Introduce Refaster rules for Sonar's java:S4635 rule (#1320)

As well as two related expressions that can be optimized.

See:
- https://sonarcloud.io/organizations/picnic-technologies/rules?open=java%3AS4635&rule_key=java%3AS4635
- 2615792731/java-checks/src/main/java/org/sonar/java/checks/StringOffsetMethodsCheck.java

error-prone-contrib/src/main/java/tech/picnic/errorprone/refasterrules/StringRules.java

@@ -244,4 +244,105 @@ final class StringRules {
       return Utf8.encodedLength(str);
     }
   }
+
+  /** Prefer {@link String#indexOf(int, int)} over less efficient alternatives. */
+  static final class StringIndexOfChar {
+    @BeforeTemplate
+    @SuppressWarnings("java:S4635" /* This violation will be rewritten. */)
+    int before(String string, int ch, int fromIndex) {
+      return string.substring(fromIndex).indexOf(ch);
+    }
+
+    @AfterTemplate
+    int after(String string, int ch, int fromIndex) {
+      return Math.max(-1, string.indexOf(ch, fromIndex) - fromIndex);
+    }
+  }
+
+  /** Prefer {@link String#indexOf(String, int)} over less efficient alternatives. */
+  static final class StringIndexOfString {
+    @BeforeTemplate
+    @SuppressWarnings("java:S4635" /* This violation will be rewritten. */)
+    int before(String string, String substring, int fromIndex) {
+      return string.substring(fromIndex).indexOf(substring);
+    }
+
+    @AfterTemplate
+    int after(String string, String substring, int fromIndex) {
+      return Math.max(-1, string.indexOf(substring, fromIndex) - fromIndex);
+    }
+  }
+
+  // XXX: Once we compile Refaster templates with JDK 21 also suggest `String#indexOf(int, int,
+  // int)` and `String#indexOf(String, int, int)`.
+
+  /** Prefer {@link String#lastIndexOf(int, int)} over less efficient alternatives. */
+  static final class StringLastIndexOfChar {
+    @BeforeTemplate
+    @SuppressWarnings("java:S4635" /* This violation will be rewritten. */)
+    int before(String string, int ch, int fromIndex) {
+      return string.substring(fromIndex).lastIndexOf(ch);
+    }
+
+    @AfterTemplate
+    int after(String string, int ch, int fromIndex) {
+      return Math.max(-1, string.lastIndexOf(ch) - fromIndex);
+    }
+  }
+
+  /** Prefer {@link String#lastIndexOf(String, int)} over less efficient alternatives. */
+  static final class StringLastIndexOfString {
+    @BeforeTemplate
+    @SuppressWarnings("java:S4635" /* This violation will be rewritten. */)
+    int before(String string, String substring, int fromIndex) {
+      return string.substring(fromIndex).lastIndexOf(substring);
+    }
+
+    @AfterTemplate
+    int after(String string, String substring, int fromIndex) {
+      return Math.max(-1, string.lastIndexOf(substring) - fromIndex);
+    }
+  }
+
+  /** Prefer {@link String#lastIndexOf(int, int)} over less efficient alternatives. */
+  static final class StringLastIndexOfCharWithIndex {
+    @BeforeTemplate
+    int before(String string, int ch, int fromIndex) {
+      return string.substring(0, fromIndex).lastIndexOf(ch);
+    }
+
+    @AfterTemplate
+    int after(String string, int ch, int fromIndex) {
+      return string.lastIndexOf(ch, fromIndex - 1);
+    }
+  }
+
+  /** Prefer {@link String#lastIndexOf(String, int)} over less efficient alternatives. */
+  // XXX: The replacement expression isn't fully equivalent: in case `substring` is empty, then
+  // the replacement yields `fromIndex - 1` rather than `fromIndex`.
+  static final class StringLastIndexOfStringWithIndex {
+    @BeforeTemplate
+    int before(String string, String substring, int fromIndex) {
+      return string.substring(0, fromIndex).lastIndexOf(substring);
+    }
+
+    @AfterTemplate
+    int after(String string, String substring, int fromIndex) {
+      return string.lastIndexOf(substring, fromIndex - 1);
+    }
+  }
+
+  /** Prefer {@link String#startsWith(String, int)} over less efficient alternatives. */
+  static final class StringStartsWith {
+    @BeforeTemplate
+    @SuppressWarnings("java:S4635" /* This violation will be rewritten. */)
+    boolean before(String string, String prefix, int fromIndex) {
+      return string.substring(fromIndex).startsWith(prefix);
+    }
+
+    @AfterTemplate
+    boolean after(String string, String prefix, int fromIndex) {
+      return string.startsWith(prefix, fromIndex);
+    }
+  }
 }

error-prone-contrib/src/test/resources/tech/picnic/errorprone/refasterrules/StringRulesTestInput.java

@@ -96,4 +96,32 @@ final class StringRulesTest implements RefasterRuleCollectionTestCase {
   int testUtf8EncodedLength() {
     return "foo".getBytes(UTF_8).length;
   }
+
+  int testStringIndexOfChar() {
+    return "foo".substring(1).indexOf('a');
+  }
+
+  int testStringIndexOfString() {
+    return "foo".substring(1).indexOf("bar");
+  }
+
+  int testStringLastIndexOfChar() {
+    return "foo".substring(1).lastIndexOf('a');
+  }
+
+  int testStringLastIndexOfString() {
+    return "foo".substring(1).lastIndexOf("bar");
+  }
+
+  int testStringLastIndexOfCharWithIndex() {
+    return "foo".substring(0, 2).lastIndexOf('a');
+  }
+
+  int testStringLastIndexOfStringWithIndex() {
+    return "foo".substring(0, 2).lastIndexOf("bar");
+  }
+
+  boolean testStringStartsWith() {
+    return "foo".substring(1).startsWith("bar");
+  }
 }

error-prone-contrib/src/test/resources/tech/picnic/errorprone/refasterrules/StringRulesTestOutput.java

@@ -96,4 +96,32 @@ final class StringRulesTest implements RefasterRuleCollectionTestCase {
   int testUtf8EncodedLength() {
     return Utf8.encodedLength("foo");
   }
+
+  int testStringIndexOfChar() {
+    return Math.max(-1, "foo".indexOf('a', 1) - 1);
+  }
+
+  int testStringIndexOfString() {
+    return Math.max(-1, "foo".indexOf("bar", 1) - 1);
+  }
+
+  int testStringLastIndexOfChar() {
+    return Math.max(-1, "foo".lastIndexOf('a') - 1);
+  }
+
+  int testStringLastIndexOfString() {
+    return Math.max(-1, "foo".lastIndexOf("bar") - 1);
+  }
+
+  int testStringLastIndexOfCharWithIndex() {
+    return "foo".lastIndexOf('a', 2 - 1);
+  }
+
+  int testStringLastIndexOfStringWithIndex() {
+    return "foo".lastIndexOf("bar", 2 - 1);
+  }
+
+  boolean testStringStartsWith() {
+    return "foo".startsWith("bar", 1);
+  }
 }