E0210
A struct or enum whose own fields name a capability-bearing type (a built-in capability, a cap type, or a cap fn, including inside written type arguments such as List<Stdout>) is not declared cap. A field typed by a type parameter, as in struct Box<T> { value: T }, does not count. Spans the type's name.
Machine-applicable fix: Insert cap before struct or enum.
Example
This program fails with E0210.
struct Logger { out: Stdout }
Fixed
The same program, fixed.
cap struct Logger { out: Stdout }
Listed in the checker notes. oskac explain E0210 prints this entry offline.