fix: support implicit @MethodSource for UnusedMethod

Author: JarvisCraft

core/src/main/java/com/google/errorprone/bugpatterns/UnusedMethod.java

@@ -303,10 +303,14 @@ private void handleMethodSource(MethodTree tree) {
         sym.getRawAttributes().stream()
             .filter(a -> a.type.tsym.getQualifiedName().equals(name))
             .findAny()
-            // get the annotation value array as a set of Names
-            .flatMap(a -> getAnnotationValue(a, "value"))
-            .map(
-                y -> asStrings(y).map(state::getName).map(Name::toString).collect(toImmutableSet()))
+            // get the annotation value array as a set of Names,
+            // normalizing unset value to the empty value.
+            .map(a -> getAnnotationValue(a, "value")
+                .map(y -> asStrings(y).map(state::getName).map(Name::toString).collect(toImmutableSet()))
+                .orElse(ImmutableSet.of())
+            )
+            // If no explicit method sources were specified, use method name instead.
+            .map(names -> names.isEmpty() ? Set.of(sym.name.toString()) : names)
             // remove all potentially unused methods referenced by the @MethodSource
             .ifPresent(
                 referencedNames ->

core/src/test/java/com/google/errorprone/bugpatterns/UnusedMethodTest.java

@@ -339,6 +339,36 @@ private static Stream<String> parameters() {
         .doTest();
   }
 
+  @Test
+  public void implicitMethodSource() {
+    helper
+        .addSourceLines(
+            "MethodSource.java",
+            """
+            package org.junit.jupiter.params.provider;
+
+            public @interface MethodSource {
+              String[] value();
+            }
+            """)
+        .addSourceLines(
+            "Test.java",
+            """
+            import java.util.stream.Stream;
+            import org.junit.jupiter.params.provider.MethodSource;
+
+            class Test {
+              @MethodSource
+              void test() {}
+
+              private static Stream<String> test() {
+                return Stream.of();
+              }
+            }
+            """)
+        .doTest();
+  }
+
   @Test
   public void qualifiedMethodSource() {
     helper