Fix String#slice with negative offset

This fix resolves the issue with String#slice(index, offset) called with negative offset.
#slice should return `nil` when offset is negative but returned a String itself:

```ruby
"foobar".slice(0, -1) #=> "foobar"
```

Author: andrykonchin

CHANGELOG.md

@@ -11,6 +11,7 @@ Compatibility:
 * Implement `StringScanner#{peek_byte,scan_byte,scan_integer,named_captures}` methods (#3788, @andrykonchin).
 * Support String patterns in `StringScanner#{exist?,scan_until,skip_until,check_until,search_full}` methods (@andrykonchin).
 * Implement `ObjectSpace::WeakKeyMap` (#3681, @andrykonchin).
+* Fix `String#slice` called with negative offset (@andrykonchin).
 
 Performance:
 

spec/ruby/core/string/shared/slice.rb

@@ -119,6 +119,13 @@
     "hello there".send(@method, -4,-3).should == nil
   end
 
+  platform_is pointer_size: 64 do
+    it "returns nil if the length is negative big value" do
+      "hello there".send(@method, 4, -(1 << 31)).should == nil
+      "hello there".send(@method, 4, -(1 << 63)).should == nil
+    end
+  end
+
   it "calls to_int on the given index and the given length" do
     "hello".send(@method, 0.5, 1).should == "h"
     "hello".send(@method, 0.5, 2.5).should == "he"

src/main/java/org/truffleruby/core/string/StringNodes.java

@@ -575,7 +575,13 @@ Object slice(Object string, int start, int length) {
         }
 
         @Specialization
-        Object slice(Object string, long start, long length) {
+        Object slice(Object string, long start, long length,
+                @Cached @Shared InlinedBranchProfile negativeLengthProfile) {
+            if (length < 0) {
+                negativeLengthProfile.enter(this);
+                return nil;
+            }
+
             int lengthInt = (int) length;
             if (lengthInt != length) {
                 lengthInt = Integer.MAX_VALUE; // go to end of string
@@ -589,8 +595,9 @@ Object slice(Object string, long start, long length) {
 
         @Specialization(guards = "wasProvided(length)")
         Object slice(Object string, long start, Object length,
-                @Cached @Shared ToLongNode toLongNode) {
-            return slice(string, start, toLongNode.execute(this, length));
+                @Cached @Shared ToLongNode toLongNode,
+                @Cached @Shared InlinedBranchProfile negativeLengthProfile) {
+            return slice(string, start, toLongNode.execute(this, length), negativeLengthProfile);
         }
 
         @Specialization(
@@ -600,8 +607,10 @@ Object slice(Object string, long start, Object length,
                         "isNotRubyString(start)",
                         "wasProvided(length)" })
         Object slice(Object string, Object start, Object length,
-                @Cached @Shared ToLongNode toLongNode) {
-            return slice(string, toLongNode.execute(this, start), toLongNode.execute(this, length));
+                @Cached @Shared ToLongNode toLongNode,
+                @Cached @Shared InlinedBranchProfile negativeLengthProfile) {
+            return slice(string, toLongNode.execute(this, start), toLongNode.execute(this, length),
+                    negativeLengthProfile);
         }
 
         // endregion