Tentative implementation of string slices (hidden under the flag --string-slices).

src/objects-debug.cc

Index: src/objects-debug.cc
diff --git a/src/objects-debug.cc b/src/objects-debug.cc
index 29632317a5e9309ee456c2e0ce9042b0b4c48cdf..62e38767608d03a48764484b05c8b36de0d49f41 100644
--- a/src/objects-debug.cc
+++ b/src/objects-debug.cc
@@ -349,6 +349,37 @@ void String::StringVerify() {
   if (IsSymbol()) {
     CHECK(!HEAP->InNewSpace(this));
   }
+  if (IsConsString()) {
+    ConsString::cast(this)->ConsStringVerify();
+  } else if (IsSlicedString()) {
+    SlicedString::cast(this)->SlicedStringVerify();
+  }
+}
+
+
+void ConsString::ConsStringVerify() {
+  CHECK(this->first()->IsString());
+  CHECK(this->second() == GetHeap()->empty_string() ||
+        this->second()->IsString());
+  if (this->IsFlat()) {
+    // A flat cons can only have a sequential string as first part because only
+    // String::SlowTryFlatten can convert a cons into a flat cons.
+    CHECK(this->first()->IsSeqString());
+  }
+}
+
+
+void SlicedString::SlicedStringVerify() {
+  CHECK(!this->parent()->IsConsString());
+  CHECK(!this->parent()->IsSlicedString());
+  if (this->IsTruncated()) {
+    CHECK(this->offset() == 0);
+    // A truncated slice cannot have an external string as parent because only
+    // String::SlowTryTruncate can convert a slice into a truncated slice.
+    // There is no way to gain direct access to the truncated parent string,
+    // therefore the truncated parent cannot be morphed into an external string.
+    CHECK(this->parent()->IsSeqString());
+  }
 }