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use crate::buffer::Cursor;
use crate::error::{self, Error};
use crate::sealed::lookahead::Sealed;
use crate::span::IntoSpans;
use crate::token::Token;
use proc_macro2::{Delimiter, Span};
use std::cell::RefCell;
/// 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
///
/// Consuming tokens from the source stream after constructing a lookahead
/// object does not also advance the lookahead object.
///
/// # Example
///
/// ```
/// use syn::{ConstParam, Ident, Lifetime, LifetimeParam, 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(LifetimeParam),
/// 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(crate) 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)` *(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(crate) fn is_delimiter(cursor: Cursor, delimiter: Delimiter) -> bool {
cursor.group(delimiter).is_some()
}
impl<F: Copy + FnOnce(TokenMarker) -> T, T: Token> Sealed for F {}
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