1 //! Parsing interface for parsing a token stream into a syntax tree node.
2 //!
3 //! Parsing in Syn is built on parser functions that take in a [`ParseStream`]
4 //! and produce a [`Result<T>`] where `T` is some syntax tree node. Underlying
5 //! these parser functions is a lower level mechanism built around the
6 //! [`Cursor`] type. `Cursor` is a cheaply copyable cursor over a range of
7 //! tokens in a token stream.
8 //!
9 //! [`ParseStream`]: type.ParseStream.html
10 //! [`Result<T>`]: type.Result.html
11 //! [`Cursor`]: ../buffer/index.html
12 //!
13 //! # Example
14 //!
15 //! Here is a snippet of parsing code to get a feel for the style of the
16 //! library. We define data structures for a subset of Rust syntax including
17 //! enums (not shown) and structs, then provide implementations of the [`Parse`]
18 //! trait to parse these syntax tree data structures from a token stream.
19 //!
20 //! Once `Parse` impls have been defined, they can be called conveniently from a
21 //! procedural macro through [`parse_macro_input!`] as shown at the bottom of
22 //! the snippet. If the caller provides syntactically invalid input to the
23 //! procedural macro, they will receive a helpful compiler error message
24 //! pointing out the exact token that triggered the failure to parse.
25 //!
26 //! [`parse_macro_input!`]: ../macro.parse_macro_input.html
27 //!
28 //! ```
29 //! extern crate proc_macro;
30 //!
31 //! use proc_macro::TokenStream;
32 //! use syn::{braced, parse_macro_input, token, Field, Ident, Result, Token};
33 //! use syn::parse::{Parse, ParseStream};
34 //! use syn::punctuated::Punctuated;
35 //!
36 //! enum Item {
37 //! Struct(ItemStruct),
38 //! Enum(ItemEnum),
39 //! }
40 //!
41 //! struct ItemStruct {
42 //! struct_token: Token![struct],
43 //! ident: Ident,
44 //! brace_token: token::Brace,
45 //! fields: Punctuated<Field, Token![,]>,
46 //! }
47 //! #
48 //! # enum ItemEnum {}
49 //!
50 //! impl Parse for Item {
51 //! fn parse(input: ParseStream) -> Result<Self> {
52 //! let lookahead = input.lookahead1();
53 //! if lookahead.peek(Token![struct]) {
54 //! input.parse().map(Item::Struct)
55 //! } else if lookahead.peek(Token![enum]) {
56 //! input.parse().map(Item::Enum)
57 //! } else {
58 //! Err(lookahead.error())
59 //! }
60 //! }
61 //! }
62 //!
63 //! impl Parse for ItemStruct {
64 //! fn parse(input: ParseStream) -> Result<Self> {
65 //! let content;
66 //! Ok(ItemStruct {
67 //! struct_token: input.parse()?,
68 //! ident: input.parse()?,
69 //! brace_token: braced!(content in input),
70 //! fields: content.parse_terminated(Field::parse_named)?,
71 //! })
72 //! }
73 //! }
74 //! #
75 //! # impl Parse for ItemEnum {
76 //! # fn parse(input: ParseStream) -> Result<Self> {
77 //! # unimplemented!()
78 //! # }
79 //! # }
80 //!
81 //! # const IGNORE: &str = stringify! {
82 //! #[proc_macro]
83 //! # };
84 //! pub fn my_macro(tokens: TokenStream) -> TokenStream {
85 //! let input = parse_macro_input!(tokens as Item);
86 //!
87 //! /* ... */
88 //! # "".parse().unwrap()
89 //! }
90 //! ```
91 //!
92 //! # The `syn::parse*` functions
93 //!
94 //! The [`syn::parse`], [`syn::parse2`], and [`syn::parse_str`] functions serve
95 //! as an entry point for parsing syntax tree nodes that can be parsed in an
96 //! obvious default way. These functions can return any syntax tree node that
97 //! implements the [`Parse`] trait, which includes most types in Syn.
