src/discouraged.rs

//! Extensions to the parsing API with niche applicability.

use super::*;

/// Extensions to the `ParseStream` API to support speculative parsing.
pub trait Speculative {
    /// Advance this parse stream to the position of a forked parse stream.
    ///
    /// This is the opposite operation to [`ParseStream::fork`]. You can fork a
    /// parse stream, perform some speculative parsing, then join the original
    /// stream to the fork to "commit" the parsing from the fork to the main
    /// stream.
    ///
    /// If you can avoid doing this, you should, as it limits the ability to
    /// generate useful errors. That said, it is often the only way to parse
    /// syntax of the form `A* B*` for arbitrary syntax `A` and `B`. The problem
    /// is that when the fork fails to parse an `A`, it's impossible to tell
    /// whether that was because of a syntax error and the user meant to provide
    /// an `A`, or that the `A`s are finished and its time to start parsing
    /// `B`s. Use with care.
    ///
    /// Also note that if `A` is a subset of `B`, `A* B*` can be parsed by
    /// parsing `B*` and removing the leading members of `A` from the
    /// repetition, bypassing the need to involve the downsides associated with
    /// speculative parsing.
    ///
    /// [`ParseStream::fork`]: ../struct.ParseBuffer.html#method.fork
    ///
    /// # Example
    ///
    /// There has been chatter about the possibility of making the colons in the
    /// turbofish syntax like `path::to::<T>` no longer required by accepting
    /// `path::to<T>` in expression position. Specifically, according to [RFC
    /// 2544], [`PathSegment`] parsing should always try to consume a following
    /// `<` token as the start of generic arguments, and reset to the `<` if
    /// that fails (e.g. the token is acting as a less-than operator).
    ///
    /// This is the exact kind of parsing behavior which requires the "fork,
    /// try, commit" behavior that [`ParseStream::fork`] discourages. With
    /// `advance_to`, we can avoid having to parse the speculatively parsed
    /// content a second time.
    ///
    /// This change in behavior can be implemented in syn by replacing just the
    /// `Parse` implementation for `PathSegment`:
    ///
    /// ```edition2018
    /// # use syn::ext::IdentExt;
    /// use syn::parse::discouraged::Speculative;
    /// # use syn::parse::{Parse, ParseStream};
    /// # use syn::{Ident, PathArguments, Result, Token};
    ///
    /// pub struct PathSegment {
    ///     pub ident: Ident,
    ///     pub arguments: PathArguments,
    /// }
    /// #
    /// # impl<T> From<T> for PathSegment
    /// # where
    /// #     T: Into<Ident>,
    /// # {
    /// #     fn from(ident: T) -> Self {
    /// #         PathSegment {
    /// #             ident: ident.into(),
    /// #             arguments: PathArguments::None,
    /// #         }
    /// #     }
    /// # }
    ///
    /// impl Parse for PathSegment {
    ///     fn parse(input: ParseStream) -> Result<Self> {
    ///         if input.peek(Token![super])
    ///             || input.peek(Token![self])
    ///             || input.peek(Token![Self])
    ///             || input.peek(Token![crate])
    ///             || input.peek(Token![extern])
    ///         {
    ///             let ident = input.call(Ident::parse_any)?;
    ///             return Ok(PathSegment::from(ident));
    ///         }
    ///
    ///         let ident = input.parse()?;
    ///         if input.peek(Token![::]) && input.peek3(Token![<]) {
    ///             return Ok(PathSegment {
    ///                 ident: ident,
    ///                 arguments: PathArguments::AngleBracketed(input.parse()?),
    ///             });
    ///         }
    ///         if input.peek(Token![<]) && !input.peek(Token![<=]) {
    ///             let fork = input.fork();
    ///             if let Ok(arguments) = fork.parse() {
    ///                 input.advance_to(&fork);
    ///                 return Ok(PathSegment {
    ///                     ident: ident,
    ///                     arguments: PathArguments::AngleBracketed(arguments),
    ///                 });
    ///             }
    ///         }
    ///         Ok(PathSegment::from(ident))
    ///     }
    /// }
    ///
    /// # syn::parse_str::<PathSegment>("a<b,c>").unwrap();
    /// ```
    ///
    /// # Drawbacks
    ///
    /// The main drawback of this style of speculative parsing is in error
    /// presentation. Even if the lookahead is the "correct" parse, the error
    /// that is shown is that of the "fallback" parse. To use the same example
    /// as the turbofish above, take the following unfinished "turbofish":
    ///
    /// ```text
    /// let _ = f<&'a fn(), for<'a> serde::>();
    /// ```
    ///
    /// If this is parsed as generic arguments, we can provide the error message
    ///
    /// ```text
    /// error: expected identifier
    ///  --> src.rs:L:C
    ///   |
    /// L | let _ = f<&'a fn(), for<'a> serde::>();
    ///   |                                    ^
    /// ```
    ///
    /// but if parsed using the above speculative parsing, it falls back to
    /// assuming that the `<` is a less-than when it fails to parse the generic
    /// arguments, and tries to interpret the `&'a` as the start of a labelled
    /// loop, resulting in the much less helpful error
    ///
    /// ```text
    /// error: expected `:`
    ///  --> src.rs:L:C
    ///   |
    /// L | let _ = f<&'a fn(), for<'a> serde::>();
    ///   |               ^^
    /// ```
    ///
    /// This can be mitigated with various heuristics (two examples: show both
    /// forks' parse errors, or show the one that consumed more tokens), but
    /// when you can control the grammar, sticking to something that can be
    /// parsed LL(3) and without the LL(*) speculative parsing this makes
    /// possible, displaying reasonable errors becomes much more simple.
    ///
    /// [RFC 2544]: https://github.com/rust-lang/rfcs/pull/2544
    /// [`PathSegment`]: ../../struct.PathSegment.html
    ///
    /// # Performance
    ///
    /// This method performs a cheap fixed amount of work that does not depend
    /// on how far apart the two streams are positioned.
    ///
    /// # Panics
    ///
    /// The forked stream in the argument of `advance_to` must have been
    /// obtained by forking `self`. Attempting to advance to any other stream
    /// will cause a panic.
    fn advance_to(&self, fork: &Self);
}

impl<'a> Speculative for ParseBuffer<'a> {
    fn advance_to(&self, fork: &Self) {
        if !private::same_scope(self.cursor(), fork.cursor()) {
            panic!("Fork was not derived from the advancing parse stream");
        }

        // See comment on `cell` in the struct definition.
        self.cell
            .set(unsafe { mem::transmute::<Cursor, Cursor<'static>>(fork.cursor()) })
    }
}