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737 lines
27 KiB
Rust
737 lines
27 KiB
Rust
//! An NFA-based parser, which is porting from rustc mbe parsing code
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//!
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//! See https://github.com/rust-lang/rust/blob/70b18bc2cbac4712020019f5bf57c00905373205/compiler/rustc_expand/src/mbe/macro_parser.rs
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//! Here is a quick intro to how the parser works, copied from rustc:
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//!
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//! A 'position' is a dot in the middle of a matcher, usually represented as a
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//! dot. For example `· a $( a )* a b` is a position, as is `a $( · a )* a b`.
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//!
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//! The parser walks through the input a character at a time, maintaining a list
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//! of threads consistent with the current position in the input string: `cur_items`.
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//!
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//! As it processes them, it fills up `eof_items` with threads that would be valid if
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//! the macro invocation is now over, `bb_items` with threads that are waiting on
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//! a Rust non-terminal like `$e:expr`, and `next_items` with threads that are waiting
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//! on a particular token. Most of the logic concerns moving the · through the
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//! repetitions indicated by Kleene stars. The rules for moving the · without
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//! consuming any input are called epsilon transitions. It only advances or calls
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//! out to the real Rust parser when no `cur_items` threads remain.
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//!
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//! Example:
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//!
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//! ```text, ignore
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//! Start parsing a a a a b against [· a $( a )* a b].
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//!
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//! Remaining input: a a a a b
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//! next: [· a $( a )* a b]
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//!
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//! - - - Advance over an a. - - -
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//!
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//! Remaining input: a a a b
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//! cur: [a · $( a )* a b]
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//! Descend/Skip (first item).
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//! next: [a $( · a )* a b] [a $( a )* · a b].
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//!
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//! - - - Advance over an a. - - -
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//!
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//! Remaining input: a a b
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//! cur: [a $( a · )* a b] [a $( a )* a · b]
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//! Follow epsilon transition: Finish/Repeat (first item)
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//! next: [a $( a )* · a b] [a $( · a )* a b] [a $( a )* a · b]
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//!
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//! - - - Advance over an a. - - - (this looks exactly like the last step)
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//!
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//! Remaining input: a b
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//! cur: [a $( a · )* a b] [a $( a )* a · b]
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//! Follow epsilon transition: Finish/Repeat (first item)
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//! next: [a $( a )* · a b] [a $( · a )* a b] [a $( a )* a · b]
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//!
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//! - - - Advance over an a. - - - (this looks exactly like the last step)
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//!
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//! Remaining input: b
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//! cur: [a $( a · )* a b] [a $( a )* a · b]
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//! Follow epsilon transition: Finish/Repeat (first item)
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//! next: [a $( a )* · a b] [a $( · a )* a b] [a $( a )* a · b]
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//!
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//! - - - Advance over a b. - - -
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//!
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//! Remaining input: ''
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//! eof: [a $( a )* a b ·]
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//! ```
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use crate::{
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expander::{Binding, Bindings, Fragment},
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parser::{Op, OpDelimited, OpDelimitedIter, RepeatKind, Separator},
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tt_iter::TtIter,
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ExpandError, MetaTemplate,
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};
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use super::ExpandResult;
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use parser::FragmentKind::*;
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use smallvec::{smallvec, SmallVec};
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use syntax::SmolStr;
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impl Bindings {
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fn push_optional(&mut self, name: &SmolStr) {
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// FIXME: Do we have a better way to represent an empty token ?
