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(delim:delimiter).is_some()
167}
168
169impl<F: Copy + FnOnce(TokenMarker) -> T, T: Token> Sealed for F {}
170