98 //!
99 //! [`syn::parse`]: ../fn.parse.html
100 //! [`syn::parse2`]: ../fn.parse2.html
101 //! [`syn::parse_str`]: ../fn.parse_str.html
102 //! [`Parse`]: trait.Parse.html
103 //!
104 //! ```
105 //! use syn::Type;
106 //!
107 //! # fn run_parser() -> syn::Result<()> {
108 //! let t: Type = syn::parse_str("std::collections::HashMap<String, Value>")?;
109 //! # Ok(())
110 //! # }
111 //! #
112 //! # run_parser().unwrap();
113 //! ```
114 //!
115 //! The [`parse_quote!`] macro also uses this approach.
116 //!
117 //! [`parse_quote!`]: ../macro.parse_quote.html
118 //!
119 //! # The `Parser` trait
120 //!
121 //! Some types can be parsed in several ways depending on context. For example
122 //! an [`Attribute`] can be either "outer" like `#[...]` or "inner" like
123 //! `#![...]` and parsing the wrong one would be a bug. Similarly [`Punctuated`]
124 //! may or may not allow trailing punctuation, and parsing it the wrong way
125 //! would either reject valid input or accept invalid input.
126 //!
127 //! [`Attribute`]: ../struct.Attribute.html
128 //! [`Punctuated`]: ../punctuated/index.html
129 //!
130 //! The `Parse` trait is not implemented in these cases because there is no good
131 //! behavior to consider the default.
132 //!
133 //! ```compile_fail
134 //! # extern crate proc_macro;
135 //! #
136 //! # use syn::punctuated::Punctuated;
137 //! # use syn::{PathSegment, Result, Token};
138 //! #
139 //! # fn f(tokens: proc_macro::TokenStream) -> Result<()> {
140 //! #
141 //! // Can't parse `Punctuated` without knowing whether trailing punctuation
142 //! // should be allowed in this context.
143 //! let path: Punctuated<PathSegment, Token![::]> = syn::parse(tokens)?;
144 //! #
145 //! # Ok(())
146 //! # }
147 //! ```
148 //!
149 //! In these cases the types provide a choice of parser functions rather than a
150 //! single `Parse` implementation, and those parser functions can be invoked
151 //! through the [`Parser`] trait.
152 //!
153 //! [`Parser`]: trait.Parser.html
154 //!
155 //! ```
156 //! extern crate proc_macro;
157 //!
158 //! use proc_macro::TokenStream;
159 //! use syn::parse::Parser;
160 //! use syn::punctuated::Punctuated;
161 //! use syn::{Attribute, Expr, PathSegment, Result, Token};
162 //!
163 //! fn call_some_parser_methods(input: TokenStream) -> Result<()> {
164 //! // Parse a nonempty sequence of path segments separated by `::` punctuation
165 //! // with no trailing punctuation.
166 //! let tokens = input.clone();
167 //! let parser = Punctuated::<PathSegment, Token![::]>::parse_separated_nonempty;
168 //! let _path = parser.parse(tokens)?;
169 //!
170 //! // Parse a possibly empty sequence of expressions terminated by commas with
171 //! // an optional trailing punctuation.
172 //! let tokens = input.clone();
173 //! let parser = Punctuated::<Expr, Token![,]>::parse_terminated;
174 //! let _args = parser.parse(tokens)?;
175 //!
176 //! // Parse zero or more outer attributes but not inner attributes.
177 //! let tokens = input.clone();
178 //! let parser = Attribute::parse_outer;
179 //! let _attrs = parser.parse(tokens)?;
180 //!
181 //! Ok(())
182 //! }
183 //! ```
184 //!
185 //! ---
186 //!