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// Insert an empty subtree for empty token
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let tt = tt::Subtree::default().into();
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self.inner.push((name.clone(), Binding::Fragment(Fragment::Tokens(tt))));
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}
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fn push_empty(&mut self, name: &SmolStr) {
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self.inner.push((name.clone(), Binding::Empty));
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}
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fn push_nested(&mut self, idx: usize, nested: Bindings) -> Result<(), ExpandError> {
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for (key, value) in nested.inner {
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if self.get_mut(&key).is_none() {
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self.inner.push((key.clone(), Binding::Nested(Vec::new())));
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}
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match self.get_mut(&key) {
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Some(Binding::Nested(it)) => {
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// insert empty nested bindings before this one
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while it.len() < idx {
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it.push(Binding::Nested(vec![]));
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}
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it.push(value);
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}
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_ => {
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return Err(ExpandError::BindingError(format!(
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"could not find binding `{}`",
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key
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)));
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}
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}
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}
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Ok(())
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}
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fn get_mut(&mut self, name: &str) -> Option<&mut Binding> {
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self.inner.iter_mut().find_map(|(n, b)| if n == name { Some(b) } else { None })
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}
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fn bindings(&self) -> impl Iterator<Item = &Binding> {
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self.inner.iter().map(|(_, b)| b)
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}
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}
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macro_rules! err {
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() => {
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ExpandError::BindingError(format!(""))
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};
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($($tt:tt)*) => {
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ExpandError::BindingError(format!($($tt)*))
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};
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}
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#[derive(Clone, Debug, Default, PartialEq, Eq)]
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pub(super) struct Match {
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pub(super) bindings: Bindings,
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/// We currently just keep the first error and count the rest to compare matches.
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pub(super) err: Option<ExpandError>,
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pub(super) err_count: usize,
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/// How many top-level token trees were left to match.
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pub(super) unmatched_tts: usize,
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/// The number of bound variables
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pub(super) bound_count: usize,
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}
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impl Match {
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fn add_err(&mut self, err: ExpandError) {
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let prev_err = self.err.take();
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self.err = prev_err.or(Some(err));
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self.err_count += 1;
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}
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}
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/// Matching errors are added to the `Match`.
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pub(super) fn match_(pattern: &MetaTemplate, input: &tt::Subtree) -> Match {
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let mut res = match_loop(pattern, &input);
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res.bound_count = count(res.bindings.bindings());
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return res;
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fn count<'a>(bindings: impl Iterator<Item = &'a Binding>) -> usize {
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bindings
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.map(|it| match it {
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Binding::Fragment(_) => 1,
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Binding::Empty => 1,
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Binding::Nested(it) => count(it.iter()),
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})
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.sum()
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}
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}
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#[derive(Debug, Clone)]
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struct MatchState<'t> {
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/// The position of the "dot" in this matcher
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dot: OpDelimitedIter<'t>,
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/// Token subtree stack
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/// When matching against matchers with nested delimited submatchers (e.g., `pat ( pat ( .. )
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/// pat ) pat`), we need to keep track of the matchers we are descending into. This stack does
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/// that where the bottom of the stack is the outermost matcher.
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stack: SmallVec<[OpDelimitedIter<'t>; 4]>,
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/// The "parent" matcher position if we are in a repetition. That is, the matcher position just
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/// before we enter the repetition.
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up: Option<Box<MatchState<'t>>>,
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/// The separator if we are in a repetition.
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sep: Option<Separator>,
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/// The KleeneOp of this sequence if we are in a repetition.
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sep_kind: Option<RepeatKind>,
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/// Number of tokens of seperator parsed
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sep_parsed: Option<usize>,
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/// Matched meta variables bindings
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bindings: SmallVec<[Bindings; 4]>,
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/// Cached result of meta variable parsing
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meta_result: Option<(TtIter<'t>, ExpandResult<Option<Fragment>>)>,
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/// Is error occuried in this state, will `poised` to "parent"
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is_error: bool,
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}
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/// Process the matcher positions of `cur_items` until it is empty. In the process, this will
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/// produce more items in `next_items`, `eof_items`, and `bb_items`.
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///
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/// For more info about the how this happens, see the module-level doc comments and the inline
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/// comments of this function.
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///
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/// # Parameters
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///
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/// - `src`: the current token of the parser.
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/// - `stack`: the "parent" frames of the token tree
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/// - `res`: the match result to store errors
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/// - `cur_items`: the set of current items to be processed. This should be empty by the end of a
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/// successful execution of this function.
