Unused (checker-framework 3.39.0-eisop1 API)

```
@Documented
@Retention(RUNTIME)
@Target(FIELD)
public @interface Unused
```
Declares that the field may not be accessed if the receiver is of the specified qualifier type (or any supertype).
This property is verified by the checker that type-checks the when element value qualifier.
Example Consider a class, Table, with a locking field, lock. The lock is used when a Table instance is shared across threads. When running in a local thread, the lock field ought not to be used.
You can declare this behavior in the following way:
```
 class Table {
   private @Unused(when=LocalToThread.class) final Lock lock;
   ...
 }
 
```
The checker for @LocalToThread would issue an error for the following code:
```
  @LocalToThread Table table = ...;
   ... table.lock ...;
 
```
See the Checker Framework Manual:
Unused fields

Required Element Summary

Required Elements
Modifier and Type	Required Element	Description
`java.lang.Class<? extends java.lang.annotation.Annotation>`	`when`	The field that is annotated with @Unused may not be accessed via a receiver that is annotated with the "when" annotation.

- Element Detail
  - when
```
java.lang.Class<? extends java.lang.annotation.Annotation> when
```
    The field that is annotated with @Unused may not be accessed via a receiver that is annotated with the "when" annotation.