187 //! *This module is available if Syn is built with the `"parsing"` feature.*
188
189 #[path = "discouraged.rs"]
190 pub mod discouraged;
191
192 use std::cell::Cell;
193 use std::fmt::{self, Debug, Display};
194 use std::marker::PhantomData;
195 use std::mem;
196 use std::ops::Deref;
197 use std::rc::Rc;
198 use std::str::FromStr;
199
200 #[cfg(all(
201 not(all(target_arch = "wasm32", any(target_os = "unknown", target_os = "wasi"))),
202 feature = "proc-macro"
203 ))]
204 use crate::proc_macro;
205 use proc_macro2::{self, Delimiter, Group, Literal, Punct, Span, TokenStream, TokenTree};
206
207 use crate::buffer::{Cursor, TokenBuffer};
208 use crate::error;
209 use crate::lookahead;
210 use crate::punctuated::Punctuated;
211 use crate::token::Token;
212
213 pub use crate::error::{Error, Result};
214 pub use crate::lookahead::{Lookahead1, Peek};
215
216 /// Parsing interface implemented by all types that can be parsed in a default
217 /// way from a token stream.
218 pub trait Parse: Sized {
parse(input: ParseStream) -> Result<Self>219 fn parse(input: ParseStream) -> Result<Self>;
220 }
221
222 /// Input to a Syn parser function.
223 ///
224 /// See the methods of this type under the documentation of [`ParseBuffer`]. For
225 /// an overview of parsing in Syn, refer to the [module documentation].
226 ///
227 /// [module documentation]: self
228 pub type ParseStream<'a> = &'a ParseBuffer<'a>;
229
230 /// Cursor position within a buffered token stream.
231 ///
232 /// This type is more commonly used through the type alias [`ParseStream`] which
233 /// is an alias for `&ParseBuffer`.
234 ///
235 /// `ParseStream` is the input type for all parser functions in Syn. They have
236 /// the signature `fn(ParseStream) -> Result<T>`.
237 ///
238 /// ## Calling a parser function
239 ///
240 /// There is no public way to construct a `ParseBuffer`. Instead, if you are
241 /// looking to invoke a parser function that requires `ParseStream` as input,
242 /// you will need to go through one of the public parsing entry points.
243 ///
244 /// - The [`parse_macro_input!`] macro if parsing input of a procedural macro;
245 /// - One of [the `syn::parse*` functions][syn-parse]; or
246 /// - A method of the [`Parser`] trait.
247 ///
248 /// [syn-parse]: index.html#the-synparse-functions
249 pub struct ParseBuffer<'a> {
250 scope: Span,
251 // Instead of Cell<Cursor<'a>> so that ParseBuffer<'a> is covariant in 'a.
252 // The rest of the code in this module needs to be careful that only a
253 // cursor derived from this `cell` is ever assigned to this `cell`.
254 //
255 // Cell<Cursor<'a>> cannot be covariant in 'a because then we could take a
256 // ParseBuffer<'a>, upcast to ParseBuffer<'short> for some lifetime shorter
257 // than 'a, and then assign a Cursor<'short> into the Cell.
258 //
259 // By extension, it would not be safe to expose an API that accepts a
260 // Cursor<'a> and trusts that it lives as long as the cursor currently in
261 // the cell.
262 cell: Cell<Cursor<'static>>,
263 marker: PhantomData<Cursor<'a>>,
264 unexpected: Cell<Option<Rc<Cell<Unexpected>>>>,
265 }
266
267 impl<'a> Drop for ParseBuffer<'a> {
drop(&mut self)268 fn drop(&mut self) {
269 if !self.is_empty() {
270 let (inner, old_span) = inner_unexpected(self);
271 if old_span.is_none() {
272 inner.set(Unexpected::Some(self.cursor().span()));
273 }
274 }
275 }
276 }
277
278 impl<'a> Display for ParseBuffer<'a> {
fmt(&self, f: &mut fmt::Formatter) -> fmt::Result279 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
280 Display::fmt(&self.cursor().token_stream(), f)
281 }
282 }
283
284 impl<'a> Debug for ParseBuffer<'a> {
fmt(&self, f: &mut fmt::Formatter) -> fmt::Result285 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
286 Debug::fmt(&self.cursor().token_stream(), f)
287 }
288 }
289
290 /// Cursor state associated with speculative parsing.
