[FEATURE] Add decode_uri expression function

This function takes a layer parameter and an optional uri part parameter.
It will return details about the data source of the layer. The available
details depend on the data provider.
If called with only a layer parameter, a map will be returned with all the
available information. If the part is also specified, only the value for
the requested part will be extracted from the map.

src/core/expression/qgsexpressionfunction.cpp

@@ -46,6 +46,7 @@
 #include "qgsfieldformatterregistry.h"
 #include "qgsfieldformatter.h"
 #include "qgsvectorlayerfeatureiterator.h"
+#include "qgsproviderregistry.h"
 
 const QString QgsExpressionFunction::helpText() const
 {
@@ -4030,6 +4031,35 @@ static QVariant fcnGetLayerProperty( const QVariantList &values, const QgsExpres
   return QVariant();
 }
 
+static QVariant fcnDecodeUri( const QVariantList &values, const QgsExpressionContext *, QgsExpression *parent, const QgsExpressionNodeFunction * )
+{
+  QgsMapLayer *layer = QgsExpressionUtils::getMapLayer( values.at( 0 ), parent );
+  if ( !layer )
+  {
+    parent->setEvalErrorString( QObject::tr( "Cannot find layer %1" ).arg( values.at( 0 ).toString() ) );
+    return QVariant();
+  }
+
+  if ( !layer->dataProvider() )
+  {
+    parent->setEvalErrorString( QObject::tr( "Layer %1 has invalid data provider" ).arg( layer->name() ) );
+    return QVariant();
+  }
+
+  const QString uriPart = values.at( 1 ).toString();
+
+  const QVariantMap decodedUri = QgsProviderRegistry::instance()->decodeUri( layer->providerType(), layer->dataProvider()->dataSourceUri() );
+
+  if ( !uriPart.isNull() )
+  {
+    return decodedUri.value( values.at( 1 ).toString() );
+  }
+  else
+  {
+    return decodedUri;
+  }
+}
+
 static QVariant fcnGetRasterBandStat( const QVariantList &values, const QgsExpressionContext *, QgsExpression *parent, const QgsExpressionNodeFunction * )
 {
   QString layerIdOrName = QgsExpressionUtils::getStringValue( values.at( 0 ), parent );
@@ -4906,6 +4936,11 @@ const QList<QgsExpressionFunction *> &QgsExpression::Functions()
     // **General** functions
     sFunctions
         << new QgsStaticExpressionFunction( QStringLiteral( "layer_property" ), 2, fcnGetLayerProperty, QStringLiteral( "General" ) )
+        << new QgsStaticExpressionFunction( QStringLiteral( "decode_uri" ),
+                                            QgsExpressionFunction::ParameterList()
+                                            << QgsExpressionFunction::Parameter( QStringLiteral( "layer" ) )
+                                            << QgsExpressionFunction::Parameter( QStringLiteral( "part" ), true ),
+                                            fcnDecodeUri, QStringLiteral( "Map Layers" ) )
         << new QgsStaticExpressionFunction( QStringLiteral( "raster_statistic" ), QgsExpressionFunction::ParameterList() << QgsExpressionFunction::Parameter( QStringLiteral( "layer" ) )
                                             << QgsExpressionFunction::Parameter( QStringLiteral( "band" ) )
                                             << QgsExpressionFunction::Parameter( QStringLiteral( "statistic" ) ), fcnGetRasterBandStat, QStringLiteral( "Rasters" ) );