Merging upstream version 1.71.1+dfsg1.

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 194 insertions, 0 deletions

diff --git a/vendor/syn-1.0.109/src/discouraged.rs b/vendor/syn-1.0.109/src/discouraged.rs
new file mode 100644
index 000000000..a46129b6a
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+++ b/vendor/syn-1.0.109/src/discouraged.rs
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+//! 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 it's 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])
+    ///         {
+    ///             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");
+        }
+
+        let (self_unexp, self_sp) = inner_unexpected(self);
+        let (fork_unexp, fork_sp) = inner_unexpected(fork);
+        if !Rc::ptr_eq(&self_unexp, &fork_unexp) {
+            match (fork_sp, self_sp) {
+                // Unexpected set on the fork, but not on `self`, copy it over.
+                (Some(span), None) => {
+                    self_unexp.set(Unexpected::Some(span));
+                }
+                // Unexpected unset. Use chain to propagate errors from fork.
+                (None, None) => {
+                    fork_unexp.set(Unexpected::Chain(self_unexp));
+
+                    // Ensure toplevel 'unexpected' tokens from the fork don't
+                    // bubble up the chain by replacing the root `unexpected`
+                    // pointer, only 'unexpected' tokens from existing group
+                    // parsers should bubble.
+                    fork.unexpected
+                        .set(Some(Rc::new(Cell::new(Unexpected::None))));
+                }
+                // Unexpected has been set on `self`. No changes needed.
+                (_, Some(_)) => {}
+            }
+        }
+
+        // See comment on `cell` in the struct definition.
+        self.cell
+            .set(unsafe { mem::transmute::<Cursor, Cursor<'static>>(fork.cursor()) });
+    }
+}