diff --git a/src/core/qgspallabeling.cpp b/src/core/qgspallabeling.cpp
index c1c455d718d..030eb50ff67 100644
--- a/src/core/qgspallabeling.cpp
+++ b/src/core/qgspallabeling.cpp
@@ -1038,13 +1038,15 @@ bool QgsPalLayerSettings::checkMinimumSizeMM( const QgsRenderContext &ct, const
   return QgsPalLabeling::checkMinimumSizeMM( ct, geom, minSize );
 }
 
-void QgsPalLayerSettings::calculateLabelSize( const QFontMetricsF *fm, QString text, double &labelX, double &labelY, QgsFeature *f, QgsRenderContext *context )
+void QgsPalLayerSettings::calculateLabelSize( const QFontMetricsF *fm, const QString &text, double &labelX, double &labelY, const QgsFeature *f, QgsRenderContext *context )
 {
   if ( !fm || !f )
   {
     return;
   }
 
+  QString textCopy( text );
+
   //try to keep < 2.12 API - handle no passed render context
   std::unique_ptr< QgsRenderContext > scopedRc;
   if ( !context )
@@ -1150,29 +1152,25 @@ void QgsPalLayerSettings::calculateLabelSize( const QFontMetricsF *fm, QString t
 
     if ( placeDirSymb == QgsPalLayerSettings::SymbolLeftRight )
     {
-      text.append( dirSym );
+      textCopy.append( dirSym );
     }
     else
     {
-      text.prepend( dirSym + QStringLiteral( "\n" ) ); // SymbolAbove or SymbolBelow
+      textCopy.prepend( dirSym + QStringLiteral( "\n" ) ); // SymbolAbove or SymbolBelow
     }
   }
 
   double w = 0.0, h = 0.0;
-  QStringList multiLineSplit = QgsPalLabeling::splitToLines( text, wrapchr, evalAutoWrapLength, useMaxLineLengthForAutoWrap );
+  const QStringList multiLineSplit = QgsPalLabeling::splitToLines( textCopy, wrapchr, evalAutoWrapLength, useMaxLineLengthForAutoWrap );
   int lines = multiLineSplit.size();
 
   double labelHeight = fm->ascent() + fm->descent(); // ignore +1 for baseline
 
   h += fm->height() + static_cast< double >( ( lines - 1 ) * labelHeight * multilineH );
 
-  for ( int i = 0; i < lines; ++i )
+  for ( const QString &line : multiLineSplit )
   {
-    double width = fm->width( multiLineSplit.at( i ) );
-    if ( width > w )
-    {
-      w = width;
-    }
+    w = std::max( w, fm->width( line ) );
   }
 
 #if 0 // XXX strk
@@ -1186,7 +1184,7 @@ void QgsPalLayerSettings::calculateLabelSize( const QFontMetricsF *fm, QString t
 #endif
 }
 
-void QgsPalLayerSettings::registerFeature( QgsFeature &f, QgsRenderContext &context, QgsLabelFeature **labelFeature, QgsGeometry obstacleGeometry )
+void QgsPalLayerSettings::registerFeature( const QgsFeature &f, QgsRenderContext &context, QgsLabelFeature **labelFeature, QgsGeometry obstacleGeometry )
 {
   // either used in QgsPalLabeling (palLayer is set) or in QgsLabelingEngine (labelFeature is set)
   Q_ASSERT( labelFeature );
@@ -1995,7 +1993,7 @@ void QgsPalLayerSettings::registerFeature( QgsFeature &f, QgsRenderContext &cont
   lf->setDataDefinedValues( dataDefinedValues );
 }
 
-void QgsPalLayerSettings::registerObstacleFeature( QgsFeature &f, QgsRenderContext &context, QgsLabelFeature **obstacleFeature, const QgsGeometry &obstacleGeometry )
+void QgsPalLayerSettings::registerObstacleFeature( const QgsFeature &f, QgsRenderContext &context, QgsLabelFeature **obstacleFeature, const QgsGeometry &obstacleGeometry )
 {
   mCurFeat = &f;
 
diff --git a/src/core/qgspallabeling.h b/src/core/qgspallabeling.h
index 255674fa895..0678baf4c51 100644
--- a/src/core/qgspallabeling.h
+++ b/src/core/qgspallabeling.h
@@ -750,8 +750,11 @@ class CORE_EXPORT QgsPalLayerSettings
     //! Z-Index of label, where labels with a higher z-index are rendered on top of labels with a lower z-index
     double zIndex;
 
