projects/linux·blob master · refs

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1// SPDX-License-Identifier: Apache-2.0 OR MIT
2
3use crate::buffer::Cursor;
4use crate::error::{self, Error};
5use crate::sealed::lookahead::Sealed;
6use crate::span::IntoSpans;
7use crate::token::{CustomToken, Token};
8use proc_macro2::{Delimiter, Span};
9use std::cell::RefCell;
10
11/// Support for checking the next token in a stream to decide how to parse.
12///
13/// An important advantage over [`ParseStream::peek`] is that here we
14/// automatically construct an appropriate error message based on the token
15/// alternatives that get peeked. If you are producing your own error message,
16/// go ahead and use `ParseStream::peek` instead.
17///
18/// Use [`ParseStream::lookahead1`] to construct this object.
19///
20/// [`ParseStream::peek`]: crate::parse::ParseBuffer::peek
21/// [`ParseStream::lookahead1`]: crate::parse::ParseBuffer::lookahead1
22///
23/// Consuming tokens from the source stream after constructing a lookahead
24/// object does not also advance the lookahead object.
25///
26/// # Example
27///
28/// ```
29/// use syn::{ConstParam, Ident, Lifetime, LifetimeParam, Result, Token, TypeParam};
30/// use syn::parse::{Parse, ParseStream};
31///
32/// // A generic parameter, a single one of the comma-separated elements inside
33/// // angle brackets in:
34/// //
35/// // fn f<T: Clone, 'a, 'b: 'a, const N: usize>() { ... }
36/// //
37/// // On invalid input, lookahead gives us a reasonable error message.
38/// //
39/// // error: expected one of: identifier, lifetime, `const`
40/// // |
41/// // 5 | fn f<!Sized>() {}
42/// // | ^
43/// enum GenericParam {
44/// Type(TypeParam),
45/// Lifetime(LifetimeParam),
46/// Const(ConstParam),
47/// }
48///
49/// impl Parse for GenericParam {
50/// fn parse(input: ParseStream) -> Result<Self> {
51/// let lookahead = input.lookahead1();
52/// if lookahead.peek(Ident) {
53/// input.parse().map(GenericParam::Type)
54/// } else if lookahead.peek(Lifetime) {
55/// input.parse().map(GenericParam::Lifetime)
56/// } else if lookahead.peek(Token![const]) {
57/// input.parse().map(GenericParam::Const)
58/// } else {
59/// Err(lookahead.error())
60/// }
61/// }
62/// }
63/// ```
64pub struct Lookahead1<'a> {
65 scope: Span,
66 cursor: Cursor<'a>,
67 comparisons: RefCell<Vec<&'static str>>,
68}
69
70pub(crate) fn new(scope: Span, cursor: Cursor) -> Lookahead1 {
71 Lookahead1 {
72 scope,
73 cursor,
74 comparisons: RefCell::new(Vec::new()),
75 }
76}
77
78fn peek_impl(
79 lookahead: &Lookahead1,
80 peek: fn(Cursor) -> bool,
81 display: fn() -> &'static str,
82) -> bool {
83 if peek(lookahead.cursor) {
84 return true;
85 }
86 lookahead.comparisons.borrow_mut().push(display());
87 false
88}
89
90impl<'a> Lookahead1<'a> {
91 /// Looks at the next token in the parse stream to determine whether it
92 /// matches the requested type of token.
93 ///
94 /// # Syntax
95 ///
96 /// Note that this method does not use turbofish syntax. Pass the peek type
97 /// inside of parentheses.
98 ///
99 /// - `input.peek(Token![struct])`
100 /// - `input.peek(Token![==])`
101 /// - `input.peek(Ident)`&emsp;*(does not accept keywords)*
102 /// - `input.peek(Ident::peek_any)`
103 /// - `input.peek(Lifetime)`
104 /// - `input.peek(token::Brace)`
105 pub fn peek<T: Peek>(&self, token: T) -> bool {
106 let _ = token;
107 peek_impl(self, T::Token::peek, T::Token::display)
108 }
109
110 /// Triggers an error at the current position of the parse stream.
111 ///
112 /// The error message will identify all of the expected token types that
113 /// have been peeked against this lookahead instance.
114 pub fn error(self) -> Error {
115 let mut comparisons = self.comparisons.into_inner();
116 comparisons.retain_mut(|display| {
117 if *display == "`)`" {
118 *display = match self.cursor.scope_delimiter() {
119 Delimiter::Parenthesis => "`)`",
120 Delimiter::Brace => "`}`",
121 Delimiter::Bracket => "`]`",
122 Delimiter::None => return false,
123 }
124 }
125 true
126 });
127 match comparisons.len() {
128 0 => {
129 if self.cursor.eof() {
130 Error::new(self.scope, "unexpected end of input")
131 } else {
132 Error::new(self.cursor.span(), "unexpected token")
133 }
134 }
135 1 => {
136 let message = format!("expected {}", comparisons[0]);
137 error::new_at(self.scope, self.cursor, message)
138 }
139 2 => {
140 let message = format!("expected {} or {}", comparisons[0], comparisons[1]);
141 error::new_at(self.scope, self.cursor, message)
142 }
143 _ => {
144 let join = comparisons.join(", ");
145 let message = format!("expected one of: {}", join);
146 error::new_at(self.scope, self.cursor, message)
147 }
148 }
149 }
150}
151
152/// Types that can be parsed by looking at just one token.
153///
154/// Use [`ParseStream::peek`] to peek one of these types in a parse stream
155/// without consuming it from the stream.
156///
157/// This trait is sealed and cannot be implemented for types outside of Syn.