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/// - `next_items`: the set of newly generated items. These are used to replenish `cur_items` in
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/// the function `parse`.
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/// - `eof_items`: the set of items that would be valid if this was the EOF.
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/// - `bb_items`: the set of items that are waiting for the black-box parser.
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/// - `error_items`: the set of items in errors, used for error-resilient parsing
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fn match_loop_inner<'t>(
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src: TtIter<'t>,
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stack: &[TtIter<'t>],
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res: &mut Match,
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cur_items: &mut SmallVec<[MatchState<'t>; 1]>,
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bb_items: &mut SmallVec<[MatchState<'t>; 1]>,
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next_items: &mut Vec<MatchState<'t>>,
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eof_items: &mut SmallVec<[MatchState<'t>; 1]>,
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error_items: &mut SmallVec<[MatchState<'t>; 1]>,
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) {
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macro_rules! try_push {
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($items: expr, $it:expr) => {
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if $it.is_error {
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error_items.push($it);
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} else {
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$items.push($it);
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}
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};
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}
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while let Some(mut item) = cur_items.pop() {
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while item.dot.is_eof() {
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match item.stack.pop() {
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Some(frame) => {
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item.dot = frame;
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item.dot.next();
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}
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None => break,
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}
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}
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let op = match item.dot.peek() {
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None => {
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// We are at or past the end of the matcher of `item`.
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if item.up.is_some() {
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if item.sep_parsed.is_none() {
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// Get the `up` matcher
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let mut new_pos = *item.up.clone().unwrap();
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// Add matches from this repetition to the `matches` of `up`
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if let Some(bindings) = new_pos.bindings.last_mut() {
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for (i, b) in item.bindings.iter_mut().enumerate() {
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bindings.push_nested(i, b.clone()).unwrap();
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}
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}
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// Move the "dot" past the repetition in `up`
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new_pos.dot.next();
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new_pos.is_error = new_pos.is_error || item.is_error;
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cur_items.push(new_pos);
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}
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// Check if we need a separator.
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// We check the separator one by one
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let sep_idx = *item.sep_parsed.as_ref().unwrap_or(&0);
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let sep_len = item.sep.as_ref().map_or(0, Separator::tt_count);
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if item.sep.is_some() && sep_idx != sep_len {
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let sep = item.sep.as_ref().unwrap();
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if src.clone().expect_separator(&sep, sep_idx) {
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item.dot.next();
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item.sep_parsed = Some(sep_idx + 1);
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try_push!(next_items, item);
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}
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}
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// We don't need a separator. Move the "dot" back to the beginning of the matcher
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// and try to match again UNLESS we are only allowed to have _one_ repetition.
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else if item.sep_kind != Some(RepeatKind::ZeroOrOne) {
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item.dot = item.dot.reset();
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item.sep_parsed = None;
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item.bindings.push(Bindings::default());
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cur_items.push(item);
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}
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} else {
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// If we are not in a repetition, then being at the end of a matcher means that we have
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// reached the potential end of the input.
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try_push!(eof_items, item);
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}
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continue;
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}
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Some(it) => it,
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};
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// We are in the middle of a matcher.
