Skip UseEnumSetOf empty conversion for static fields to avoid circular class-init (#1157)

[timtebeek]

• Fixes UseEnumSetOf introduces circular class-init ClassCastException #1157

UseEnumSetOf rewrites an empty Set.of() to EnumSet.noneOf(E.class). In a static field initializer, EnumSet.noneOf reflectively reads E's constants during class initialization. When two enums have a circular static dependency (one enum's constants carry a value from a sibling enum that also holds an EnumSet.noneOf(...) static field), this can re-enter a not-yet-initialized enum and throw a misleading ClassCastException("... not an enum") at runtime — even though the code compiles cleanly.

Fix

Skip the empty Set.of() → EnumSet.noneOf(E.class) conversion when the assignment target is a static field. Static field initializers run during <clinit>, which is the only place the re-entrant class-init hazard exists; local variables and instance fields are unaffected and still convert (e.g. the existing dontHaveArgs test). The non-empty Set.of(E.X, ...) case is untouched since it does not use the reflective noneOf path.

• This mirrors the approach of UseMapOf rewrites the initializer of a HashMap-typed variable to Map.of(), leaving uncompilable code #1148 (skip when the assignment target is a concrete HashMap).

[protocol7]

Quick as usual, thanks!