[][src]Module syntax::tokenstream

Token Streams

TokenStreams represent syntactic objects before they are converted into ASTs. A TokenStream is, roughly speaking, a sequence (eg stream) of TokenTrees, which are themselves a single Token or a Delimited subsequence of tokens.


TokenStreams are persistent data structures constructed as ropes with reference counted-children. In general, this means that calling an operation on a TokenStream (such as slice) produces an entirely new TokenStream from the borrowed reference to the original. This essentially coerces TokenStreams into 'views' of their subparts, and a borrowed TokenStream is sufficient to build an owned TokenStream without taking ownership of the original.



The TokenStream type is large enough to represent a single TokenTree without allocation. ThinTokenStream is smaller, but needs to allocate to represent a single TokenTree. We must use ThinTokenStream in TokenTree::Delimited to avoid infinite size due to recursion.


Token Streams




When the main rust parser encounters a syntax-extension invocation, it parses the arguments to the invocation as a token-tree. This is a very loose structure, such that all sorts of different AST-fragments can be passed to syntax extensions using a uniform type.