syn/
lookahead.rs

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1use crate::buffer::Cursor;
2use crate::error::{self, Error};
3use crate::sealed::lookahead::Sealed;
4use crate::span::IntoSpans;
5use crate::token::Token;
6use proc_macro2::{Delimiter, Span};
7use std::cell::RefCell;
8
9/// Support for checking the next token in a stream to decide how to parse.
10///
11/// An important advantage over [`ParseStream::peek`] is that here we
12/// automatically construct an appropriate error message based on the token
13/// alternatives that get peeked. If you are producing your own error message,
14/// go ahead and use `ParseStream::peek` instead.
15///
16/// Use [`ParseStream::lookahead1`] to construct this object.
17///
18/// [`ParseStream::peek`]: crate::parse::ParseBuffer::peek
19/// [`ParseStream::lookahead1`]: crate::parse::ParseBuffer::lookahead1
20///
21/// Consuming tokens from the source stream after constructing a lookahead
22/// object does not also advance the lookahead object.
23///
24/// # Example
25///
26/// ```
27/// use syn::{ConstParam, Ident, Lifetime, LifetimeDef, Result, Token, TypeParam};
28/// use syn::parse::{Parse, ParseStream};
29///
30/// // A generic parameter, a single one of the comma-separated elements inside
31/// // angle brackets in:
32/// //
33/// //     fn f<T: Clone, 'a, 'b: 'a, const N: usize>() { ... }
34/// //
35/// // On invalid input, lookahead gives us a reasonable error message.
36/// //
37/// //     error: expected one of: identifier, lifetime, `const`
38/// //       |
39/// //     5 |     fn f<!Sized>() {}
40/// //       |          ^
41/// enum GenericParam {
42///     Type(TypeParam),
43///     Lifetime(LifetimeDef),
44///     Const(ConstParam),
45/// }
46///
47/// impl Parse for GenericParam {
48///     fn parse(input: ParseStream) -> Result<Self> {
49///         let lookahead = input.lookahead1();
50///         if lookahead.peek(Ident) {
51///             input.parse().map(GenericParam::Type)
52///         } else if lookahead.peek(Lifetime) {
53///             input.parse().map(GenericParam::Lifetime)
54///         } else if lookahead.peek(Token![const]) {
55///             input.parse().map(GenericParam::Const)
56///         } else {
57///             Err(lookahead.error())
58///         }
59///     }
60/// }
61/// ```
62pub struct Lookahead1<'a> {
63    scope: Span,
64    cursor: Cursor<'a>,
65    comparisons: RefCell<Vec<&'static str>>,
66}
67
68pub fn new(scope: Span, cursor: Cursor) -> Lookahead1 {
69    Lookahead1 {
70        scope,
71        cursor,
72        comparisons: RefCell::new(Vec::new()),
73    }
74}
75
76fn peek_impl(
77    lookahead: &Lookahead1,
78    peek: fn(Cursor) -> bool,
79    display: fn() -> &'static str,
80) -> bool {
81    if peek(lookahead.cursor) {
82        return true;
83    }
84    lookahead.comparisons.borrow_mut().push(display());
85    false
86}
87
88impl<'a> Lookahead1<'a> {
89    /// Looks at the next token in the parse stream to determine whether it
90    /// matches the requested type of token.
91    ///
92    /// # Syntax
93    ///
94    /// Note that this method does not use turbofish syntax. Pass the peek type
95    /// inside of parentheses.
96    ///
97    /// - `input.peek(Token![struct])`
98    /// - `input.peek(Token![==])`
99    /// - `input.peek(Ident)`&emsp;*(does not accept keywords)*
100    /// - `input.peek(Ident::peek_any)`
101    /// - `input.peek(Lifetime)`
102    /// - `input.peek(token::Brace)`
103    pub fn peek<T: Peek>(&self, token: T) -> bool {
104        let _ = token;
105        peek_impl(self, T::Token::peek, T::Token::display)
106    }
107
108    /// Triggers an error at the current position of the parse stream.
109    ///
110    /// The error message will identify all of the expected token types that
111    /// have been peeked against this lookahead instance.
112    pub fn error(self) -> Error {
113        let comparisons = self.comparisons.borrow();
114        match comparisons.len() {
115            0 => {
116                if self.cursor.eof() {
117                    Error::new(self.scope, "unexpected end of input")
118                } else {
119                    Error::new(self.cursor.span(), "unexpected token")
120                }
121            }
122            1 => {
123                let message = format!("expected {}", comparisons[0]);
124                error::new_at(self.scope, self.cursor, message)
125            }
126            2 => {
127                let message = format!("expected {} or {}", comparisons[0], comparisons[1]);
128                error::new_at(self.scope, self.cursor, message)
129            }
130            _ => {
131                let join = comparisons.join(", ");
132                let message = format!("expected one of: {}", join);
133                error::new_at(self.scope, self.cursor, message)
134            }
135        }
136    }
137}
138
139/// Types that can be parsed by looking at just one token.
140///
141/// Use [`ParseStream::peek`] to peek one of these types in a parse stream
142/// without consuming it from the stream.
143///
144/// This trait is sealed and cannot be implemented for types outside of Syn.
145///
146/// [`ParseStream::peek`]: crate::parse::ParseBuffer::peek
147pub trait Peek: Sealed {
148    // Not public API.
149    #[doc(hidden)]
150    type Token: Token;
151}
152
153impl<F: Copy + FnOnce(TokenMarker) -> T, T: Token> Peek for F {
154    type Token = T;
155}
156
157pub enum TokenMarker {}
158
159impl<S> IntoSpans<S> for TokenMarker {
160    fn into_spans(self) -> S {
161        match self {}
162    }
163}
164
165pub fn is_delimiter(cursor: Cursor, delimiter: Delimiter) -> bool {
166    cursor.group(delimiter).is_some()
167}
168
169impl<F: Copy + FnOnce(TokenMarker) -> T, T: Token> Sealed for F {}
syn/lookahead.rs

syn/
lookahead.rs