291 ///
292 /// This type is the input of the closure provided to [`ParseStream::step`].
293 ///
294 /// [`ParseStream::step`]: ParseBuffer::step
295 ///
296 /// # Example
297 ///
298 /// ```
299 /// use proc_macro2::TokenTree;
300 /// use syn::Result;
301 /// use syn::parse::ParseStream;
302 ///
303 /// // This function advances the stream past the next occurrence of `@`. If
304 /// // no `@` is present in the stream, the stream position is unchanged and
305 /// // an error is returned.
306 /// fn skip_past_next_at(input: ParseStream) -> Result<()> {
307 /// input.step(|cursor| {
308 /// let mut rest = *cursor;
309 /// while let Some((tt, next)) = rest.token_tree() {
310 /// match &tt {
311 /// TokenTree::Punct(punct) if punct.as_char() == '@' => {
312 /// return Ok(((), next));
313 /// }
314 /// _ => rest = next,
315 /// }
316 /// }
317 /// Err(cursor.error("no `@` was found after this point"))
318 /// })
319 /// }
320 /// #
321 /// # fn remainder_after_skipping_past_next_at(
322 /// # input: ParseStream,
323 /// # ) -> Result<proc_macro2::TokenStream> {
324 /// # skip_past_next_at(input)?;
325 /// # input.parse()
326 /// # }
327 /// #
328 /// # use syn::parse::Parser;
329 /// # let remainder = remainder_after_skipping_past_next_at
330 /// # .parse_str("a @ b c")
331 /// # .unwrap();
332 /// # assert_eq!(remainder.to_string(), "b c");
333 /// ```
334 #[derive(Copy, Clone)]
335 pub struct StepCursor<'c, 'a> {
336 scope: Span,
337 // This field is covariant in 'c.
338 cursor: Cursor<'c>,
339 // This field is contravariant in 'c. Together these make StepCursor
340 // invariant in 'c. Also covariant in 'a. The user cannot cast 'c to a
341 // different lifetime but can upcast into a StepCursor with a shorter
342 // lifetime 'a.
343 //
344 // As long as we only ever construct a StepCursor for which 'c outlives 'a,
345 // this means if ever a StepCursor<'c, 'a> exists we are guaranteed that 'c
346 // outlives 'a.
347 marker: PhantomData<fn(Cursor<'c>) -> Cursor<'a>>,
348 }
349
350 impl<'c, 'a> Deref for StepCursor<'c, 'a> {
351 type Target = Cursor<'c>;
352
deref(&self) -> &Self::Target353 fn deref(&self) -> &Self::Target {
354 &self.cursor
355 }
356 }
357
358 impl<'c, 'a> StepCursor<'c, 'a> {
359 /// Triggers an error at the current position of the parse stream.
360 ///
361 /// The `ParseStream::step` invocation will return this same error without
362 /// advancing the stream state.
error<T: Display>(self, message: T) -> Error363 pub fn error<T: Display>(self, message: T) -> Error {
364 error::new_at(self.scope, self.cursor, message)
365 }
366 }
367
advance_step_cursor<'c, 'a>(proof: StepCursor<'c, 'a>, to: Cursor<'c>) -> Cursor<'a>368 pub(crate) fn advance_step_cursor<'c, 'a>(proof: StepCursor<'c, 'a>, to: Cursor<'c>) -> Cursor<'a> {
369 // Refer to the comments within the StepCursor definition. We use the
370 // fact that a StepCursor<'c, 'a> exists as proof that 'c outlives 'a.
371 // Cursor is covariant in its lifetime parameter so we can cast a
372 // Cursor<'c> to one with the shorter lifetime Cursor<'a>.
