Protobuf

C++
* Fix warnings raised by clang 11 (8664)
* Make StringPiece constructible from std::string_view (8707)
* Add missing capability attributes for LLVM 12 (8714)
* Stop using std::iterator (deprecated in C++17). (8741)
* Move field_access_listener from libprotobuf-lite to libprotobuf (8775)
* Fix 7047 Safely handle setlocale (8735)
* Remove deprecated version of SetTotalBytesLimit() (8794)
* Support arena allocation of google::protobuf::AnyMetadata (8758)
* Fix undefined symbol error around SharedCtor() (8827)
* Fix default value of enum(int) in json_util with proto2 (8835)
* Better Smaller ByteSizeLong
* Introduce event filters for inject_field_listener_events
* Reduce memory usage of DescriptorPool
* For lazy fields copy serialized form when allowed.
* Re-introduce the InlinedStringField class
* v2 access listener
* Reduce padding in the proto's ExtensionRegistry map.
* GetExtension performance optimizations
* Make tracker a static variable rather than call static functions
* Support extensions in field access listener
* Annotate MergeFrom for field access listener
* Fix incomplete types for field access listener
* Add map_entry/new_map_entry to SpecificField in MessageDifferencer. They
record the map items which are different in MessageDifferencer's reporter.
* Reduce binary size due to fieldless proto messages
* TextFormat: ParseInfoTree supports getting field end location in addition to
start.
* Fix repeated enum extension size in field listener
* Enable Any Text Expansion for Descriptors::DebugString()
* Switch from int{8,16,32,64} to int{8,16,32,64}_t

Java
* Fix errorprone conflict (8723)
* Removing deprecated TimeUtil class. (8749)
* Optimized FieldDescriptor.valueOf() to avoid array copying.
* Removing deprecated TimeUtil class.
* Add Durations.parseUnchecked(String) and Timestamps.parseUnchecked(String)
* FieldMaskUtil: Add convenience method to mask the fields out of a given proto.

JavaScript
* Optimize binary parsing of repeated float64
* Fix for optimization when reading doubles from binary wire format
* Replace toArray implementation with toJSON.

PHP
* Migrate PHP & Ruby to ABSL wyhash (8854)

Ruby
* Move DSL implementation from C to pure Ruby (8850)

Other
* [csharp] ByteString.CreateCodedInput should use ArraySegment offset and count (8740)
* [ObjC] Add support for using the proto package to prefix symbols. (8760)
* field_presence.md: fix Go example (8788)

**Python**
* Note: This is the last release to support Python 2.7. Future releases will require Python >= 3.5.

**C++**
* Introduce FieldAccessListener.
* Stop emitting boilerplate {Copy/Merge}From in each ProtoBuf class
* Fixed some uninitialized variable warnings in generated_message_reflection.cc.

**Kotlin**
* Fix duplicate proto files error (8699)

**Java**
* Fixed parser to check that we are at a proper limit when a sub-message has
finished parsing.

**General**
* Support M1 (8557)

****Kotlin****
* Fix duplicate class error (8653)

****PHP****
* Fixed SEGV in sub-message getters for well-known types when message is unset
(8670)

PHP
* Fixed PHP memory leaks and arginfo errors. (8614)
* Fixed JSON parser to allow multiple values from the same oneof as long as all but one are null.

Ruby
* Fixed memory bug: properly root repeated/map field when assigning. (8639)
* Fixed JSON parser to allow multiple values from the same oneof as long as all but one are null.

Protocol Compiler
* Fix the generated source information for reserved values in Enums.

C++
* Fix -Wunused-parameter in map<string, int> fields (fixes 8494) (8500)
* Use byteswap.h when building against musl libc (8503)
* Fix -Wundefined-inline error when using SharedCtor() or SharedDtor() (8532)
* Fix bug where `Descriptor::DebugString()` printed proto3 synthetic oneofs.
* Provide stable versions of `SortAndUnique()`.
* Make sure to cache proto3 optional message fields when they are cleared.
* Expose UnsafeArena methods to Reflection.
* Use std::string::empty() rather than std::string::size() > 0.

Kotlin
* Introduce support for Kotlin protos (8272)
* Restrict extension setter and getter operators to non-nullable T.

Java
* updating GSON and Guava to more recent versions (8524)
* Reduce the time spent evaluating isExtensionNumber by storing the extension
ranges in a TreeMap for faster queries. This is particularly relevant for
protos which define a large number of extension ranges, for example when
each tag is defined as an extension.
* Fix java bytecode estimation logic for optional fields.
* Optimize Descriptor.isExtensionNumber.

Python
* Add MethodDescriptor.CopyToProto() (8327)
* Remove unused python_protobuf.{cc,h} (8513)
* Start publishing python aarch64 manylinux wheels normally (8530)
* Fix constness issue detected by MSVC standard conforming mode (8568)
* Make JSON parsing match C++ and Java when multiple fields from the same
oneof are present and all but one is null.

Ruby
* Add support for proto3 json_name in compiler and field definitions (8356)
* Fixed memory leak of Ruby arena objects. (8461)
* Fix source gem compilation (8471)
* Fix various exceptions in Ruby on 64-bit Windows (8563)
* Fix crash when calculating Message hash values on 64-bit Windows (8565)

Conformance Tests
* Added a conformance test for the case of multiple fields from the same
oneof.

Other
* Use a newer version of rules_proto, with the new rule `proto_descriptor_set` (8469)