Protocol Compiler

• The proto compiler no longer requires a .proto filename when it is not
  generating code.
• Added flag --deterministic_output to protoc --encode=....
• Fixed deadlock when using google.protobuf.Any embedded in aggregate options.

C++

• Arenas are now unconditionally enabled. cc_enable_arenas no longer has
  any effect.
• Removed inlined string support, which is incompatible with arenas.
• Fix a memory corruption bug in reflection when mixing optional and
  non-optional fields.
• Make SpaceUsed() calculation more thorough for map fields.
• Add stack overflow protection for text format with unknown field values.
• FieldPath::FollowAll() now returns a bool to signal if an out-of-bounds
  error was encountered.
• Performance improvements for Map.
• Minor formatting fix when dumping a descriptor to .proto format with
  DebugString.
• UBSAN fix in RepeatedField (#2073).
• When running under ASAN, skip a test that makes huge allocations.
• Fixed a crash that could happen when creating more than 256 extensions in
  a single message.
• Fix a crash in BuildFile when passing in invalid descriptor proto.
• Parser security fix when operating with CodedInputStream.
• Warn against the use of AllowUnknownExtension.
• Migrated to C++11 for-range loops instead of index-based loops where
  possible. This fixes a lot of warnings when compiling with -Wsign-compare.
• Fix segment fault for proto3 optional (#7805)
• Adds a CMake option to build libprotoc separately (#7949)

Java

• Bugfix in mergeFrom() when a oneof has multiple message fields.
• Fix RopeByteString.RopeInputStream.read() returning -1 when told to read
  0 bytes when not at EOF.
• Redefine remove(Object) on primitive repeated field Lists to avoid
  autoboxing.
• Support "\u" escapes in textformat string literals.
• Trailing empty spaces are no longer ignored for FieldMask.
• Fix FieldMaskUtil.subtract to recursively remove mask.
• Mark enums with @java.lang.Deprecated if the proto enum has option
  deprecated = true;.
• Adding forgotten duration.proto to the lite library (#7738)

Python

• Print google.protobuf.NullValue as null instead of "NULL_VALUE" when it is
  used outside WKT Value/Struct.
• Fix bug occurring when attempting to deep copy an enum type in python 3.
• Add a setuptools extension for generating Python protobufs (#7783)
• Remove uses of pkg_resources in non-namespace packages. (#7902)
• [bazel/py] Omit google/init.py from the Protobuf runtime. (#7908)
• Removed the unnecessary setuptools package dependency for Python package (#7511)
• Fix PyUnknownFields memory leak (#7928)

PHP

• Added support for "==" to the PHP C extension (#7883)
• Added == operators for Map and Array. (#7900)
• Native C well-known types (#7944)
• Optimized away hex2bin() call in generated code (#8006)
• New version of upb, and a new hash function wyhash in third_party. (#8000)
• add missing hasOneof method to check presence of oneof fields (#8003)

Go

• Update go_package options to reference google.golang.org/protobuf module.

C#

• annotate ByteString.CopyFrom(ReadOnlySpan) as SecuritySafeCritical (#7701)
• Fix C# optional field reflection when there are regular fields too (#7705)
• Fix parsing negative Int32Value that crosses segment boundary (#8035)

Javascript

• JS: parse (un)packed fields conditionally (#7379)