Instead, only reference count the SyntaxArena that the RawSyntax nodes
live in. The user of RawSyntax nodes must guarantee that the SyntaxArena
stays alive as long as the RawSyntax nodes are being accessed.
During parse time, the SyntaxTreeCreator holds on to the SyntaxArena
in which it creates RawSyntax nodes. When inspecting a syntax tree,
the root SyntaxData node keeps the SyntaxArena alive. The change should
be mostly invisible to the users of the public libSyntax API.
This change significantly decreases the overall reference-counting
overhead. Since we were not able to free individual RawSyntax nodes
anyway, performing the reference-counting on the level of the
SyntaxArena feels natural.
Do the same thing that we are already doing for trivia: Since RawSyntax
nodes always live inside a SyntaxArena, we don't need to tail-allocate
an OwnedString to store the token's text. Instead we can just copy it
to the SyntaxArena. If we copy the entire source buffer to the syntax
arena at the start of parsing, this means that no more copies are
required later on. Plus we also avoid ref-counting the OwnedString which
should also increase performance.
Instead of creating syntax nodes directly, modify the parser to invoke an abstract interface 'SyntaxParseActions' while it is parsing the source code.
This decouples the act of parsing from the act of forming a syntax tree representation.
'SyntaxTreeCreator' is an implementation of SyntaxParseActions that handles the logic of creating a syntax tree.
To enforce the layering separation of parsing and syntax tree creation, a static library swiftSyntaxParse is introduced to compose the two.
This decoupling is important for introducing a syntax parser library for SwiftSyntax to directly access parsing.
