Remove deprecated QgsCoordinateTransform ctor ...

... and add a getter for coordinateTransform to QgsMapLayer

The reason is that I think we should reduce the dataProvider API
usage and rely on the higher level QgsMapLayer API whenever it
is possible, QgsMapLayer checks for data provider validity
and returns a default constructed QgsCoordinateTransform instead
of crashing.

python/core/auto_generated/qgscoordinatetransform.sip.in

@@ -50,7 +50,6 @@ transforms coordinates from the layer's coordinate system to the map canvas.
 Default constructor, creates an invalid QgsCoordinateTransform.
 %End
 
-
     explicit QgsCoordinateTransform( const QgsCoordinateReferenceSystem &source,
                                      const QgsCoordinateReferenceSystem &destination,
                                      const QgsCoordinateTransformContext &context );

python/core/auto_generated/qgsmaplayer.sip.in

@@ -617,6 +617,15 @@ Returns the layer's spatial reference system.
 Sets layer's spatial reference system
 %End
 
+    QgsCoordinateTransformContext transformContext( ) const;
+%Docstring
+Returns the layer data provider coordinate transform context
+or a default transform context if the layer does not have a valid data provider.
+
+.. versionadded:: 3.10
+%End
+
+
     static QString formatLayerName( const QString &name );
 %Docstring
 A convenience function to capitalize and format a layer ``name``.

src/core/processing/qgsprocessingutils.cpp

@@ -638,9 +638,7 @@ QgsRectangle QgsProcessingUtils::combineLayerExtents( const QList<QgsMapLayer *>
     if ( crs.isValid() )
     {
       //transform layer extent to target CRS
-      Q_NOWARN_DEPRECATED_PUSH
-      QgsCoordinateTransform ct( layer->crs(), crs );
-      Q_NOWARN_DEPRECATED_POP
+      QgsCoordinateTransform ct( layer->crs(), crs, QgsProject::instance()->transformContext() );
       try
       {
         QgsRectangle reprojExtent = ct.transformBoundingBox( layer->extent() );

src/core/qgscoordinatetransform.cpp

@@ -52,20 +52,6 @@ QgsCoordinateTransform::QgsCoordinateTransform()
   d = new QgsCoordinateTransformPrivate();
 }
 
-QgsCoordinateTransform::QgsCoordinateTransform( const QgsCoordinateReferenceSystem &source, const QgsCoordinateReferenceSystem &destination )
-{
-  d = new QgsCoordinateTransformPrivate( source, destination, QgsCoordinateTransformContext() );
-
-  if ( !d->checkValidity() )
-    return;
-
-  if ( !setFromCache( d->mSourceCRS, d->mDestCRS, d->mSourceDatumTransform, d->mDestinationDatumTransform ) )
-  {
-    d->initialize();
-    addToCache();
-  }
-}
-
 QgsCoordinateTransform::QgsCoordinateTransform( const QgsCoordinateReferenceSystem &source, const QgsCoordinateReferenceSystem &destination, const QgsCoordinateTransformContext &context )
 {
   mContext = context;

src/core/qgscoordinatetransform.h

@@ -64,19 +64,6 @@ class CORE_EXPORT QgsCoordinateTransform
     //! Default constructor, creates an invalid QgsCoordinateTransform.
     QgsCoordinateTransform();
 
-    /**
-     * Constructs a QgsCoordinateTransform using QgsCoordinateReferenceSystem objects.
-     * \param source source CRS, typically of the layer's coordinate system
-     * \param destination CRS, typically of the map canvas coordinate system
-     * correctly handle the user's datum transform setup. Instead the constructor
-     * variant which accepts a QgsCoordinateTransformContext or QgsProject
-     * argument should be used instead. It is highly likely that this constructor
-     * will be removed in future QGIS versions.
-     * \note Not available in Python bindings.
-     * \deprecated Use of this constructor is strongly discouraged, as it will not
-     */
-    Q_DECL_DEPRECATED explicit QgsCoordinateTransform( const QgsCoordinateReferenceSystem &source, const QgsCoordinateReferenceSystem &destination ) SIP_SKIP;
-
     /**
      * Constructs a QgsCoordinateTransform to transform from the \a source
      * to \a destination coordinate reference system.

src/core/qgsmaplayer.cpp

@@ -782,6 +782,11 @@ void QgsMapLayer::setCrs( const QgsCoordinateReferenceSystem &srs, bool emitSign
     emit crsChanged();
 }
 