-    // called from register feature hook
-    void calculateLabelSize( const QFontMetricsF *fm, QString text, double &labelX, double &labelY, QgsFeature *f = nullptr, QgsRenderContext *context = nullptr );
+    /**
+     * Calculates the space required to render the provided \a text in map units.
+     * Results will be written to \a labelX and \a labelY.
+     */
+    void calculateLabelSize( const QFontMetricsF *fm, const QString &text, double &labelX, double &labelY, const QgsFeature *f = nullptr, QgsRenderContext *context = nullptr );
 
     /**
      * Register a feature for labeling.
@@ -766,7 +769,7 @@ class CORE_EXPORT QgsPalLayerSettings
      * the feature's original geometry will be used as an obstacle for labels. Not available
      * in Python bindings.
      */
-    void registerFeature( QgsFeature &f, QgsRenderContext &context,
+    void registerFeature( const QgsFeature &f, QgsRenderContext &context,
                           QgsLabelFeature **labelFeature SIP_PYARGREMOVE = nullptr,
                           QgsGeometry obstacleGeometry SIP_PYARGREMOVE = QgsGeometry() );
 
@@ -823,7 +826,7 @@ class CORE_EXPORT QgsPalLayerSettings
     void setFormat( const QgsTextFormat &format ) { mFormat = format; }
 
     // temporary stuff: set when layer gets prepared or labeled
-    QgsFeature *mCurFeat = nullptr;
+    const QgsFeature *mCurFeat = nullptr;
     QgsFields mCurFields;
     int fieldIndex;
     const QgsMapToPixel *xform = nullptr;
@@ -835,7 +838,6 @@ class CORE_EXPORT QgsPalLayerSettings
     int mFeaturesToLabel = 0; // total features that will probably be labeled, may be less (figured before PAL)
     int mFeatsSendingToPal = 0; // total features tested for sending into PAL (relative to maxNumLabels)
     int mFeatsRegPal = 0; // number of features registered in PAL, when using limitNumLabels
-
   private:
 
     friend class QgsVectorLayer;  // to allow calling readFromLayerCustomProperties()
@@ -899,7 +901,7 @@ class CORE_EXPORT QgsPalLayerSettings
     /**
      * Registers a feature as an obstacle only (no label rendered)
      */
-    void registerObstacleFeature( QgsFeature &f, QgsRenderContext &context, QgsLabelFeature **obstacleFeature, const QgsGeometry &obstacleGeometry = QgsGeometry() );
+    void registerObstacleFeature( const QgsFeature &f, QgsRenderContext &context, QgsLabelFeature **obstacleFeature, const QgsGeometry &obstacleGeometry = QgsGeometry() );
 
     QMap<Property, QVariant> dataDefinedValues;
 
diff --git a/src/core/qgsrulebasedlabeling.cpp b/src/core/qgsrulebasedlabeling.cpp
index c54b7723cce..8911ea17ec5 100644
--- a/src/core/qgsrulebasedlabeling.cpp
+++ b/src/core/qgsrulebasedlabeling.cpp
@@ -37,7 +37,7 @@ bool QgsRuleBasedLabelProvider::prepare( const QgsRenderContext &context, QSet<Q
   return true;
 }
 
-void QgsRuleBasedLabelProvider::registerFeature( QgsFeature &feature, QgsRenderContext &context, const QgsGeometry &obstacleGeometry )
+void QgsRuleBasedLabelProvider::registerFeature( const QgsFeature &feature, QgsRenderContext &context, const QgsGeometry &obstacleGeometry )
 {
   // will register the feature to relevant sub-providers
   mRules->rootRule()->registerFeature( feature, context, mSubProviders, obstacleGeometry );
@@ -316,7 +316,7 @@ void QgsRuleBasedLabeling::Rule::prepare( const QgsRenderContext &context, QSet<
   }
 }
 
-QgsRuleBasedLabeling::Rule::RegisterResult QgsRuleBasedLabeling::Rule::registerFeature( QgsFeature &feature, QgsRenderContext &context, QgsRuleBasedLabeling::RuleToProviderMap &subProviders, const QgsGeometry &obstacleGeometry )
+QgsRuleBasedLabeling::Rule::RegisterResult QgsRuleBasedLabeling::Rule::registerFeature( const QgsFeature &feature, QgsRenderContext &context, QgsRuleBasedLabeling::RuleToProviderMap &subProviders, const QgsGeometry &obstacleGeometry )
 {
   if ( !isFilterOK( feature, context )
        || !isScaleOK( context.rendererScale() ) )
@@ -363,7 +363,7 @@ QgsRuleBasedLabeling::Rule::RegisterResult QgsRuleBasedLabeling::Rule::registerF
     return Filtered;
 }
 
