In order to do this, we need to save and restore parser state easily. The
important pieces of state are:
* lexer position;
* lexical scope stack.
Lexer position can be saved/restored easily. We don't need to store the tokens
for the function body because swift does not have a preprocessor and we can
easily re-lex everything we need. We just store the lexer state for the
beginning and the end of the body.
To save the lexical scope stack, we had to change the underlying data
structure. Originally, the parser used the ScopedHashTable, which supports
only a stack of scopes. But we need a *tree* of scopes. I implemented
TreeScopedHashTable based on ScopedHashTable. It has an optimization for
pushing/popping scopes in a stack fashion -- these scopes will not be allocated
on the heap. While ‘detached’ scopes that we want to re-enter later, and all
their parent scopes, are moved to the heap.
In parseIntoTranslationUnit() we do a second pass over the 'structural AST'
that does not contain function bodies to actually parse them from saved token
ranges.
Swift SVN r5886