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match op {
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OpDelimited::Op(Op::Repeat { tokens, kind, separator }) => {
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if matches!(kind, RepeatKind::ZeroOrMore | RepeatKind::ZeroOrOne) {
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let mut new_item = item.clone();
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new_item.dot.next();
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let mut vars = Vec::new();
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let bindings = new_item.bindings.last_mut().unwrap();
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collect_vars(&mut vars, tokens);
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for var in vars {
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bindings.push_empty(&var);
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}
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cur_items.push(new_item);
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}
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cur_items.push(MatchState {
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dot: tokens.iter_delimited(None),
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stack: Default::default(),
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up: Some(Box::new(item)),
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sep: separator.clone(),
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sep_kind: Some(*kind),
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sep_parsed: None,
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bindings: smallvec![Bindings::default()],
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meta_result: None,
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is_error: false,
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})
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}
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OpDelimited::Op(Op::Subtree { tokens, delimiter }) => {
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if let Ok(subtree) = src.clone().expect_subtree() {
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if subtree.delimiter_kind() == delimiter.map(|it| it.kind) {
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item.stack.push(item.dot);
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item.dot = tokens.iter_delimited(delimiter.as_ref());
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cur_items.push(item);
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}
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}
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}
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OpDelimited::Op(Op::Var { kind, name, .. }) => {
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if let Some(kind) = kind {
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let mut fork = src.clone();
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let match_res = match_meta_var(kind.as_str(), &mut fork);
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match match_res.err {
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None => {
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// Some meta variables are optional (e.g. vis)
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if match_res.value.is_some() {
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item.meta_result = Some((fork, match_res));
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try_push!(bb_items, item);
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} else {
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item.bindings.last_mut().unwrap().push_optional(name);
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item.dot.next();
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cur_items.push(item);
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}
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}
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Some(err) => {
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res.add_err(err);
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match match_res.value {
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Some(fragment) => {
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item.bindings
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.last_mut()
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.unwrap()
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.inner
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.push((name.clone(), Binding::Fragment(fragment)));
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}
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_ => {}
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}
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item.is_error = true;
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error_items.push(item);
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}
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}
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}
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}
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OpDelimited::Op(Op::Leaf(leaf)) => {
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if let Err(err) = match_leaf(&leaf, &mut src.clone()) {
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res.add_err(err);
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item.is_error = true;
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} else {
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item.dot.next();
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}
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try_push!(next_items, item);
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}
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OpDelimited::Open => {
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if matches!(src.clone().next(), Some(tt::TokenTree::Subtree(..))) {
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item.dot.next();
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try_push!(next_items, item);
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}
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}
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OpDelimited::Close => {
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let is_delim_closed = src.peek_n(0).is_none() && !stack.is_empty();
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if is_delim_closed {
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item.dot.next();
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try_push!(next_items, item);
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}
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}
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}
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}
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}
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fn match_loop(pattern: &MetaTemplate, src: &tt::Subtree) -> Match {
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let mut src = TtIter::new(src);
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let mut stack: SmallVec<[TtIter; 1]> = SmallVec::new();
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let mut res = Match::default();
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let mut error_reover_item = None;
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let mut cur_items = smallvec![MatchState {
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dot: pattern.iter_delimited(None),
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stack: Default::default(),
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up: None,
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sep: None,
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sep_kind: None,
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sep_parsed: None,
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bindings: smallvec![Bindings::default()],
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is_error: false,
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meta_result: None,
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}];
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let mut next_items = vec![];
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loop {
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let mut bb_items = SmallVec::new();
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let mut eof_items = SmallVec::new();
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let mut error_items = SmallVec::new();
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stdx::always!(next_items.is_empty());
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match_loop_inner(
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src.clone(),
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&stack,
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&mut res,
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&mut cur_items,
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&mut bb_items,
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&mut next_items,
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&mut eof_items,
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&mut error_items,
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);
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stdx::always!(cur_items.is_empty());
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if error_items.len() > 0 {
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error_reover_item = error_items.pop();
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} else if eof_items.len() > 0 {
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error_reover_item = Some(eof_items[0].clone());
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}
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// We need to do some post processing after the `match_loop_inner`.
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// If we reached the EOF, check that there is EXACTLY ONE possible matcher. Otherwise,
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// either the parse is ambiguous (which should never happen) or there is a syntax error.
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if src.peek_n(0).is_none() && stack.is_empty() {
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if eof_items.len() == 1 {
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// remove all errors, because it is the correct answer !
