Correct, the Report Definition Language is just XML. And, as with any other file, you can just open it (File/Open/File…) in Visual Studio, just like in a text editor. In order to see a designer you need to give Visual Studio a bit of context about what sort of file it is by making it part of a Report Server Project.
- Install SSDT for Visual Studio if you haven’t already done so
- Create a new project using the ‘Report Server Project’ template
- Copy/move the .rdl file(s) you have into the folder for the newly created project (eg ..\Visual Studio 2015\Projects\My Report Project\My Report Project)
- Use the “Add Existing Item…” context menu option in Solution Explorer to add the .rdl to the project
- Open report designer by double-clicking the newly added report project item(s)