The padding is always positive, remove dead code

Author: eregon

src/main/java/org/truffleruby/core/time/RubyDateFormatter.java

@@ -614,7 +614,7 @@ public static boolean formatCanBeFast(Token[] compiledPattern) {
 
     @ExplodeLoop
     public static Rope formatToRopeFast(Token[] compiledPattern, ZonedDateTime dt,
-            RopeNodes.ConcatNode concatNode, RopeNodes.SubstringNode substringNode) {
+            RopeNodes.ConcatNode concatNode) {
         Rope rope = null;
 
         for (Token token : compiledPattern) {
@@ -661,6 +661,7 @@ public static Rope formatToRopeFast(Token[] compiledPattern, ZonedDateTime dt,
                     // This fast-path only handles the '%6N' format, so output will always be 6 characters long.
                     final int length = 6;
                     final int padding = length - microSecondRope.characterLength();
+                    assert padding >= 0;
 
                     // `padding` is guaranteed to be >= 0 because `nano` can be at most 9 digits long before the
                     // conversion to microseconds. The division further constrains the rope to be at most 6 digits long.

src/main/java/org/truffleruby/core/time/TimeNodes.java

@@ -423,14 +423,9 @@ protected RubyString timeStrftime(RubyTime time, Object format,
                 @Cached RopeNodes.EqualNode equalNode,
                 @Cached("formatCanBeFast(pattern)") boolean canUseFast,
                 @Cached ConditionProfile yearIsFastProfile,
-                @Cached RopeNodes.ConcatNode concatNode,
-                @Cached RopeNodes.SubstringNode substringNode) {
+                @Cached RopeNodes.ConcatNode concatNode) {
             if (canUseFast && yearIsFastProfile.profile(yearIsFast(time))) {
-                return makeStringNode.fromRope(RubyDateFormatter.formatToRopeFast(
-                        pattern,
-                        time.dateTime,
-                        concatNode,
-                        substringNode));
+                return makeStringNode.fromRope(RubyDateFormatter.formatToRopeFast(pattern, time.dateTime, concatNode));
             } else {
                 return makeStringNode.fromBuilderUnsafe(formatTime(time, pattern), CodeRange.CR_UNKNOWN);
             }
@@ -440,15 +435,10 @@ protected RubyString timeStrftime(RubyTime time, Object format,
         @Specialization(guards = "libFormat.isRubyString(format)")
         protected RubyString timeStrftime(RubyTime time, Object format,
                 @CachedLibrary(limit = "2") RubyStringLibrary libFormat,
-                @Cached RopeNodes.ConcatNode concatNode,
-                @Cached RopeNodes.SubstringNode substringNode) {
+                @Cached RopeNodes.ConcatNode concatNode) {
             final Token[] pattern = compilePattern(libFormat.getRope(format));
             if (formatCanBeFast(pattern) && yearIsFast(time)) {
-                return makeStringNode.fromRope(RubyDateFormatter.formatToRopeFast(
-                        pattern,
-                        time.dateTime,
-                        concatNode,
-                        substringNode));
+                return makeStringNode.fromRope(RubyDateFormatter.formatToRopeFast(pattern, time.dateTime, concatNode));
             } else {
                 return makeStringNode.fromBuilderUnsafe(formatTime(time, pattern), CodeRange.CR_UNKNOWN);
             }