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res = Match::default();
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res.bindings = eof_items[0].bindings[0].clone();
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} else {
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// Error recovery
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if error_reover_item.is_some() {
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res.bindings = error_reover_item.unwrap().bindings[0].clone();
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}
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res.add_err(ExpandError::UnexpectedToken);
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}
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return res;
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}
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// If there are no possible next positions AND we aren't waiting for the black-box parser,
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// then there is a syntax error.
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//
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// Another possibility is that we need to call out to parse some rust nonterminal
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// (black-box) parser. However, if there is not EXACTLY ONE of these, something is wrong.
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if (bb_items.is_empty() && next_items.is_empty())
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|| (!bb_items.is_empty() && !next_items.is_empty())
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|| bb_items.len() > 1
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{
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res.unmatched_tts += src.len();
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while let Some(it) = stack.pop() {
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src = it;
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res.unmatched_tts += src.len();
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}
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res.add_err(err!("leftover tokens"));
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if let Some(mut error_reover_item) = error_reover_item {
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res.bindings = error_reover_item.bindings.remove(0);
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}
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return res;
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}
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// Dump all possible `next_items` into `cur_items` for the next iteration.
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else if !next_items.is_empty() {
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// Now process the next token
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cur_items.extend(next_items.drain(..));
|
|
|
|
match src.next() {
|
|
Some(tt::TokenTree::Subtree(subtree)) => {
|
|
stack.push(src.clone());
|
|
src = TtIter::new(subtree);
|
|
}
|
|
None if !stack.is_empty() => src = stack.pop().unwrap(),
|
|
_ => (),
|
|
}
|
|
}
|
|
// Finally, we have the case where we need to call the black-box parser to get some
|
|
// nonterminal.
|
|
else {
|
|
stdx::always!(bb_items.len() == 1);
|
|
let mut item = bb_items.pop().unwrap();
|
|
|
|
if let Some(OpDelimited::Op(Op::Var { name, .. })) = item.dot.peek() {
|
|
let (iter, match_res) = item.meta_result.take().unwrap();
|
|
let bindings = item.bindings.last_mut().unwrap();
|
|
match match_res.value {
|
|
Some(fragment) => {
|
|
bindings.inner.push((name.clone(), Binding::Fragment(fragment)));
|
|
}
|
|
None if match_res.err.is_none() => bindings.push_optional(name),
|
|
_ => {}
|
|
}
|
|
if let Some(err) = match_res.err {
|
|
res.add_err(err);
|
|
}
|
|
src = iter.clone();
|
|
item.dot.next();
|
|
} else {
|
|
unreachable!()
|
|
}
|
|
cur_items.push(item);
|
|
}
|
|
stdx::always!(!cur_items.is_empty());
|
|
}
|
|
}
|
|
|
|
fn match_leaf(lhs: &tt::Leaf, src: &mut TtIter) -> Result<(), ExpandError> {
