4 files changed, 42 insertions, 6 deletions
diff --git a/test/-ext-/string/test_capacity.rb b/test/-ext-/string/test_capacity.rb
index 2c6c51fdda..a23892142a 100644
--- a/test/-ext-/string/test_capacity.rb
+++ b/test/-ext-/string/test_capacity.rb
@@ -2,16 +2,17 @@
 require 'test/unit'
 require '-test-/string'
 require 'rbconfig/sizeof'
+require 'objspace'
 
 class Test_StringCapacity < Test::Unit::TestCase
   def test_capacity_embedded
-    assert_equal GC::INTERNAL_CONSTANTS[:BASE_SLOT_SIZE] - embed_header_size - 1, capa('foo')
+    assert_equal pool_slot_size(0) - embed_header_size - 1, capa('foo')
     assert_equal max_embed_len, capa('1' * max_embed_len)
     assert_equal max_embed_len, capa('1' * (max_embed_len - 1))
   end
 
   def test_capacity_shared
-    sym = ("a" * GC::INTERNAL_CONSTANTS[:BASE_SLOT_SIZE]).to_sym
+    sym = ("a" * pool_slot_size(0)).to_sym
     assert_equal 0, capa(sym.to_s)
   end
 
@@ -23,7 +24,7 @@ class Test_StringCapacity < Test::Unit::TestCase
   def test_s_new_capacity
     assert_equal("", String.new(capacity: 1000))
     assert_equal(String, String.new(capacity: 1000).class)
-    assert_equal(10_000 - 1, capa(String.new(capacity: 10_000))) # Real capa doesn't account for termlen
+    assert_equal(10_000, capa(String.new(capacity: 10_000)))
 
     assert_equal("", String.new(capacity: -1000))
     assert_equal(capa(String.new(capacity: -10000)), capa(String.new(capacity: -1000)))
@@ -47,7 +48,7 @@ class Test_StringCapacity < Test::Unit::TestCase
 
   def test_capacity_frozen
     s = String.new("I am testing", capacity: 1000)
-    s << "a" * GC::INTERNAL_CONSTANTS[:BASE_SLOT_SIZE]
+    s << "a" * pool_slot_size(0)
     s.freeze
     assert_equal(s.length, capa(s))
   end
@@ -66,7 +67,11 @@ class Test_StringCapacity < Test::Unit::TestCase
   end
 
   def embed_header_size
-    3 * RbConfig::SIZEOF['void*']
+    GC::INTERNAL_CONSTANTS[:RBASIC_SIZE] + RbConfig::SIZEOF['void*']
+  end
+
+  def pool_slot_size(_idx = 0)
+    Integer(ObjectSpace.dump("")[/"slot_size":(\d+)/, 1])
   end
 
   def max_embed_len
diff --git a/test/-ext-/string/test_fstring.rb b/test/-ext-/string/test_fstring.rb
index f737c9a269..fcec6be543 100644
--- a/test/-ext-/string/test_fstring.rb
+++ b/test/-ext-/string/test_fstring.rb
@@ -20,6 +20,10 @@ class Test_String_Fstring < Test::Unit::TestCase
     RUBY
   end
 
+  def test_rb_enc_interned_str_null_encoding
+    assert_equal Encoding::ASCII_8BIT, Bug::String.rb_enc_interned_str(nil).encoding
+  end
+
   def test_rb_enc_str_new_autoloaded_encoding
     assert_separately([], <<~RUBY)
       require '-test-/string'
@@ -28,6 +32,10 @@ class Test_String_Fstring < Test::Unit::TestCase
     RUBY
   end
 
+  def test_rb_enc_str_new_null_encoding
+    assert_equal Encoding::ASCII_8BIT, Bug::String.rb_enc_str_new(nil).encoding
+  end
+
   def test_instance_variable
     str = __method__.to_s * 3
     str.instance_variable_set(:@test, 42)
diff --git a/test/-ext-/string/test_interned_str.rb b/test/-ext-/string/test_interned_str.rb
index 340dba41e8..a81cb59aa5 100644
--- a/test/-ext-/string/test_interned_str.rb
+++ b/test/-ext-/string/test_interned_str.rb
@@ -9,4 +9,9 @@ class Test_RbInternedStr < Test::Unit::TestCase
     src << "b" * 20
     assert_equal "a" * 20, interned_str
   end
+
+  def test_interned_str_encoding
+    src = :ascii.name
+    assert_equal Encoding::US_ASCII, Bug::String.rb_interned_str_dup(src).encoding
+  end
 end
diff --git a/test/-ext-/string/test_set_len.rb b/test/-ext-/string/test_set_len.rb
index e3eff75d9b..41e14a293a 100644
--- a/test/-ext-/string/test_set_len.rb
+++ b/test/-ext-/string/test_set_len.rb
@@ -5,7 +5,7 @@ require "-test-/string"
 class Test_StrSetLen < Test::Unit::TestCase
   def setup
     # Make string long enough so that it is not embedded
-    @range_end = ("0".ord + GC::INTERNAL_CONSTANTS[:BASE_SLOT_SIZE]).chr
+    @range_end = ("0".ord + GC.stat_heap(0, :slot_size)).chr
     @s0 = [*"0"..@range_end].join("").freeze
     @s1 = Bug::String.new(@s0)
   end
@@ -63,4 +63,22 @@ class Test_StrSetLen < Test::Unit::TestCase
     assert_not_predicate str, :ascii_only?
     assert_equal u, str
   end
+
+  def test_valid_encoding_after_resized
+    s = "\0\0".force_encoding(Encoding::UTF_16BE)
+    str = Bug::String.new(s)
+    assert_predicate str, :valid_encoding?
+    str.resize(1)
+    assert_not_predicate str, :valid_encoding?
+    str.resize(2)
+    assert_predicate str, :valid_encoding?
+    str.resize(3)
+    assert_not_predicate str, :valid_encoding?
+
+    s = "\xDB\x00\xDC\x00".force_encoding(Encoding::UTF_16BE)
+    str = Bug::String.new(s)
+    assert_predicate str, :valid_encoding?
+    str.resize(2)
+    assert_not_predicate str, :valid_encoding?
+  end
 end