Allow -> as placeholder separator

mug-errorprone/src/main/java/com/google/mu/errorprone/FormatStringUtils.java

@@ -4,6 +4,7 @@
 import static com.google.common.base.CharMatcher.whitespace;
 import static com.google.common.collect.ImmutableList.toImmutableList;
 import static com.google.mu.util.Optionals.optionally;
+import static com.google.mu.util.Substring.before;
 import static com.google.mu.util.Substring.consecutive;
 import static com.google.mu.util.Substring.first;
 import static com.google.mu.util.Substring.firstOccurrence;
@@ -33,17 +34,16 @@
 final class FormatStringUtils {
   static final Substring.Pattern PLACEHOLDER_PATTERN =
       consecutive(CharMatcher.noneOf("{}")::matches).immediatelyBetween("{", INCLUSIVE, "}", INCLUSIVE);
-  private static final Substring.Pattern PLACEHOLDER_SEPARATOR =
+  private static final Substring.Pattern PLACEHOLDER_NAME_END =
       Stream.of("=", "->").map(Substring::first).collect(firstOccurrence());
   static final Substring.RepeatingPattern PLACEHOLDER_NAMES_PATTERN =
       consecutive(CharMatcher.noneOf("{}")::matches).immediatelyBetween("{", "}").repeatedly();
 
   static ImmutableList<String> placeholderVariableNames(String formatString) {
-    Substring.Pattern beforeSeparator = Substring.before(PLACEHOLDER_SEPARATOR);
     return PLACEHOLDER_NAMES_PATTERN
         .from(formatString)
         // for Cloud resource name syntax
-        .map(n -> beforeSeparator.from(n).map(whitespace()::trimTrailingFrom).orElse(n))
+        .map(n -> before(PLACEHOLDER_NAME_END).from(n).map(whitespace()::trimTrailingFrom).orElse(n))
         .collect(toImmutableList());
   }