158///
159/// [`ParseStream::peek`]: crate::parse::ParseBuffer::peek
160pub trait Peek: Sealed {
161 // Not public API.
162 #[doc(hidden)]
163 type Token: Token;
164}
165
166/// Pseudo-token used for peeking the end of a parse stream.
167///
168/// This type is only useful as an argument to one of the following functions:
169///
170/// - [`ParseStream::peek`][crate::parse::ParseBuffer::peek]
171/// - [`ParseStream::peek2`][crate::parse::ParseBuffer::peek2]
172/// - [`ParseStream::peek3`][crate::parse::ParseBuffer::peek3]
173/// - [`Lookahead1::peek`]
174///
175/// The peek will return `true` if there are no remaining tokens after that
176/// point in the parse stream.
177///
178/// # Example
179///
180/// Suppose we are parsing attributes containing core::fmt inspired formatting
181/// arguments:
182///
183/// - `#[fmt("simple example")]`
184/// - `#[fmt("interpolation e{}ample", self.x)]`
185/// - `#[fmt("interpolation e{x}ample")]`
186///
187/// and we want to recognize the cases where no interpolation occurs so that
188/// more efficient code can be generated.
189///
190/// The following implementation uses `input.peek(Token![,]) &&
191/// input.peek2(End)` to recognize the case of a trailing comma without
192/// consuming the comma from the parse stream, because if it isn't a trailing
193/// comma, that same comma needs to be parsed as part of `args`.
194///
195/// ```
196/// use proc_macro2::TokenStream;
197/// use quote::quote;
198/// use syn::parse::{End, Parse, ParseStream, Result};
199/// use syn::{parse_quote, Attribute, LitStr, Token};
200///
201/// struct FormatArgs {
202/// template: LitStr, // "...{}..."
203/// args: TokenStream, // , self.x
204/// }
205///
206/// impl Parse for FormatArgs {
207/// fn parse(input: ParseStream) -> Result<Self> {
208/// let template: LitStr = input.parse()?;
209///
210/// let args = if input.is_empty()
211/// || input.peek(Token![,]) && input.peek2(End)
212/// {
213/// input.parse::<Option<Token![,]>>()?;
214/// TokenStream::new()
215/// } else {
216/// input.parse()?
217/// };
218///
219/// Ok(FormatArgs {
220/// template,
221/// args,
222/// })
223/// }
224/// }
225///
226/// fn main() -> Result<()> {
227/// let attrs: Vec<Attribute> = parse_quote! {
228/// #[fmt("simple example")]
229/// #[fmt("interpolation e{}ample", self.x)]
230/// #[fmt("interpolation e{x}ample")]
231/// };
232///
233/// for attr in &attrs {
234/// let FormatArgs { template, args } = attr.parse_args()?;
235/// let requires_fmt_machinery =
236/// !args.is_empty() || template.value().contains(['{', '}']);
237/// let out = if requires_fmt_machinery {
238/// quote! {
239/// ::core::write!(__formatter, #template #args)
240/// }
241/// } else {
242/// quote! {
243/// __formatter.write_str(#template)
244/// }
245/// };
246/// println!("{}", out);
247/// }
248/// Ok(())
249/// }
250/// ```
251///
252/// Implementing this parsing logic without `peek2(End)` is more clumsy because
253/// we'd need a parse stream actually advanced past the comma before being able
254/// to find out whether there is anything after it. It would look something
255/// like:
256///
257/// ```
258/// # use proc_macro2::TokenStream;
259/// # use syn::parse::{ParseStream, Result};
260/// # use syn::Token;
261/// #
262/// # fn parse(input: ParseStream) -> Result<()> {
263/// use syn::parse::discouraged::Speculative as _;
264///
265/// let ahead = input.fork();
266/// ahead.parse::<Option<Token![,]>>()?;
267/// let args = if ahead.is_empty() {
268/// input.advance_to(&ahead);
269/// TokenStream::new()
270/// } else {
271/// input.parse()?
272/// };
273/// # Ok(())
274/// # }
275/// ```
276///
277/// or:
278///
279/// ```
280/// # use proc_macro2::TokenStream;
281/// # use syn::parse::{ParseStream, Result};
282/// # use syn::Token;
283/// #
284/// # fn parse(input: ParseStream) -> Result<()> {
285/// use quote::ToTokens as _;
286///
287/// let comma: Option<Token![,]> = input.parse()?;
288/// let mut args = TokenStream::new();
289/// if !input.is_empty() {
290/// comma.to_tokens(&mut args);
291/// input.parse::<TokenStream>()?.to_tokens(&mut args);
292/// }
293/// # Ok(())
294/// # }
295/// ```
296pub struct End;
297
298impl Copy for End {}
299
300impl Clone for End {
301 fn clone(&self) -> Self {
302 *self
303 }
304}
305
306impl Peek for End {
307 type Token = Self;
308}
309
310impl CustomToken for End {
311 fn peek(cursor: Cursor) -> bool {
312 cursor.eof()
313 }
314
315 fn display() -> &'static str {
316 "`)`" // Lookahead1 error message will fill in the expected close delimiter
317 }
318}
319
320impl<F: Copy + FnOnce(TokenMarker) -> T, T: Token> Peek for F {
321 type Token = T;
322}
323
324pub enum TokenMarker {}
325
326impl<S> IntoSpans<S> for TokenMarker {
327 fn into_spans(self) -> S {
328 match self {}
329 }
330}
331
332impl<F: Copy + FnOnce(TokenMarker) -> T, T: Token> Sealed for F {}
333
334impl Sealed for End {}