+QgsCoordinateTransformContext QgsMapLayer::transformContext() const
+{
+  return dataProvider() ? dataProvider()->transformContext() : QgsCoordinateTransformContext();
+}
+
 QString QgsMapLayer::formatLayerName( const QString &name )
 {
   QString layerName( name );

src/core/qgsmaplayer.h

@@ -612,6 +612,14 @@ class CORE_EXPORT QgsMapLayer : public QObject
     //! Sets layer's spatial reference system
     void setCrs( const QgsCoordinateReferenceSystem &srs, bool emitSignal = true );
 
+    /**
+     * Returns the layer data provider coordinate transform context
+     * or a default transform context if the layer does not have a valid data provider.
+    \since QGIS 3.10
+     */
+    QgsCoordinateTransformContext transformContext( ) const;
+
+
     /**
      * A convenience function to capitalize and format a layer \a name.
      *

src/core/qgsvectorfilewriter.cpp

@@ -2495,9 +2495,7 @@ QgsVectorFileWriter::writeAsVectorFormat( QgsVectorLayer *layer,
   QgsCoordinateTransform ct;
   if ( destCRS.isValid() && layer )
   {
-    Q_NOWARN_DEPRECATED_PUSH
-    ct = QgsCoordinateTransform( layer->crs(), destCRS );
-    Q_NOWARN_DEPRECATED_POP
+    ct = QgsCoordinateTransform( layer->crs(), destCRS, layer->transformContext() );
   }
 
   QgsVectorFileWriter::WriterError error = writeAsVectorFormat( layer, fileName, fileEncoding, ct, driverName, onlySelected,

src/core/qgsvectorlayerexporter.cpp

@@ -351,9 +351,7 @@ QgsVectorLayerExporter::exportLayer( QgsVectorLayer *layer,
   // Create our transform
   if ( destCRS.isValid() )
   {
-    Q_NOWARN_DEPRECATED_PUSH
-    ct = QgsCoordinateTransform( layer->crs(), destCRS );
-    Q_NOWARN_DEPRECATED_POP
+    ct = QgsCoordinateTransform( layer->crs(), destCRS, layer->transformContext() );
   }
 
   // Check for failure

src/core/raster/qgsrasterfilewriter.cpp

@@ -231,9 +231,7 @@ QgsRasterFileWriter::WriterError QgsRasterFileWriter::writeDataRaster( const Qgs
       QgsRasterProjector *projector = pipe->projector();
       if ( projector && projector->destinationCrs() != projector->sourceCrs() )
       {
-        Q_NOWARN_DEPRECATED_PUSH
-        QgsCoordinateTransform ct( projector->destinationCrs(), projector->sourceCrs() );
-        Q_NOWARN_DEPRECATED_POP
+        QgsCoordinateTransform ct( projector->destinationCrs(), projector->sourceCrs(), srcProvider->transformContext() );
         srcExtent = ct.transformBoundingBox( outputExtent );
       }
       if ( !srcProvider->extent().contains( srcExtent ) )

tests/src/python/test_qgsvectorlayer.py

@@ -57,6 +57,7 @@
                        QgsTextFormat,
                        QgsVectorLayerSelectedFeatureSource,
                        QgsExpression,
+                       QgsCoordinateTransformContext,
                        NULL)
 from qgis.gui import (QgsAttributeTableModel,
                       QgsGui
@@ -3166,6 +3167,20 @@ def testMaximumValue(self):
         """
         pass
 
+
+class TestQgsVectorLayerTransformContext(unittest.TestCase):
+
+    def testTransformContext(self):
+        vl = QgsVectorLayer(
+            'Point?crs=epsg:4326&field=pk:integer&field=cnt:integer&field=name:string(0)&field=name2:string(0)&field=num_char:string&key=pk',
+            'test', 'memory')
+        ctx = QgsCoordinateTransformContext()
+        ctx.addSourceDestinationDatumTransform(QgsCoordinateReferenceSystem(4326), QgsCoordinateReferenceSystem(3857), 1234, 1235)
+        vl.dataProvider().setTransformContext(ctx)
+        self.assertTrue(vl.isValid())
+        self.assertEqual(vl.transformContext(), vl.dataProvider().transformContext())
+        self.assertEqual(list(vl.transformContext().sourceDestinationDatumTransforms().keys())[0], ('EPSG:4326', 'EPSG:3857'))
+
 # TODO:
 # - fetch rect: feat with changed geometry: 1. in rect, 2. out of rect
 # - more join tests