1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169
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, 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)` *(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 {}