1 files changed, 168 insertions, 0 deletions
diff --git a/syn/src/lookahead.rs b/syn/src/lookahead.rs
new file mode 100644
index 0000000..6a67909
--- /dev/null
+++ b/syn/src/lookahead.rs
@@ -0,0 +1,168 @@
+use std::cell::RefCell;
+
+use proc_macro2::{Delimiter, Span};
+
+use crate::buffer::Cursor;
+use crate::error::{self, Error};
+use crate::sealed::lookahead::Sealed;
+use crate::span::IntoSpans;
+use crate::token::Token;
+
+/// Support for checking the next token in a stream to decide how to parse.
+///
+/// An important advantage over [`ParseStream::peek`] is that here we
+/// automatically construct an appropriate error message based on the token
+/// alternatives that get peeked. If you are producing your own error message,
+/// go ahead and use `ParseStream::peek` instead.
+///
+/// Use [`ParseStream::lookahead1`] to construct this object.
+///
+/// [`ParseStream::peek`]: crate::parse::ParseBuffer::peek
+/// [`ParseStream::lookahead1`]: crate::parse::ParseBuffer::lookahead1
+///
+/// # Example
+///
+/// ```
+/// use syn::{ConstParam, Ident, Lifetime, LifetimeDef, Result, Token, TypeParam};
+/// use syn::parse::{Parse, ParseStream};
+///
+/// // A generic parameter, a single one of the comma-separated elements inside
+/// // angle brackets in:
+/// //
+/// //     fn f<T: Clone, 'a, 'b: 'a, const N: usize>() { ... }
+/// //
+/// // On invalid input, lookahead gives us a reasonable error message.
+/// //
+/// //     error: expected one of: identifier, lifetime, `const`
+/// //       |
+/// //     5 |     fn f<!Sized>() {}
+/// //       |          ^
+/// enum GenericParam {
+///     Type(TypeParam),
+///     Lifetime(LifetimeDef),
+///     Const(ConstParam),
+/// }
+///
+/// impl Parse for GenericParam {
+///     fn parse(input: ParseStream) -> Result<Self> {
+///         let lookahead = input.lookahead1();
+///         if lookahead.peek(Ident) {
+///             input.parse().map(GenericParam::Type)
+///         } else if lookahead.peek(Lifetime) {
+///             input.parse().map(GenericParam::Lifetime)
+///         } else if lookahead.peek(Token![const]) {
+///             input.parse().map(GenericParam::Const)
+///         } else {
+///             Err(lookahead.error())
+///         }
+///     }
+/// }
+/// ```
+pub struct Lookahead1<'a> {
+    scope: Span,
+    cursor: Cursor<'a>,
+    comparisons: RefCell<Vec<&'static str>>,
+}
+
+pub fn new(scope: Span, cursor: Cursor) -> Lookahead1 {
+    Lookahead1 {
+        scope,
+        cursor,
+        comparisons: RefCell::new(Vec::new()),
+    }
+}
+
+fn peek_impl(
+    lookahead: &Lookahead1,
+    peek: fn(Cursor) -> bool,
+    display: fn() -> &'static str,
+) -> bool {
+    if peek(lookahead.cursor) {
+        return true;
+    }
+    lookahead.comparisons.borrow_mut().push(display());
+    false
+}
+
+impl<'a> Lookahead1<'a> {
+    /// Looks at the next token in the parse stream to determine whether it
+    /// matches the requested type of token.
+    ///
+    /// # Syntax
+    ///
+    /// Note that this method does not use turbofish syntax. Pass the peek type
+    /// inside of parentheses.
+    ///
+    /// - `input.peek(Token![struct])`
+    /// - `input.peek(Token![==])`
+    /// - `input.peek(Ident)`&emsp;*(does not accept keywords)*
+    /// - `input.peek(Ident::peek_any)`
+    /// - `input.peek(Lifetime)`
+    /// - `input.peek(token::Brace)`
+    pub fn peek<T: Peek>(&self, token: T) -> bool {
+        let _ = token;
+        peek_impl(self, T::Token::peek, T::Token::display)
+    }
+
+    /// Triggers an error at the current position of the parse stream.
+    ///
+    /// The error message will identify all of the expected token types that
+    /// have been peeked against this lookahead instance.
+    pub fn error(self) -> Error {
+        let comparisons = self.comparisons.borrow();
+        match comparisons.len() {
+            0 => {
+                if self.cursor.eof() {
+                    Error::new(self.scope, "unexpected end of input")
+                } else {
+                    Error::new(self.cursor.span(), "unexpected token")
+                }
+            }
+            1 => {
+                let message = format!("expected {}", comparisons[0]);
+                error::new_at(self.scope, self.cursor, message)
+            }
+            2 => {
+                let message = format!("expected {} or {}", comparisons[0], comparisons[1]);
+                error::new_at(self.scope, self.cursor, message)
+            }
+            _ => {
+                let join = comparisons.join(", ");
+                let message = format!("expected one of: {}", join);
+                error::new_at(self.scope, self.cursor, message)
+            }
+        }
+    }
+}
+
+/// Types that can be parsed by looking at just one token.
+///
+/// Use [`ParseStream::peek`] to peek one of these types in a parse stream
+/// without consuming it from the stream.
+///
+/// This trait is sealed and cannot be implemented for types outside of Syn.
+///
+/// [`ParseStream::peek`]: crate::parse::ParseBuffer::peek
+pub trait Peek: Sealed {
+    // Not public API.
+    #[doc(hidden)]
+    type Token: Token;
+}
+
+impl<F: Copy + FnOnce(TokenMarker) -> T, T: Token> Peek for F {
+    type Token = T;
+}
+
+pub enum TokenMarker {}
+
+impl<S> IntoSpans<S> for TokenMarker {
+    fn into_spans(self) -> S {
+        match self {}
+    }
+}
+
+pub fn is_delimiter(cursor: Cursor, delimiter: Delimiter) -> bool {
+    cursor.group(delimiter).is_some()
+}
+
+impl<F: Copy + FnOnce(TokenMarker) -> T, T: Token> Sealed for F {}