//! 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`]: ParseBuffer::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`:
///
/// ```
/// # 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,
/// 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,
/// 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`]: crate::PathSegment
///
/// # 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 !crate::buffer::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()) })
}
}