Module rustc_expand::mbe::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: cur_items.

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 $e:expr, and 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.

Example:

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 ·]

Structs

MatcherPos

Represents a single “position” (aka “matcher position”, aka “item”), as described in the module documentation.

MatcherTtFrame

An unzipping of TokenTrees… see the stack field of MatcherPos.

Enums

MatcherPosHandle

NamedMatch

NamedMatch is a pattern-match result for a single token::MATCH_NONTERMINAL: so it is associated with a single ident in a parse, and all MatchedNonterminals in the NamedMatch have the same non-terminal type (expr, item, etc). Each leaf in a single NamedMatch corresponds to a single token::MATCH_NONTERMINAL in the TokenTree that produced it.

ParseResult

Represents the possible results of an attempted parse.

TokenTreeOrTokenTreeSlice

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.

Functions

count_names

Count how many metavars are named in the given matcher ms.

create_matches

len Vecs (initially shared and empty) that will store matches of metavars.

initial_matcher_pos

Generates the top-level matcher position in which the “dot” is before the first token of the matcher ms.

inner_parse_loop

Process the matcher positions of cur_items until it is empty. In the process, this will produce more items in next_items, eof_items, and bb_items.

nameize

Takes a sequence of token trees ms representing a matcher which successfully matched input and an iterator of items that matched input and produces a NamedParseResult.

parse_tt

Use the given sequence of token trees (ms) as a matcher. Match the token stream from the given parser against it and return the match.

token_name_eq

Performs a token equality check, ignoring syntax context (that is, an unhygienic comparison)

Type Definitions

NamedMatchVec

NamedParseResult

A ParseResult where the Success variant contains a mapping of MacroRulesNormalizedIdents to NamedMatches. This represents the mapping of metavars to the token trees they bind to.