Avoid interface calls in hot loop

Before, every charAt would emit (on android):
```
    0x00002104    adrp x17, #+0x1000 (addr 0x3000)
    0x00002108    ldr w17, [x17, #20]
    0x0000210c    ldr x0, [x0, #128]
    0x00002110    ldr x0, [x0, #328]
    0x00002114    ldr lr, [x0, #24]
    0x00002118    blr lr <-- Call into String.charAt(int)
```
Now, it emits the inlined implementation of charAt (branch is for possibly compressed strings):
```
    0x000020b4    ldur w16, [x4, #-8]
    0x000020b8    tbnz w16, #0, #+0xc (addr 0x20c4)
    0x000020bc    ldrb w4, [x4, x0]
    0x000020c0    b #+0x8 (addr 0x20c8)
    0x000020c4    ldrh w4, [x4, x0, lsl #1]
```

PiperOrigin-RevId: 590821750

java/core/src/main/java/com/google/protobuf/Utf8.java

@@ -214,24 +214,24 @@ static class UnpairedSurrogateException extends IllegalArgumentException {
    * @throws IllegalArgumentException if {@code sequence} contains ill-formed UTF-16 (unpaired
    *     surrogates)
    */
-  static int encodedLength(CharSequence sequence) {
+  static int encodedLength(String string) {
     // Warning to maintainers: this implementation is highly optimized.
-    int utf16Length = sequence.length();
+    int utf16Length = string.length();
     int utf8Length = utf16Length;
     int i = 0;
 
     // This loop optimizes for pure ASCII.
-    while (i < utf16Length && sequence.charAt(i) < 0x80) {
+    while (i < utf16Length && string.charAt(i) < 0x80) {
       i++;
     }
 
     // This loop optimizes for chars less than 0x800.
     for (; i < utf16Length; i++) {
-      char c = sequence.charAt(i);
+      char c = string.charAt(i);
       if (c < 0x800) {
         utf8Length += ((0x7f - c) >>> 31); // branch free!
       } else {
-        utf8Length += encodedLengthGeneral(sequence, i);
+        utf8Length += encodedLengthGeneral(string, i);
         break;
       }
     }
@@ -244,19 +244,19 @@ static int encodedLength(CharSequence sequence) {
     return utf8Length;
   }
 
-  private static int encodedLengthGeneral(CharSequence sequence, int start) {
-    int utf16Length = sequence.length();
+  private static int encodedLengthGeneral(String string, int start) {
+    int utf16Length = string.length();
     int utf8Length = 0;
     for (int i = start; i < utf16Length; i++) {
-      char c = sequence.charAt(i);
+      char c = string.charAt(i);
       if (c < 0x800) {
         utf8Length += (0x7f - c) >>> 31; // branch free!
       } else {
         utf8Length += 2;
         // jdk7+: if (Character.isSurrogate(c)) {
         if (Character.MIN_SURROGATE <= c && c <= Character.MAX_SURROGATE) {
           // Check that we have a well-formed surrogate pair.
-          int cp = Character.codePointAt(sequence, i);
+          int cp = Character.codePointAt(string, i);
           if (cp < MIN_SUPPLEMENTARY_CODE_POINT) {
             throw new UnpairedSurrogateException(i, utf16Length);
           }
@@ -267,7 +267,7 @@ private static int encodedLengthGeneral(CharSequence sequence, int start) {
     return utf8Length;
   }
 
-  static int encode(CharSequence in, byte[] out, int offset, int length) {
+  static int encode(String in, byte[] out, int offset, int length) {
     return processor.encodeUtf8(in, out, offset, length);
   }
   // End Guava UTF-8 methods.
@@ -326,9 +326,9 @@ static String decodeUtf8(byte[] bytes, int index, int size)
    *
    * @param in the source string to be encoded
    * @param out the target buffer to receive the encoded string.
-   * @see Utf8#encode(CharSequence, byte[], int, int)
+   * @see Utf8#encode(String, byte[], int, int)
    */
-  static void encodeUtf8(CharSequence in, ByteBuffer out) {
+  static void encodeUtf8(String in, ByteBuffer out) {
     processor.encodeUtf8(in, out);
   }
 