373 let _ = proof;
374 unsafe { mem::transmute::<Cursor<'c>, Cursor<'a>>(to) }
375 }
376
new_parse_buffer( scope: Span, cursor: Cursor, unexpected: Rc<Cell<Unexpected>>, ) -> ParseBuffer377 pub(crate) fn new_parse_buffer(
378 scope: Span,
379 cursor: Cursor,
380 unexpected: Rc<Cell<Unexpected>>,
381 ) -> ParseBuffer {
382 ParseBuffer {
383 scope,
384 // See comment on `cell` in the struct definition.
385 cell: Cell::new(unsafe { mem::transmute::<Cursor, Cursor<'static>>(cursor) }),
386 marker: PhantomData,
387 unexpected: Cell::new(Some(unexpected)),
388 }
389 }
390
391 #[derive(Clone)]
392 pub(crate) enum Unexpected {
393 None,
394 Some(Span),
395 Chain(Rc<Cell<Unexpected>>),
396 }
397
398 impl Default for Unexpected {
default() -> Self399 fn default() -> Self {
400 Unexpected::None
401 }
402 }
403
404 // We call this on Cell<Unexpected> and Cell<Option<T>> where temporarily
405 // swapping in a None is cheap.
cell_clone<T: Default + Clone>(cell: &Cell<T>) -> T406 fn cell_clone<T: Default + Clone>(cell: &Cell<T>) -> T {
407 let prev = cell.take();
408 let ret = prev.clone();
409 cell.set(prev);
410 ret
411 }
412
inner_unexpected(buffer: &ParseBuffer) -> (Rc<Cell<Unexpected>>, Option<Span>)413 fn inner_unexpected(buffer: &ParseBuffer) -> (Rc<Cell<Unexpected>>, Option<Span>) {
414 let mut unexpected = get_unexpected(buffer);
415 loop {
416 match cell_clone(&unexpected) {
417 Unexpected::None => return (unexpected, None),
418 Unexpected::Some(span) => return (unexpected, Some(span)),
419 Unexpected::Chain(next) => unexpected = next,
420 }
421 }
422 }
423
get_unexpected(buffer: &ParseBuffer) -> Rc<Cell<Unexpected>>424 pub(crate) fn get_unexpected(buffer: &ParseBuffer) -> Rc<Cell<Unexpected>> {
425 cell_clone(&buffer.unexpected).unwrap()
426 }
427
428 impl<'a> ParseBuffer<'a> {
429 /// Parses a syntax tree node of type `T`, advancing the position of our
430 /// parse stream past it.
parse<T: Parse>(&self) -> Result<T>431 pub fn parse<T: Parse>(&self) -> Result<T> {
432 T::parse(self)
433 }
434
435 /// Calls the given parser function to parse a syntax tree node of type `T`
436 /// from this stream.
437 ///
438 /// # Example
439 ///
440 /// The parser below invokes [`Attribute::parse_outer`] to parse a vector of
441 /// zero or more outer attributes.
442 ///
443 /// [`Attribute::parse_outer`]: crate::Attribute::parse_outer
444 ///
445 /// ```
446 /// use syn::{Attribute, Ident, Result, Token};
447 /// use syn::parse::{Parse, ParseStream};
448 ///
449 /// // Parses a unit struct with attributes.
450 /// //
451 /// // #[path = "s.tmpl"]
452 /// // struct S;
453 /// struct UnitStruct {
454 /// attrs: Vec<Attribute>,
455 /// struct_token: Token![struct],
456 /// name: Ident,
457 /// semi_token: Token![;],
458 /// }
459 ///
460 /// impl Parse for UnitStruct {
461 /// fn parse(input: ParseStream) -> Result<Self> {
462 /// Ok(UnitStruct {
463 /// attrs: input.call(Attribute::parse_outer)?,
464 /// struct_token: input.parse()?,
465 /// name: input.parse()?,
466 /// semi_token: input.parse()?,
467 /// })
468 /// }
469 /// }
470 /// ```
call<T>(&self, function: fn(ParseStream) -> Result<T>) -> Result<T>471 pub fn call<T>(&self, function: fn(ParseStream) -> Result<T>) -> Result<T> {
472 function(self)
473 }
474
475 /// Looks at the next token in the parse stream to determine whether it
476 /// matches the requested type of token.
