Function parse_tree 

fn parse_tree<'a>(
    tree: &'a TokenTree,
    outer_iter: &mut TokenStreamIter<'a>,
    part: RulePart,
    sess: &Session,
    node_id: NodeId,
    features: &Features,
    edition: Edition,
) -> TokenTree

Takes a tokenstream::TokenTree and returns a self::TokenTree. Specifically, this takes a generic TokenTree, such as is used in the rest of the compiler, and returns a TokenTree for use in parsing a macro.

Converting the given tree may involve reading more tokens.

Parameters

• tree: the tree we wish to convert.
• outer_iter: an iterator over trees. We may need to read more tokens from it in order to finish converting tree
• part: same as parse.
• sess: the parsing session. Any errors will be emitted to this session.
• features: language features so we can do feature gating.