-bool QgsRuleBasedLabeling::Rule::isFilterOK( QgsFeature &f, QgsRenderContext &context ) const
+bool QgsRuleBasedLabeling::Rule::isFilterOK( const QgsFeature &f, QgsRenderContext &context ) const
 {
   if ( ! mFilter || mElseRule )
     return true;
diff --git a/src/core/qgsrulebasedlabeling.h b/src/core/qgsrulebasedlabeling.h
index 8f3f2fb0d5a..dd18d87ece1 100644
--- a/src/core/qgsrulebasedlabeling.h
+++ b/src/core/qgsrulebasedlabeling.h
@@ -282,7 +282,7 @@ class CORE_EXPORT QgsRuleBasedLabeling : public QgsAbstractVectorLayerLabeling
          * register individual features
          * \note not available in Python bindings
          */
-        RegisterResult registerFeature( QgsFeature &feature, QgsRenderContext &context, RuleToProviderMap &subProviders, const QgsGeometry &obstacleGeometry = QgsGeometry() ) SIP_SKIP;
+        RegisterResult registerFeature( const QgsFeature &feature, QgsRenderContext &context, RuleToProviderMap &subProviders, const QgsGeometry &obstacleGeometry = QgsGeometry() ) SIP_SKIP;
 
         /**
          * Returns true if this rule or any of its children requires advanced composition effects
@@ -302,7 +302,7 @@ class CORE_EXPORT QgsRuleBasedLabeling : public QgsAbstractVectorLayerLabeling
          * \param context   The context in which the rendering happens
          * \returns          True if the feature shall be rendered
          */
-        bool isFilterOK( QgsFeature &f, QgsRenderContext &context ) const;
+        bool isFilterOK( const QgsFeature &f, QgsRenderContext &context ) const;
 
         /**
          * Check if this rule applies for a given \a scale.
@@ -395,7 +395,7 @@ class CORE_EXPORT QgsRuleBasedLabelProvider : public QgsVectorLayerLabelProvider
 
     bool prepare( const QgsRenderContext &context, QSet<QString> &attributeNames ) override;
 
-    void registerFeature( QgsFeature &feature, QgsRenderContext &context, const QgsGeometry &obstacleGeometry = QgsGeometry() ) override;
+    void registerFeature( const QgsFeature &feature, QgsRenderContext &context, const QgsGeometry &obstacleGeometry = QgsGeometry() ) override;
 
     //! create a label provider
     virtual QgsVectorLayerLabelProvider *createProvider( QgsVectorLayer *layer, const QString &providerId, bool withFeatureLoop, const QgsPalLayerSettings *settings );
diff --git a/src/core/qgsvectorlayerlabelprovider.cpp b/src/core/qgsvectorlayerlabelprovider.cpp
index 075d8984b1f..77bf7fec29f 100644
--- a/src/core/qgsvectorlayerlabelprovider.cpp
+++ b/src/core/qgsvectorlayerlabelprovider.cpp
@@ -258,9 +258,10 @@ QList<QgsLabelFeature *> QgsVectorLayerLabelProvider::labelFeatures( QgsRenderCo
   return mLabels;
 }
 
-void QgsVectorLayerLabelProvider::registerFeature( QgsFeature &feature, QgsRenderContext &context, const QgsGeometry &obstacleGeometry )
+void QgsVectorLayerLabelProvider::registerFeature( const QgsFeature &feature, QgsRenderContext &context, const QgsGeometry &obstacleGeometry )
 {
   QgsLabelFeature *label = nullptr;
+
   mSettings.registerFeature( feature, context, &label, obstacleGeometry );
   if ( label )
     mLabels << label;
diff --git a/src/core/qgsvectorlayerlabelprovider.h b/src/core/qgsvectorlayerlabelprovider.h
index 8b64790a8e7..a08546d588e 100644
--- a/src/core/qgsvectorlayerlabelprovider.h
+++ b/src/core/qgsvectorlayerlabelprovider.h
@@ -74,7 +74,7 @@ class CORE_EXPORT QgsVectorLayerLabelProvider : public QgsAbstractLabelProvider
      * symbol, the obstacle geometry should represent the bounds of the offset symbol). If not set,
      * the feature's original geometry will be used as an obstacle for labels.
      */
-    virtual void registerFeature( QgsFeature &feature, QgsRenderContext &context, const QgsGeometry &obstacleGeometry = QgsGeometry() );
+    virtual void registerFeature( const QgsFeature &feature, QgsRenderContext &context, const QgsGeometry &obstacleGeometry = QgsGeometry() );
 
     /**
      * Returns the geometry for a point feature which should be used as an obstacle for labels. This