rust/crates/mbe/src/tt_iter.rs

//! A "Parser" structure for token trees. We use this when parsing a declarative
//! macro definition into a list of patterns and templates.

use syntax::SyntaxKind;
use tt::buffer::TokenBuffer;

use crate::{to_parser_input::to_parser_input, ExpandError, ExpandResult};

#[derive(Debug, Clone)]
pub(crate) struct TtIter<'a> {
    pub(crate) inner: std::slice::Iter<'a, tt::TokenTree>,
}

impl<'a> TtIter<'a> {
    pub(crate) fn new(subtree: &'a tt::Subtree) -> TtIter<'a> {
        TtIter { inner: subtree.token_trees.iter() }
    }

    pub(crate) fn expect_char(&mut self, char: char) -> Result<(), ()> {
        match self.next() {
            Some(&tt::TokenTree::Leaf(tt::Leaf::Punct(tt::Punct { char: c, .. }))) if c == char => {
                Ok(())
            }
            _ => Err(()),
        }
    }

    pub(crate) fn expect_any_char(&mut self, chars: &[char]) -> Result<(), ()> {
        match self.next() {
            Some(tt::TokenTree::Leaf(tt::Leaf::Punct(tt::Punct { char: c, .. })))
                if chars.contains(c) =>
            {
                Ok(())
            }
            _ => Err(()),
        }
    }

    pub(crate) fn expect_subtree(&mut self) -> Result<&'a tt::Subtree, ()> {
        match self.next() {
            Some(tt::TokenTree::Subtree(it)) => Ok(it),
            _ => Err(()),
        }
    }

    pub(crate) fn expect_leaf(&mut self) -> Result<&'a tt::Leaf, ()> {
        match self.next() {
            Some(tt::TokenTree::Leaf(it)) => Ok(it),
            _ => Err(()),
        }
    }

    pub(crate) fn expect_ident(&mut self) -> Result<&'a tt::Ident, ()> {
        match self.expect_leaf()? {
            tt::Leaf::Ident(it) if it.text != "_" => Ok(it),
            _ => Err(()),
        }
    }

    pub(crate) fn expect_ident_or_underscore(&mut self) -> Result<&'a tt::Ident, ()> {
        match self.expect_leaf()? {
            tt::Leaf::Ident(it) => Ok(it),
            _ => Err(()),
        }
    }

    pub(crate) fn expect_literal(&mut self) -> Result<&'a tt::Leaf, ()> {
        let it = self.expect_leaf()?;
        match it {
            tt::Leaf::Literal(_) => Ok(it),
            tt::Leaf::Ident(ident) if ident.text == "true" || ident.text == "false" => Ok(it),
            _ => Err(()),
        }
    }

    pub(crate) fn expect_punct(&mut self) -> Result<&'a tt::Punct, ()> {
        match self.expect_leaf()? {
            tt::Leaf::Punct(it) => Ok(it),
            _ => Err(()),
        }
    }

    pub(crate) fn expect_fragment(
        &mut self,
        entry_point: parser::PrefixEntryPoint,
    ) -> ExpandResult<Option<tt::TokenTree>> {
        let buffer = TokenBuffer::from_tokens(self.inner.as_slice());
        let parser_input = to_parser_input(&buffer);
        let tree_traversal = entry_point.parse(&parser_input);

        let mut cursor = buffer.begin();
        let mut error = false;
        for step in tree_traversal.iter() {
            match step {
                parser::Step::Token { kind, mut n_input_tokens } => {
                    if kind == SyntaxKind::LIFETIME_IDENT {
                        n_input_tokens = 2;
                    }
                    for _ in 0..n_input_tokens {
                        cursor = cursor.bump_subtree();
                    }
                }
                parser::Step::Enter { .. } | parser::Step::Exit => (),
                parser::Step::Error { .. } => error = true,
            }
        }

        let err = if error || !cursor.is_root() {
            Some(ExpandError::BindingError(format!("expected {entry_point:?}").into()))
        } else {
            None
        };

        let mut curr = buffer.begin();
        let mut res = vec![];

        if cursor.is_root() {
            while curr != cursor {
                if let Some(token) = curr.token_tree() {
                    res.push(token);
                }
                curr = curr.bump();
            }
        }
        self.inner = self.inner.as_slice()[res.len()..].iter();
        let res = match res.len() {
            1 => Some(res[0].cloned()),
            0 => None,
            _ => Some(tt::TokenTree::Subtree(tt::Subtree {
                delimiter: None,
                token_trees: res.into_iter().map(|it| it.cloned()).collect(),
            })),
        };
        ExpandResult { value: res, err }
    }

    pub(crate) fn peek_n(&self, n: usize) -> Option<&tt::TokenTree> {
        self.inner.as_slice().get(n)
    }
}

impl<'a> Iterator for TtIter<'a> {
    type Item = &'a tt::TokenTree;
    fn next(&mut self) -> Option<Self::Item> {
        self.inner.next()
    }

    fn size_hint(&self) -> (usize, Option<usize>) {
        self.inner.size_hint()
    }
}

impl<'a> std::iter::ExactSizeIterator for TtIter<'a> {}