src/next/parse.rs

//! Parsing interface for parsing a token stream into a syntax tree node.

use std::cell::Cell;
use std::fmt::Display;
use std::marker::PhantomData;
use std::mem;
use std::ops::Deref;

use buffer::Cursor;
use proc_macro2::{Ident, Span};

use super::error;

pub use super::error::{Error, Result};
pub use super::lookahead::{Lookahead1, Peek};

/// Parsing interface implemented by all types that can be parsed in a default
/// way from a token stream.
pub trait Parse: Sized {
    fn parse(input: ParseStream) -> Result<Self>;
}

/// Input to a Syn parser function.
pub type ParseStream<'a> = &'a ParseBuffer<'a>;

/// Cursor position within a buffered token stream.
#[derive(Clone)]
pub struct ParseBuffer<'a> {
    scope: Span,
    cell: Cell<Cursor<'static>>,
    marker: PhantomData<Cursor<'a>>,
}

// Not public API.
#[doc(hidden)]
#[derive(Copy, Clone)]
pub struct StepCursor<'c, 'a> {
    scope: Span,
    cursor: Cursor<'c>,
    marker: PhantomData<fn(Cursor<'c>) -> Cursor<'a>>,
}

impl<'c, 'a> Deref for StepCursor<'c, 'a> {
    type Target = Cursor<'c>;

    fn deref(&self) -> &Self::Target {
        &self.cursor
    }
}

impl<'c, 'a> StepCursor<'c, 'a> {
    // Not public API.
    #[doc(hidden)]
    pub fn advance(self, other: Cursor<'c>) -> Cursor<'a> {
        unsafe { mem::transmute::<Cursor<'c>, Cursor<'a>>(other) }
    }

    // Not public API.
    #[doc(hidden)]
    pub fn error<T: Display>(self, message: T) -> Error {
        error::new_at(self.scope, self.cursor, message)
    }
}

impl<'a> ParseBuffer<'a> {
    // Not public API.
    #[doc(hidden)]
    pub fn new(scope: Span, cursor: Cursor<'a>) -> Self {
        let extend = unsafe { mem::transmute::<Cursor<'a>, Cursor<'static>>(cursor) };
        ParseBuffer {
            scope: scope,
            cell: Cell::new(extend),
            marker: PhantomData,
        }
    }

    pub fn cursor(&self) -> Cursor<'a> {
        self.cell.get()
    }

    pub fn is_empty(&self) -> bool {
        self.cursor().eof()
    }

    pub fn lookahead1(&self) -> Lookahead1<'a> {
        Lookahead1::new(self.scope, self.cursor())
    }

    pub fn parse<T: Parse>(&self) -> Result<T> {
        T::parse(self)
    }

    // Not public API.
    #[doc(hidden)]
    pub fn step_cursor<F, R>(&self, function: F) -> Result<R>
    where
        F: for<'c> FnOnce(StepCursor<'c, 'a>) -> Result<(R, Cursor<'c>)>,
    {
        match function(StepCursor {
            scope: self.scope,
            cursor: self.cell.get(),
            marker: PhantomData,
        }) {
            Ok((ret, cursor)) => {
                self.cell.set(cursor);
                Ok(ret)
            }
            Err(err) => Err(err),
        }
    }
}

impl Parse for Ident {
    fn parse(input: ParseStream) -> Result<Self> {
        input.step_cursor(|cursor| {
            if let Some((ident, rest)) = cursor.ident() {
                match ident.to_string().as_str() {
                    "_"
                    // Based on https://doc.rust-lang.org/grammar.html#keywords
                    // and https://github.com/rust-lang/rfcs/blob/master/text/2421-unreservations-2018.md
                    | "abstract" | "as" | "become" | "box" | "break" | "const"
                    | "continue" | "crate" | "do" | "else" | "enum" | "extern" | "false" | "final"
                    | "fn" | "for" | "if" | "impl" | "in" | "let" | "loop" | "macro" | "match"
                    | "mod" | "move" | "mut" | "override" | "priv" | "proc" | "pub"
                    | "ref" | "return" | "Self" | "self" | "static" | "struct"
                    | "super" | "trait" | "true" | "type" | "typeof" | "unsafe" | "unsized" | "use"
                    | "virtual" | "where" | "while" | "yield" => {}
                    _ => return Ok((ident, rest)),
                }
            }
            Err(cursor.error("expected identifier"))
        })
    }
}

// In reality the impl would be for Punctuated.
impl<T: Parse> Parse for Vec<T> {
    fn parse(input: ParseStream) -> Result<Self> {
        let mut vec = Vec::new();
        while !input.is_empty() {
            let t = input.parse::<T>()?;
            vec.push(t);
        }
        Ok(vec)
    }
}