ext:: tt:: macro_parser
This is an NFA-based parser, which calls out to the main rust parser for named non-terminals (which it commits to fully when it hits one in a grammar). There's a set of current NFA threads and a set of next ones. Instead of NTs, we have a special case for Kleene star. The big-O, in pathological cases, is worse than traditional use of NFA or Earley parsing, but it's an easier fit for Macro-by-Example-style rules.
(In order to prevent the pathological case, we'd need to lazily construct the resulting
NamedMatches at the very end. It'd be a pain, and require more memory to keep around old
items, but it would also save overhead)
We don't say this parser uses the Earley algorithm, because it's unnecessarily inaccurate. The macro parser restricts itself to the features of finite state automata. Earley parsers can be described as an extension of NFAs with completion rules, prediction rules, and recursion.
Quick intro to how the parser works:
A 'position' is a dot in the middle of a matcher, usually represented as a
dot. For example
· a $( a )* a b is a position, as is
a $( · a )* a b.
The parser walks through the input a character at a time, maintaining a list
of threads consistent with the current position in the input string:
As it processes them, it fills up
eof_items with threads that would be valid if
the macro invocation is now over,
bb_items with threads that are waiting on
a Rust non-terminal like
next_items with threads that are waiting
on a particular token. Most of the logic concerns moving the · through the
repetitions indicated by Kleene stars. The rules for moving the · without
consuming any input are called epsilon transitions. It only advances or calls
out to the real Rust parser when no
cur_items threads remain.
Start parsing a a a a b against [· a $( a )* a b]. Remaining input: a a a a b next: [· a $( a )* a b] - - - Advance over an a. - - - Remaining input: a a a b cur: [a · $( a )* a b] Descend/Skip (first item). next: [a $( · a )* a b] [a $( a )* · a b]. - - - Advance over an a. - - - Remaining input: a a b cur: [a $( a · )* a b] [a $( a )* a · b] Follow epsilon transition: Finish/Repeat (first item) next: [a $( a )* · a b] [a $( · a )* a b] [a $( a )* a · b] - - - Advance over an a. - - - (this looks exactly like the last step) Remaining input: a b cur: [a $( a · )* a b] [a $( a )* a · b] Follow epsilon transition: Finish/Repeat (first item) next: [a $( a )* · a b] [a $( · a )* a b] [a $( a )* a · b] - - - Advance over an a. - - - (this looks exactly like the last step) Remaining input: b cur: [a $( a · )* a b] [a $( a )* a · b] Follow epsilon transition: Finish/Repeat (first item) next: [a $( a )* · a b] [a $( · a )* a b] [a $( a )* a · b] - - - Advance over a b. - - - Remaining input: '' eof: [a $( a )* a b ·]
Represents a single "position" (aka "matcher position", aka "item"), as described in the module documentation.
An unzipping of
Represents the possible results of an attempted parse.
Either a sequence of token trees or a single one. This is used as the representation of the sequence of tokens that make up a matcher.
Count how many metavars are named in the given matcher
The token is an identifier, but not
Generates the top-level matcher position in which the "dot" is before the first token of the
Process the matcher positions of
Checks whether a non-terminal may begin with a particular token.
Takes a sequence of token trees
Use the given sequence of token trees (
Generates an appropriate parsing failure message. For EOF, this is "unexpected end...". For other tokens, this is "unexpected token...".
A call to the "black-box" parser to parse some Rust non-terminal.
Performs a token equality check, ignoring syntax context (that is, an unhygienic comparison)