Protobuf

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
* 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
* [python-runtime] 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
* 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(ruby): Fix various exceptions in Ruby on 64-bit Windows (8563)
* fix(ruby): 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
* Opensourcing kotlin protos (8272)
* Use a newer version of rules_proto, with the new rule `proto_descriptor_set` (8469)

Java
* Refactoring java full runtime to reuse sub-message builders and prepare to
migrate parsing logic from parse constructor to builder.
* Move proto wireformat parsing functionality from the private "parsing
constructor" to the Builder class.
* Change the Lite runtime to prefer merging from the wireformat into mutable
messages rather than building up a new immutable object before merging. This
way results in fewer allocations and copy operations.
* Make message-type extensions merge from wire-format instead of building up
instances and merging afterwards. This has much better performance.
* Fix TextFormat parser to build up recurring (but supposedly not repeated)
sub-messages directly from text rather than building a new sub-message and
merging the fully formed message into the existing field.
* This release addresses a [Security Advisory for Java users](https://github.com/protocolbuffers/protobuf/security/advisories/GHSA-h4h5-3hr4-j3g2)

Java
* Improve performance characteristics of UnknownFieldSet parsing (9371)

C++
* Fix compiler warnings issue found in conformance_test_runner 8189 (8190)
* Fix MinGW-w64 build issues. (8286)
* [Protoc] C++ Resolved an issue where NO_DESTROY and CONSTINIT are in incorrect order (8296)
* Fix PROTOBUF_CONSTINIT macro redefinition (8323)
* Delete StringPiecePod (8353)
* Fix gcc error: comparison of unsigned expression in '>= 0' is always … (8309)
* Fix cmake install on iOS (8301)
* Create a CMake option to control whether or not RTTI is enabled (8347)
* Fix endian.h location on FreeBSD (8351)
* Refactor util::Status (8354)
* Make util::Status more similar to absl::Status (8405)
* Fix -Wsuggest-destructor-override for generated C++ proto classes. (8408)
* Refactor StatusOr and StringPiece (8406)
* Refactor uint128 (8416)
* The ::pb namespace is no longer exposed due to conflicts.
* Allow MessageDifferencer::TreatAsSet() (and friends) to override previous
calls instead of crashing.
* Reduce the size of generated proto headers for protos with `string` or
`bytes` fields.
* Move arena() operation on uncommon path to out-of-line routine
* For iterator-pair function parameter types, take both iterators by value.
* Code-space savings and perhaps some modest performance improvements in
RepeatedPtrField.
* Eliminate nullptr check from every tag parse.
* Remove unused _$name$_cached_byte_size_ fields.
* Serialize extension ranges together when not broken by a proto field in the
middle.
* Do out-of-line allocation and deallocation of string object in ArenaString.
* Streamline ParseContext::ParseMessage<T> to avoid code bloat and improve
performance.
* New member functions RepeatedField::Assign, RepeatedPtrField::{Add, Assign}.
* Fix undefined behavior warning due to innocuous uninitialization of value
on an error path.
* Avoid expensive inlined code space for encoding message length for messages
>= 128 bytes and instead do a procedure call to a shared out-of-line routine.
* util::DefaultFieldComparator will be final in a future version of protobuf.
Subclasses should inherit from SimpleFieldComparator instead.

C
* Add .NET 5 target and improve WriteString performance with SIMD (8147)

Java
* deps: update JUnit and Truth (8319)
* Detect invalid overflow of byteLimit and return InvalidProtocolBufferException as documented.
* Exceptions thrown while reading from an InputStream in parseFrom are now
included as causes.
* Support potentially more efficient proto parsing from RopeByteStrings.
* Clarify runtime of ByteString.Output.toStringBuffer().
* Added UnsafeByteOperations to protobuf-lite (8426)

JavaScript
* Make Any.pack() chainable.

Python
* Fix some constness / char literal issues being found by MSVC standard conforming mode (8344)
* Switch on "new" buffer API (8339)
* Enable crosscompiling aarch64 python wheels under dockcross manylinux docker image (8280)
* Fixed a bug in text format where a trailing colon was printed for repeated field.
* When TextFormat encounters a duplicate message map key, replace the current
one instead of merging.

Objective-C
* Move the class map to a CFDictionary. (8328)

PHP
* read_property() handler is not supposed to return NULL (8362)
* Changed parameter type from long to integer (7613)
* fix: README supported PHP version for C extension (8236)

Ruby
* Fixed quadratic memory usage when appending to arrays. (8364)
* Fixed memory leak of Ruby arena objects. (8461)
* Add support for proto3 json_name in compiler and field definitions. (8356)

Other
* Some doc on AOT compilation and protobuf (8294)
* [CMake] Ability to pass options to protoc executable from cmake (8374)
* Add --fatal_warnings flag to treat warnings as errors (8131)
* [bazel] Remove deprecated way to depend on googletest (8396)
* add error returns missing from protoc to prevent it from exiting with… (8409)

Ruby
* Fixed memory leak of Ruby arena objects. (8462)
* Add support for proto3 json_name in compiler and field definitions. (8356)