PHP

The C extension is completely rewritten. The new C extension has significantly
better parsing performance and fixes a handful of conformance issues. It will
also make it easier to add support for more features like proto2 and proto3 presence.
The new C extension does not support PHP 5.x, which is the reason for the major
version bump. PHP 5.x users can still use pure-PHP.

C++

Removed deprecated unsafe arena string accessors
Enabled heterogeneous lookup for std::string keys in maps.
Removed implicit conversion from StringPiece to std::string
Fix use-after-destroy bug when the Map is allocated in the arena.
Improved the randomness of map ordering
Added stack overflow protection for text format with unknown fields
Use std::hash for proto maps to help with portability.
Added more Windows macros to proto whitelist.
Arena constructors for map entry messages are now marked "explicit"
(for regular messages they were already explicit).
Fix subtle aliasing bug in RepeatedField::Add
Fix mismatch between MapEntry ByteSize and Serialize with respect to unset
fields.

JSON format conformance fixes:
- Reject lowercase t for Timestamp json format.
- Print full_name directly for extensions (no camelCase).
- Reject boolean values for integer fields.
- Reject NaN, Infinity, -Infinity that is not quoted.
- Base64 fixes for bytes fields: accept URL-safe base64 and missing padding.
Bugfix for fields/files named "async" or "await".
Improved the error message when AttributeError is returned from getattr
in EnumTypeWrapper.

Fixed a bug where setting optional proto3 enums with setFooValue() would
not mark the value as present.
Add Subtract function to FieldMaskUtil.

Dropped support for netstandard1.0 (replaced by support for netstandard1.1).
This was required to modernize the parsing stack to use the Span<byte>
type internally. (#7351)
Add ParseFrom(ReadOnlySequence<byte>) method to enable GC friendly
parsing with reduced allocations and buffer copies. (#7351)
Add support for serialization directly to a IBufferWriter<byte> or
to a Span<byte> to enable GC friendly serialization.
The new API is available as extension methods on the IMessage type. (#7576)
Add GOOGLE_PROTOBUF_REFSTRUCT_COMPATIBILITY_MODE define to make
generated code compatible with old C# compilers (pre-roslyn compilers
from .NET framework and old versions of mono) that do not support
ref structs. Users that are still on a legacy stack that does
not support C# 7.2 compiler might need to use the new define
in their projects to be able to build the newly generated code. (#7490)
Due to the major overhaul of parsing and serialization internals (#7351 and #7576),
it is recommended to regenerate your generated code to achieve the best
performance (the legacy generated code will still work, but might incur
a slight performance penalty).