477 ///
478 /// Does not advance the position of the parse stream.
479 ///
480 /// # Syntax
481 ///
482 /// Note that this method does not use turbofish syntax. Pass the peek type
483 /// inside of parentheses.
484 ///
485 /// - `input.peek(Token![struct])`
486 /// - `input.peek(Token![==])`
487 /// - `input.peek(Ident)` *(does not accept keywords)*
488 /// - `input.peek(Ident::peek_any)`
489 /// - `input.peek(Lifetime)`
490 /// - `input.peek(token::Brace)`
491 ///
492 /// # Example
493 ///
494 /// In this example we finish parsing the list of supertraits when the next
495 /// token in the input is either `where` or an opening curly brace.
496 ///
497 /// ```
498 /// use syn::{braced, token, Generics, Ident, Result, Token, TypeParamBound};
499 /// use syn::parse::{Parse, ParseStream};
500 /// use syn::punctuated::Punctuated;
501 ///
502 /// // Parses a trait definition containing no associated items.
503 /// //
504 /// // trait Marker<'de, T>: A + B<'de> where Box<T>: Clone {}
505 /// struct MarkerTrait {
506 /// trait_token: Token![trait],
507 /// ident: Ident,
508 /// generics: Generics,
509 /// colon_token: Option<Token![:]>,
510 /// supertraits: Punctuated<TypeParamBound, Token![+]>,
511 /// brace_token: token::Brace,
512 /// }
513 ///
514 /// impl Parse for MarkerTrait {
515 /// fn parse(input: ParseStream) -> Result<Self> {
516 /// let trait_token: Token![trait] = input.parse()?;
517 /// let ident: Ident = input.parse()?;
518 /// let mut generics: Generics = input.parse()?;
519 /// let colon_token: Option<Token![:]> = input.parse()?;
520 ///
521 /// let mut supertraits = Punctuated::new();
522 /// if colon_token.is_some() {
523 /// loop {
524 /// supertraits.push_value(input.parse()?);
525 /// if input.peek(Token![where]) || input.peek(token::Brace) {
526 /// break;
527 /// }
528 /// supertraits.push_punct(input.parse()?);
529 /// }
530 /// }
531 ///
532 /// generics.where_clause = input.parse()?;
533 /// let content;
534 /// let empty_brace_token = braced!(content in input);
535 ///
536 /// Ok(MarkerTrait {
537 /// trait_token,
538 /// ident,
539 /// generics,
540 /// colon_token,
541 /// supertraits,
542 /// brace_token: empty_brace_token,
543 /// })
544 /// }
545 /// }
546 /// ```
peek<T: Peek>(&self, token: T) -> bool547 pub fn peek<T: Peek>(&self, token: T) -> bool {
548 let _ = token;
549 T::Token::peek(self.cursor())
550 }
551
552 /// Looks at the second-next token in the parse stream.
553 ///
554 /// This is commonly useful as a way to implement contextual keywords.
555 ///
556 /// # Example
557 ///
558 /// This example needs to use `peek2` because the symbol `union` is not a
559 /// keyword in Rust. We can't use just `peek` and decide to parse a union if
560 /// the very next token is `union`, because someone is free to write a `mod
561 /// union` and a macro invocation that looks like `union::some_macro! { ...
562 /// }`. In other words `union` is a contextual keyword.
563 ///
564 /// ```
565 /// use syn::{Ident, ItemUnion, Macro, Result, Token};
566 /// use syn::parse::{Parse, ParseStream};
567 ///
568 /// // Parses either a union or a macro invocation.
