1use crate::buffer::Cursor;
2use crate::error::{self, Error};
3use crate::sealed::lookahead::Sealed;
4use crate::span::IntoSpans;
5use crate::token::{CustomToken, 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, LifetimeParam, 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(LifetimeParam),
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(crate) 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 mut comparisons = self.comparisons.into_inner();
114 comparisons.retain_mut(|display| {
115 if *display == "`)`" {
116 *display = match self.cursor.scope_delimiter() {
117 Delimiter::Parenthesis => "`)`",
118 Delimiter::Brace => "`}`",
119 Delimiter::Bracket => "`]`",
120 Delimiter::None => return false,
121 }
122 }
123 true
124 });
125 match comparisons.len() {
126 0 => {
127 if self.cursor.eof() {
128 Error::new(self.scope, "unexpected end of input")
129 } else {
130 Error::new(self.cursor.span(), "unexpected token")
131 }
132 }
133 1 => {
134 let message = format!("expected {}", comparisons[0]);
135 error::new_at(self.scope, self.cursor, message)
136 }
137 2 => {
138 let message = format!("expected {} or {}", comparisons[0], comparisons[1]);
139 error::new_at(self.scope, self.cursor, message)
140 }
141 _ => {
142 let join = comparisons.join(", ");
143 let message = format!("expected one of: {}", join);
144 error::new_at(self.scope, self.cursor, message)
145 }
146 }
147 }
148}
149
150/// Types that can be parsed by looking at just one token.
151///
152/// Use [`ParseStream::peek`] to peek one of these types in a parse stream
153/// without consuming it from the stream.
154///
155/// This trait is sealed and cannot be implemented for types outside of Syn.
156///
157/// [`ParseStream::peek`]: crate::parse::ParseBuffer::peek
158pub trait Peek: Sealed {
159 // Not public API.
160 #[doc(hidden)]
161 type Token: Token;
162}
163
164/// Pseudo-token used for peeking the end of a parse stream.
165///
166/// This type is only useful as an argument to one of the following functions:
167///
168/// - [`ParseStream::peek`][crate::parse::ParseBuffer::peek]
169/// - [`ParseStream::peek2`][crate::parse::ParseBuffer::peek2]
170/// - [`ParseStream::peek3`][crate::parse::ParseBuffer::peek3]
171/// - [`Lookahead1::peek`]
172///
173/// The peek will return `true` if there are no remaining tokens after that
174/// point in the parse stream.
175///
176/// # Example
177///
178/// Suppose we are parsing attributes containing core::fmt inspired formatting
179/// arguments:
180///
181/// - `#[fmt("simple example")]`
182/// - `#[fmt("interpolation e{}ample", self.x)]`
183/// - `#[fmt("interpolation e{x}ample")]`
184///
185/// and we want to recognize the cases where no interpolation occurs so that
186/// more efficient code can be generated.
187///
188/// The following implementation uses `input.peek(Token![,]) &&
189/// input.peek2(End)` to recognize the case of a trailing comma without
190/// consuming the comma from the parse stream, because if it isn't a trailing
191/// comma, that same comma needs to be parsed as part of `args`.
192///
193/// ```
194/// use proc_macro2::TokenStream;
195/// use quote::quote;
196/// use syn::parse::{End, Parse, ParseStream, Result};
197/// use syn::{parse_quote, Attribute, LitStr, Token};
198///
199/// struct FormatArgs {
200/// template: LitStr, // "...{}..."
201/// args: TokenStream, // , self.x
202/// }
203///
204/// impl Parse for FormatArgs {
205/// fn parse(input: ParseStream) -> Result<Self> {
206/// let template: LitStr = input.parse()?;
207///
208/// let args = if input.is_empty()
209/// || input.peek(Token![,]) && input.peek2(End)
210/// {
211/// input.parse::<Option<Token![,]>>()?;
212/// TokenStream::new()
213/// } else {
214/// input.parse()?
215/// };
216///
217/// Ok(FormatArgs {
218/// template,
219/// args,
220/// })
221/// }
222/// }
223///
224/// fn main() -> Result<()> {
225/// let attrs: Vec<Attribute> = parse_quote! {
226/// #[fmt("simple example")]
227/// #[fmt("interpolation e{}ample", self.x)]
228/// #[fmt("interpolation e{x}ample")]
229/// };
230///
231/// for attr in &attrs {
232/// let FormatArgs { template, args } = attr.parse_args()?;
233/// let requires_fmt_machinery =
234/// !args.is_empty() || template.value().contains(['{', '}']);
235/// let out = if requires_fmt_machinery {
236/// quote! {
237/// ::core::write!(__formatter, #template #args)
238/// }
239/// } else {
240/// quote! {
241/// __formatter.write_str(#template)
242/// }
243/// };
244/// println!("{}", out);
245/// }
246/// Ok(())
247/// }
248/// ```
249///
250/// Implementing this parsing logic without `peek2(End)` is more clumsy because
251/// we'd need a parse stream actually advanced past the comma before being able
252/// to find out whether there is anything after it. It would look something
253/// like:
254///
255/// ```
256/// # use proc_macro2::TokenStream;
257/// # use syn::parse::{ParseStream, Result};
258/// # use syn::Token;
259/// #
260/// # fn parse(input: ParseStream) -> Result<()> {
261/// use syn::parse::discouraged::Speculative as _;
262///
263/// let ahead = input.fork();
264/// ahead.parse::<Option<Token![,]>>()?;
265/// let args = if ahead.is_empty() {
266/// input.advance_to(&ahead);
267/// TokenStream::new()
268/// } else {
269/// input.parse()?
270/// };
271/// # Ok(())
272/// # }
273/// ```
274///
275/// or:
276///
277/// ```
278/// # use proc_macro2::TokenStream;
279/// # use syn::parse::{ParseStream, Result};
280/// # use syn::Token;
281/// #
282/// # fn parse(input: ParseStream) -> Result<()> {
283/// use quote::ToTokens as _;
284///
285/// let comma: Option<Token![,]> = input.parse()?;
286/// let mut args = TokenStream::new();
287/// if !input.is_empty() {
288/// comma.to_tokens(&mut args);
289/// input.parse::<TokenStream>()?.to_tokens(&mut args);
290/// }
291/// # Ok(())
292/// # }
293/// ```
294pub struct End;
295
296impl Copy for End {}
297
298impl Clone for End {
299 fn clone(&self) -> Self {
300 *self
301 }
302}
303
304impl Peek for End {
305 type Token = Self;
306}
307
308impl CustomToken for End {
309 fn peek(cursor: Cursor) -> bool {
310 cursor.eof()
311 }
312
313 fn display() -> &'static str {
314 "`)`" // Lookahead1 error message will fill in the expected close delimiter
315 }
316}
317
318impl<F: Copy + FnOnce(TokenMarker) -> T, T: Token> Peek for F {
319 type Token = T;
320}
321
322pub enum TokenMarker {}
323
324impl<S> IntoSpans<S> for TokenMarker {
325 fn into_spans(self) -> S {
326 match self {}
327 }
328}
329
330impl<F: Copy + FnOnce(TokenMarker) -> T, T: Token> Sealed for F {}
331
332impl Sealed for End {}
333