Add tests to `String#byteslice` memory allocation behaviour. (#1909)

test/ruby/test_string_memory.rb

@@ -0,0 +1,55 @@
+# frozen_string_literal: false
+require 'test/unit'
+require 'objspace'
+
+class TestStringMemory < Test::Unit::TestCase
+  def capture_allocations(klass)
+    allocations = []
+
+    GC.start
+    GC.disable
+    generation = GC.count
+
+    ObjectSpace.trace_object_allocations do
+      yield
+
+      ObjectSpace.each_object(klass) do |instance|
+        allocations << instance if ObjectSpace.allocation_generation(instance) == generation
+      end
+    end
+
+    return allocations
+  ensure
+    GC.enable
+  end
+
+  def test_byteslice_prefix
+    string = "a" * 100_000
+
+    allocations = capture_allocations(String) do
+      string.byteslice(0, 50_000)
+    end
+
+    assert_equal 1, allocations.size
+  end
+
+  def test_byteslice_postfix
+    string = ("a" * 100_000).freeze
+
+    allocations = capture_allocations(String) do
+      string.byteslice(50_000, 100_000)
+    end
+
+    assert_equal 1, allocations.size
+  end
+
+  def test_byteslice_postfix_twice
+    string = ("a" * 100_000).freeze
+
+    allocations = capture_allocations(String) do
+      string.byteslice(50_000, 100_000).byteslice(25_000, 50_000)
+    end
+
+    assert_equal 2, allocations.size
+  end
+end