569 /// enum UnionOrMacro {
570 /// // union MaybeUninit<T> { uninit: (), value: T }
571 /// Union(ItemUnion),
572 /// // lazy_static! { ... }
573 /// Macro(Macro),
574 /// }
575 ///
576 /// impl Parse for UnionOrMacro {
577 /// fn parse(input: ParseStream) -> Result<Self> {
578 /// if input.peek(Token![union]) && input.peek2(Ident) {
579 /// input.parse().map(UnionOrMacro::Union)
580 /// } else {
581 /// input.parse().map(UnionOrMacro::Macro)
582 /// }
583 /// }
584 /// }
585 /// ```
peek2<T: Peek>(&self, token: T) -> bool586 pub fn peek2<T: Peek>(&self, token: T) -> bool {
587 let _ = token;
588 self.cursor().skip().map_or(false, T::Token::peek)
589 }
590
591 /// Looks at the third-next token in the parse stream.
peek3<T: Peek>(&self, token: T) -> bool592 pub fn peek3<T: Peek>(&self, token: T) -> bool {
593 let _ = token;
594 self.cursor()
595 .skip()
596 .and_then(Cursor::skip)
597 .map_or(false, T::Token::peek)
598 }
599
600 /// Parses zero or more occurrences of `T` separated by punctuation of type
601 /// `P`, with optional trailing punctuation.
602 ///
603 /// Parsing continues until the end of this parse stream. The entire content
604 /// of this parse stream must consist of `T` and `P`.
605 ///
606 /// # Example
607 ///
608 /// ```
609 /// # use quote::quote;
610 /// #
611 /// use syn::{parenthesized, token, Ident, Result, Token, Type};
612 /// use syn::parse::{Parse, ParseStream};
613 /// use syn::punctuated::Punctuated;
614 ///
615 /// // Parse a simplified tuple struct syntax like:
616 /// //
617 /// // struct S(A, B);
618 /// struct TupleStruct {
619 /// struct_token: Token![struct],
620 /// ident: Ident,
621 /// paren_token: token::Paren,
622 /// fields: Punctuated<Type, Token![,]>,
623 /// semi_token: Token![;],
624 /// }
625 ///
626 /// impl Parse for TupleStruct {
627 /// fn parse(input: ParseStream) -> Result<Self> {
628 /// let content;
629 /// Ok(TupleStruct {
630 /// struct_token: input.parse()?,
631 /// ident: input.parse()?,
632 /// paren_token: parenthesized!(content in input),
633 /// fields: content.parse_terminated(Type::parse)?,
634 /// semi_token: input.parse()?,
635 /// })
636 /// }
637 /// }
638 /// #
639 /// # let input = quote! {
640 /// # struct S(A, B);
641 /// # };
642 /// # syn::parse2::<TupleStruct>(input).unwrap();
643 /// ```
parse_terminated<T, P: Parse>( &self, parser: fn(ParseStream) -> Result<T>, ) -> Result<Punctuated<T, P>>644 pub fn parse_terminated<T, P: Parse>(
645 &self,
646 parser: fn(ParseStream) -> Result<T>,
647 ) -> Result<Punctuated<T, P>> {
648 Punctuated::parse_terminated_with(self, parser)
649 }
650
651 /// Returns whether there are tokens remaining in this stream.
652 ///
653 /// This method returns true at the end of the content of a set of
654 /// delimiters, as well as at the very end of the complete macro input.
655 ///
656 /// # Example
657 ///
658 /// ```
659 /// use syn::{braced, token, Ident, Item, Result, Token};
660 /// use syn::parse::{Parse, ParseStream};
661 ///
662 /// // Parses a Rust `mod m { ... }` containing zero or more items.
