[][src]Struct syn::parse::Lookahead1

pub struct Lookahead1<'a> { /* fields omitted */ }

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.


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 {

impl Parse for GenericParam {
    fn parse(input: ParseStream) -> Result<Self> {
        let lookahead = input.lookahead1();
        if lookahead.peek(Ident) {
        } else if lookahead.peek(Lifetime) {
        } else if lookahead.peek(Token![const]) {
        } else {


impl<'a> Lookahead1<'a>[src]

pub fn peek<T: Peek>(&self, token: T) -> bool[src]

Looks at the next token in the parse stream to determine whether it matches the requested type of token.


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 error(self) -> Error[src]

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.

Auto Trait Implementations

impl<'a> !RefUnwindSafe for Lookahead1<'a>

impl<'a> !Send for Lookahead1<'a>

impl<'a> !Sync for Lookahead1<'a>

impl<'a> Unpin for Lookahead1<'a>

impl<'a> !UnwindSafe for Lookahead1<'a>

Blanket Implementations

impl<T> Any for T where
    T: 'static + ?Sized

impl<T> Borrow<T> for T where
    T: ?Sized

impl<T> BorrowMut<T> for T where
    T: ?Sized

impl<T> From<T> for T[src]

impl<T, U> Into<U> for T where
    U: From<T>, 

impl<T, U> TryFrom<U> for T where
    U: Into<T>, 

type Error = Infallible

The type returned in the event of a conversion error.

impl<T, U> TryInto<U> for T where
    U: TryFrom<T>, 

type Error = <U as TryFrom<T>>::Error

The type returned in the event of a conversion error.