@@ -724,7 +724,7 @@ final String decodeUtf8Default(ByteBuffer buffer, int index, int size)
      *     {@code bytes.length - offset}
      * @return the new offset, equivalent to {@code offset + Utf8.encodedLength(sequence)}
      */
-    abstract int encodeUtf8(CharSequence in, byte[] out, int offset, int length);
+    abstract int encodeUtf8(String in, byte[] out, int offset, int length);
 
     /**
      * Encodes an input character sequence ({@code in}) to UTF-8 in the target buffer ({@code out}).
@@ -743,7 +743,7 @@ final String decodeUtf8Default(ByteBuffer buffer, int index, int size)
      * @throws ArrayIndexOutOfBoundsException if {@code in} encoded in UTF-8 is longer than {@code
      *     out.remaining()}
      */
-    final void encodeUtf8(CharSequence in, ByteBuffer out) {
+    final void encodeUtf8(String in, ByteBuffer out) {
       if (out.hasArray()) {
         final int offset = out.arrayOffset();
         int endIndex = Utf8.encode(in, out.array(), offset + out.position(), out.remaining());
@@ -756,13 +756,13 @@ final void encodeUtf8(CharSequence in, ByteBuffer out) {
     }
 
     /** Encodes the input character sequence to a direct {@link ByteBuffer} instance. */
-    abstract void encodeUtf8Direct(CharSequence in, ByteBuffer out);
+    abstract void encodeUtf8Direct(String in, ByteBuffer out);
 
     /**
      * Encodes the input character sequence to a {@link ByteBuffer} instance using the {@link
      * ByteBuffer} API, rather than potentially faster approaches.
      */
-    final void encodeUtf8Default(CharSequence in, ByteBuffer out) {
+    final void encodeUtf8Default(String in, ByteBuffer out) {
       final int inLength = in.length();
       int outIx = out.position();
       int inIx = 0;
@@ -1013,7 +1013,7 @@ String decodeUtf8Direct(ByteBuffer buffer, int index, int size)
     }
 
     @Override
-    int encodeUtf8(CharSequence in, byte[] out, int offset, int length) {
+    int encodeUtf8(String in, byte[] out, int offset, int length) {
       int utf16Length = in.length();
       int j = offset;
       int i = 0;
@@ -1065,7 +1065,7 @@ int encodeUtf8(CharSequence in, byte[] out, int offset, int length) {
     }
 
     @Override
-    void encodeUtf8Direct(CharSequence in, ByteBuffer out) {
+    void encodeUtf8Direct(String in, ByteBuffer out) {
       // For safe processing, we have to use the ByteBuffer API.
       encodeUtf8Default(in, out);
     }
@@ -1442,7 +1442,7 @@ String decodeUtf8Direct(ByteBuffer buffer, int index, int size)
     }
 
     @Override
-    int encodeUtf8(final CharSequence in, final byte[] out, final int offset, final int length) {
+    int encodeUtf8(final String in, final byte[] out, final int offset, final int length) {
       long outIx = offset;
       final long outLimit = outIx + length;
       final int inLimit = in.length();
@@ -1503,7 +1503,7 @@ int encodeUtf8(final CharSequence in, final byte[] out, final int offset, final
     }
 
     @Override
-    void encodeUtf8Direct(CharSequence in, ByteBuffer out) {
+    void encodeUtf8Direct(String in, ByteBuffer out) {
       final long address = addressOffset(out);
       long outIx = address + out.position();
       final long outLimit = address + out.limit();

java/core/src/test/java/com/google/protobuf/Utf8Test.java

@@ -194,7 +194,7 @@ private void assertEncoding_insufficientSpace(String message) {
 
   private static byte[] encodeToByteArray(String message, int length, Utf8.Processor processor) {
     byte[] output = new byte[length];
-    processor.encodeUtf8(message, output, 0, output.length);
+    var unused = processor.encodeUtf8(message, output, 0, output.length);
     return output;
   }