UnnecessaryDefaultInEnumSwitch: NPE with default case in switch expression

[pkoenig10]

Description of the problem / feature request:

When using a switch expression with an unnecessary default case, UnnecessaryDefaultInEnumSwitch throws a NullPointerException.

Bugs: what's the simplest, easiest way to reproduce this bug? Please provide a minimal example if possible.

enum Case { ONE, TWO }
void m(Case c) {
  switch (c) {
    case ONE -> {}
    case TWO -> {}
    default -> {}
  }
}

This results in the following error:

An unhandled exception was thrown by the Error Prone static analysis plugin.
     Please report this at https://github.com/google/error-prone/issues/new and include the following:
  
     error-prone version: unknown version
     BugPattern: UnnecessaryDefaultInEnumSwitch
     Stack Trace:
     java.lang.NullPointerException: Cannot invoke "java.util.List.isEmpty()" because "defaultStatements" is null
  	at com.google.errorprone.bugpatterns.UnnecessaryDefaultInEnumSwitch.trivialDefault(UnnecessaryDefaultInEnumSwitch.java:218)
  	at com.google.errorprone.bugpatterns.UnnecessaryDefaultInEnumSwitch.fixDefault(UnnecessaryDefaultInEnumSwitch.java:111)
  	at com.google.errorprone.bugpatterns.UnnecessaryDefaultInEnumSwitch.matchSwitch(UnnecessaryDefaultInEnumSwitch.java:102)
  	at com.google.errorprone.scanner.ErrorProneScanner.processMatchers(ErrorProneScanner.java:450)

This is caused by the fact that CaseTree#getStatements returns null for cases with CaseKind.RULE.

error-prone/core/src/main/java/com/google/errorprone/bugpatterns/UnnecessaryDefaultInEnumSwitch.java

Lines 108 to 110 in 37893c8

	private Description fixDefault(
	SwitchTree switchTree, CaseTree caseBeforeDefault, CaseTree defaultCase, VisitorState state) {
	List<? extends StatementTree> defaultStatements = defaultCase.getStatements();

error-prone/core/src/main/java/com/google/errorprone/bugpatterns/UnnecessaryDefaultInEnumSwitch.java

Lines 182 to 184 in 37893c8

	private Description fixUnrecognized(
	SwitchTree switchTree, CaseTree defaultCase, VisitorState state) {
	List<? extends StatementTree> defaultStatements = defaultCase.getStatements();

The logic in UnnecessaryDefaultInEnumSwitch needs to be updated to support cases with CaseKind.RULE, where callers need to use CaseTree#getBody instead.

There's also some required work to correctly handle case labels with multiple expressions, similar to what #2026 does for MissingCasesInEnumSwitch.

What version of Error Prone are you using?

2.4.0