|
|
let rhs = match src.expect_leaf() {
|
|
Ok(l) => l,
|
|
Err(()) => {
|
|
return Err(err!("expected leaf: `{}`", lhs));
|
|
}
|
|
};
|
|
match (lhs, rhs) {
|
|
(
|
|
tt::Leaf::Punct(tt::Punct { char: lhs, .. }),
|
|
tt::Leaf::Punct(tt::Punct { char: rhs, .. }),
|
|
) if lhs == rhs => (),
|
|
(
|
|
tt::Leaf::Ident(tt::Ident { text: lhs, .. }),
|
|
tt::Leaf::Ident(tt::Ident { text: rhs, .. }),
|
|
) if lhs == rhs => (),
|
|
(
|
|
tt::Leaf::Literal(tt::Literal { text: lhs, .. }),
|
|
tt::Leaf::Literal(tt::Literal { text: rhs, .. }),
|
|
) if lhs == rhs => (),
|
|
_ => {
|
|
return Err(ExpandError::UnexpectedToken);
|
|
}
|
|
}
|
|
|
|
Ok(())
|
|
}
|
|
|
|
fn match_meta_var(kind: &str, input: &mut TtIter) -> ExpandResult<Option<Fragment>> {
|
|
let fragment = match kind {
|
|
"path" => Path,
|
|
"expr" => Expr,
|
|
"ty" => Type,
|
|
"pat" => Pattern,
|
|
"stmt" => Statement,
|
|
"block" => Block,
|
|
"meta" => MetaItem,
|
|
"item" => Item,
|
|
_ => {
|
|
let tt_result = match kind {
|
|
"ident" => input
|
|
.expect_ident()
|
|
.map(|ident| Some(tt::Leaf::from(ident.clone()).into()))
|
|
.map_err(|()| err!("expected ident")),
|
|
"tt" => input.expect_tt().map(Some).map_err(|()| err!()),
|
|
"lifetime" => input
|
|
.expect_lifetime()
|
|
.map(|tt| Some(tt))
|
|
.map_err(|()| err!("expected lifetime")),
|
|
"literal" => {
|
|
let neg = input.eat_char('-');
|
|
input
|
|
.expect_literal()
|
|
.map(|literal| {
|
|
let lit = tt::Leaf::from(literal.clone());
|
|
match neg {
|
|
None => Some(lit.into()),
|
|
Some(neg) => Some(tt::TokenTree::Subtree(tt::Subtree {
|
|
delimiter: None,
|
|
token_trees: vec![neg, lit.into()],
|
|
})),
|
|
}
|
|
})
|
|
.map_err(|()| err!())
|
|
}
|
|
// `vis` is optional
|
|
"vis" => match input.eat_vis() {
|
|
Some(vis) => Ok(Some(vis)),
|
|
None => Ok(None),
|
|
},
|
|
_ => Err(ExpandError::UnexpectedToken),
|
|
};
|
|
return tt_result.map(|it| it.map(Fragment::Tokens)).into();
|
|
}
|
|
};
|
|
let result = input.expect_fragment(fragment);
|
|
result.map(|tt| if kind == "expr" { tt.map(Fragment::Ast) } else { tt.map(Fragment::Tokens) })
|
|
}
|
|
|
|
fn collect_vars(buf: &mut Vec<SmolStr>, pattern: &MetaTemplate) {
|
|
for op in pattern.iter() {
|
|
match op {
|
|
Op::Var { name, .. } => buf.push(name.clone()),
|
|
Op::Leaf(_) => (),
|
|
Op::Subtree { tokens, .. } => collect_vars(buf, tokens),
|
|
Op::Repeat { tokens, .. } => collect_vars(buf, tokens),
|
|
}
|
|
}
|
|
}
|
|
|
|
impl<'a> TtIter<'a> {
|
|
fn expect_separator(&mut self, separator: &Separator, idx: usize) -> bool {
|
|
let mut fork = self.clone();
|
|
let ok = match separator {
|
|
Separator::Ident(lhs) if idx == 0 => match fork.expect_ident() {
|
|
Ok(rhs) => rhs.text == lhs.text,
|
|
_ => false,
|
|
},
|
|
Separator::Literal(lhs) if idx == 0 => match fork.expect_literal() {
|
|
Ok(rhs) => match rhs {
|
|
tt::Leaf::Literal(rhs) => rhs.text == lhs.text,
|
|
tt::Leaf::Ident(rhs) => rhs.text == lhs.text,
|
|
tt::Leaf::Punct(_) => false,
|
|
},
|
|
_ => false,
|
|
},
|
|
Separator::Puncts(lhss) if idx < lhss.len() => match fork.expect_punct() {
|
|
Ok(rhs) => rhs.char == lhss[idx].char,
|
|
_ => false,
|
|
},
|
|
_ => false,
|
|
};
|
|
if ok {
|
|
*self = fork;
|
|
}
|
|
ok
|
|
}
|
|
|
|
fn expect_tt(&mut self) -> Result<tt::TokenTree, ()> {
|
|
match self.peek_n(0) {
|
|
Some(tt::TokenTree::Leaf(tt::Leaf::Punct(punct))) if punct.char == '\'' => {
|
|
return self.expect_lifetime();
|
|
}
|
|
_ => (),
|
|
}
|
|
|
|
let tt = self.next().ok_or_else(|| ())?.clone();
|
|
let punct = match tt {
|
|
tt::TokenTree::Leaf(tt::Leaf::Punct(punct)) if punct.spacing == tt::Spacing::Joint => {
|
|
punct
|
|
}
|
|
_ => return Ok(tt),
|
|
};
|
|
|
|
let (second, third) = match (self.peek_n(0), self.peek_n(1)) {
|
|
(
|
|
Some(tt::TokenTree::Leaf(tt::Leaf::Punct(p2))),
|
|
Some(tt::TokenTree::Leaf(tt::Leaf::Punct(p3))),
|
|
) if p2.spacing == tt::Spacing::Joint => (p2.char, Some(p3.char)),
|
|
(Some(tt::TokenTree::Leaf(tt::Leaf::Punct(p2))), _) => (p2.char, None),
|
|
_ => return Ok(tt),
|
|
};
|
|
|
|
match (punct.char, second, third) {
|
|
('.', '.', Some('.'))
