You also need to clone the binary Git repository: https://git.eclipse.org/r/#/admin/projects/platform/eclipse.platform.swt.binaries
and import the project for your platform into your workspace.
Ensure that the fragment matching your windowingSystem.operatingSystem.cpuArchitecture
(e.g. org.eclipse.swt.gtk.linux.x86_64) is open in your workspace.
The fragments provide the platform-specific native libraries.
Assertions are added to the code. These don't run in production, but they do when:
Assertions look like:
assert expression ; assert expression : msg ;
See the Readme.md in the main directory of the Git repository for this project to learn more about SWT development.