[FEATURE] Expression variables for project CRS

Adds @project_crs and @project_crs_definition variables
for retrieving the current project CRS

Author: nyalldawson

src/core/qgsexpression.cpp

@@ -5712,6 +5712,8 @@ void QgsExpression::initVariableHelp()
   gVariableHelpTexts.insert( QStringLiteral( "project_path" ), QCoreApplication::translate( "variable_help", "Full path (including file name) of current project." ) );
   gVariableHelpTexts.insert( QStringLiteral( "project_folder" ), QCoreApplication::translate( "variable_help", "Folder for current project." ) );
   gVariableHelpTexts.insert( QStringLiteral( "project_filename" ), QCoreApplication::translate( "variable_help", "Filename of current project." ) );
+  gVariableHelpTexts.insert( QStringLiteral( "project_crs" ), QCoreApplication::translate( "variable_help", "Coordinate reference system of project (eg 'EPSG:4326')." ) );
+  gVariableHelpTexts.insert( QStringLiteral( "project_crs_definition" ), QCoreApplication::translate( "variable_help", "Coordinate reference system of project (full definition)." ) );
 
   //layer variables
   gVariableHelpTexts.insert( QStringLiteral( "layer_name" ), QCoreApplication::translate( "variable_help", "Name of current layer." ) );

src/core/qgsexpressioncontext.cpp

@@ -644,6 +644,9 @@ QgsExpressionContextScope* QgsExpressionContextUtils::projectScope()
   scope->addVariable( QgsExpressionContextScope::StaticVariable( QStringLiteral( "project_path" ), project->fileInfo().filePath(), true ) );
   scope->addVariable( QgsExpressionContextScope::StaticVariable( QStringLiteral( "project_folder" ), project->fileInfo().dir().path(), true ) );
   scope->addVariable( QgsExpressionContextScope::StaticVariable( QStringLiteral( "project_filename" ), project->fileInfo().fileName(), true ) );
+  QgsCoordinateReferenceSystem projectCrs = project->crs();
+  scope->addVariable( QgsExpressionContextScope::StaticVariable( QStringLiteral( "project_crs" ), projectCrs.authid(), true ) );
+  scope->addVariable( QgsExpressionContextScope::StaticVariable( QStringLiteral( "project_crs_definition" ), projectCrs.toProj4(), true ) );
 
   scope->addFunction( QStringLiteral( "project_color" ), new GetNamedProjectColor() );
   return scope;