Why do errors no longer appear in Eclipse's package explorer?

Question

My project used to be fine - errors showed up in the file margin, the editor breadcrumb, the problems view and in the package explorer tree as little red X marks.

Somewh

Answer 1

None of the answers so far applied to my situation, but they steered me towards this solution:

Project -> Properties -> Project Facets -> convert to faceted form...

and check the applicable Project Facets.