* Add more testing for ReadOnlySpan parsing
* Update `AssertReadFromParseContext` to also run assertions for a
`ParseContext` initialized from a `ReadOnlySpan` of the given input.
* Remove passing `ReadOnlySpan` parameters by ref since they are not
altered and the type isn't large enough to benefit from performance
improvements for passing by `ref` or `in`
* Update `ParseContext` `Initialize` overloads for
`ReadOnlySpan<byte>`, going down to two (since nothing was calling the
overload that changed the recursion limit). The first now calls the
second to set the `buffer` and `state` on the context, and has
documentation to note that it is used to create a `ParserInternalState`
with default settings.
* Move additional `ReadOnlySpan<byte>` `ParseContext` overloads next to
the existing overload
* Pass `ReadOnlySpan<byte>` parameters by `ref`
* Update `MessageParsingHelpers` to add checks for parsing from
`ReadOnlySpan<byte>`, remove tests from `CodedInputStreamTest`
This is important when parsing descriptor sets that contain extensions.
(The alternative is to get the descriptor bytes again for individual
elements, e.g. message descriptors, and reparse them with the
appropriate extensions. It's really ugly.)
UnknownField::AddAll() is called (multiple times) from UnknownField::MergeFrom(). The `extras` parameter is one of: `varintList`, `fixed32List`, `fixed64List`, `lengthDelimitedList`, or `groupList`. All of these members can be null, and are appropriately checked in other usage. If attempting to parse a proto with unknown extensions, and exception is thrown (NRE).
This adds the appropriate null check inside UnknownField::AddAll().
Jensaarai
Adam Cozzette
Jan Tattermusch
Joshua Haberman
Derek Perez
James Newton-King
Jon Skeet
CauhxMilloy