|
|
| ('.', '.', Some('='))
|
|
| ('<', '<', Some('='))
|
|
| ('>', '>', Some('=')) => {
|
|
let tt2 = self.next().unwrap().clone();
|
|
let tt3 = self.next().unwrap().clone();
|
|
Ok(tt::Subtree { delimiter: None, token_trees: vec![tt, tt2, tt3] }.into())
|
|
}
|
|
('-', '=', _)
|
|
| ('-', '>', _)
|
|
| (':', ':', _)
|
|
| ('!', '=', _)
|
|
| ('.', '.', _)
|
|
| ('*', '=', _)
|
|
| ('/', '=', _)
|
|
| ('&', '&', _)
|
|
| ('&', '=', _)
|
|
| ('%', '=', _)
|
|
| ('^', '=', _)
|
|
| ('+', '=', _)
|
|
| ('<', '<', _)
|
|
| ('<', '=', _)
|
|
| ('=', '=', _)
|
|
| ('=', '>', _)
|
|
| ('>', '=', _)
|
|
| ('>', '>', _)
|
|
| ('|', '=', _)
|
|
| ('|', '|', _) => {
|
|
let tt2 = self.next().unwrap().clone();
|
|
Ok(tt::Subtree { delimiter: None, token_trees: vec![tt, tt2] }.into())
|
|
}
|
|
_ => Ok(tt),
|
|
}
|
|
}
|
|
|
|
fn expect_lifetime(&mut self) -> Result<tt::TokenTree, ()> {
|
|
let punct = self.expect_punct()?;
|
|
if punct.char != '\'' {
|
|
return Err(());
|
|
}
|
|
let ident = self.expect_ident()?;
|
|
|
|
Ok(tt::Subtree {
|
|
delimiter: None,
|
|
token_trees: vec![
|
|
tt::Leaf::Punct(*punct).into(),
|
|
tt::Leaf::Ident(ident.clone()).into(),
|
|
],
|
|
}
|
|
.into())
|
|
}
|
|
|
|
fn eat_vis(&mut self) -> Option<tt::TokenTree> {
|
|
let mut fork = self.clone();
|
|
match fork.expect_fragment(Visibility) {
|
|
ExpandResult { value: tt, err: None } => {
|
|
*self = fork;
|
|
tt
|
|
}
|
|
ExpandResult { value: _, err: Some(_) } => None,
|
|
}
|
|
}
|
|
|
|
fn eat_char(&mut self, c: char) -> Option<tt::TokenTree> {
|
|
let mut fork = self.clone();
|
|
match fork.expect_char(c) {
|
|
Ok(_) => {
|
|
let tt = self.next().cloned();
|
|
*self = fork;
|
|
tt
|
|
}
|
|
Err(_) => None,
|
|
}
|
|
}
|
|
}
|