EclipseResourceFileSystemAccess2 file generation causes global rebuild of all build configs

@cdietrich This is the issue you requested during consulting.

Issue
In a workspace we have a project with our Xtext nature, and an unrelated CDT project. On generating code from our Xtext project, the CDT project is rebuilt. There are no project references, generated files don't overlap with the CDT project, and there is no externally caused refresh of the workspace files (e.g. no modification of the workspace files via java.io.File API).

If we replace the IFileSystemAccess2 based file generation by plain IFile writing, then the unwanted build does not happen.

Root cause
Writing generated files viaEclipseResourceFileSystemAccess2 triggers the callback implementation in BuilderParticipant after files have been created, changed or deleted. Every such file change invokes BuildContext.needRebuild(). Via IncrementalProjectBuilder that finally sets the rebuildRequested flag of the global BuildManager. Unfortunately, if that flag is set, all build configs are rebuild, and not just the one that relates to the Xtext project.

We don't understand why the global rebuild needs to be triggered there. If we replace the IFileSystemAccess2 by plain IFile writing, then the generated files cause additional builds via resource delta notifications, which is sufficient (since it only affects the projects containing the generated files, but not other projects).

Start with the callback in org.eclipse.xtext.builder/src/org/eclipse/xtext/builder/BuilderParticipant.java, especially the BuildContext.needRebuild() call, and trace how it reaches IncrementalProjectBuilder and the global BuildManager rebuildRequested flag. Reproduce code generation in an Xtext project alongside an unrelated CDT project. Done means generated files still trigger the needed build without rebuilding unrelated build configurations.