663 /// struct Mod {
664 /// mod_token: Token![mod],
665 /// name: Ident,
666 /// brace_token: token::Brace,
667 /// items: Vec<Item>,
668 /// }
669 ///
670 /// impl Parse for Mod {
671 /// fn parse(input: ParseStream) -> Result<Self> {
672 /// let content;
673 /// Ok(Mod {
674 /// mod_token: input.parse()?,
675 /// name: input.parse()?,
676 /// brace_token: braced!(content in input),
677 /// items: {
678 /// let mut items = Vec::new();
679 /// while !content.is_empty() {
680 /// items.push(content.parse()?);
681 /// }
682 /// items
683 /// },
684 /// })
685 /// }
686 /// }
687 /// ```
is_empty(&self) -> bool688 pub fn is_empty(&self) -> bool {
689 self.cursor().eof()
690 }
691
692 /// Constructs a helper for peeking at the next token in this stream and
693 /// building an error message if it is not one of a set of expected tokens.
694 ///
695 /// # Example
696 ///
697 /// ```
698 /// use syn::{ConstParam, Ident, Lifetime, LifetimeDef, Result, Token, TypeParam};
699 /// use syn::parse::{Parse, ParseStream};
700 ///
701 /// // A generic parameter, a single one of the comma-separated elements inside
702 /// // angle brackets in:
703 /// //
704 /// // fn f<T: Clone, 'a, 'b: 'a, const N: usize>() { ... }
705 /// //
706 /// // On invalid input, lookahead gives us a reasonable error message.
707 /// //
708 /// // error: expected one of: identifier, lifetime, `const`
709 /// // |
710 /// // 5 | fn f<!Sized>() {}
711 /// // | ^
712 /// enum GenericParam {
713 /// Type(TypeParam),
714 /// Lifetime(LifetimeDef),
715 /// Const(ConstParam),
716 /// }
717 ///
718 /// impl Parse for GenericParam {
719 /// fn parse(input: ParseStream) -> Result<Self> {
720 /// let lookahead = input.lookahead1();
721 /// if lookahead.peek(Ident) {
722 /// input.parse().map(GenericParam::Type)
723 /// } else if lookahead.peek(Lifetime) {
724 /// input.parse().map(GenericParam::Lifetime)
725 /// } else if lookahead.peek(Token![const]) {
726 /// input.parse().map(GenericParam::Const)
727 /// } else {
728 /// Err(lookahead.error())
729 /// }
730 /// }
731 /// }
732 /// ```
lookahead1(&self) -> Lookahead1<'a>733 pub fn lookahead1(&self) -> Lookahead1<'a> {
734 lookahead::new(self.scope, self.cursor())
735 }
736
737 /// Forks a parse stream so that parsing tokens out of either the original
738 /// or the fork does not advance the position of the other.
739 ///
740 /// # Performance
741 ///
742 /// Forking a parse stream is a cheap fixed amount of work and does not
743 /// involve copying token buffers. Where you might hit performance problems
744 /// is if your macro ends up parsing a large amount of content more than
745 /// once.
746 ///
747 /// ```
748 /// # use syn::{Expr, Result};
749 /// # use syn::parse::ParseStream;
750 /// #
751 /// # fn bad(input: ParseStream) -> Result<Expr> {
752 /// // Do not do this.
753 /// if input.fork().parse::<Expr>().is_ok() {
754 /// return input.parse::<Expr>();
755 /// }
756 /// # unimplemented!()
757 /// # }
758 /// ```
759 ///
760 /// As a rule, avoid parsing an unbounded amount of tokens out of a forked
761 /// parse stream. Only use a fork when the amount of work performed against
762 /// the fork is small and bounded.
763 ///
764 /// When complex speculative parsing against the forked stream is
765 /// unavoidable, use [`parse::discouraged::Speculative`] to advance the
766 /// original stream once the fork's parse is determined to have been
767 /// successful.
768 ///
769 /// For a lower level way to perform speculative parsing at the token level,
770 /// consider using [`ParseStream::step`] instead.
771 ///
772 /// [`parse::discouraged::Speculative`]: discouraged::Speculative
773 /// [`ParseStream::step`]: ParseBuffer::step
774 ///
775 /// # Example
776 ///
777 /// The parse implementation shown here parses possibly restricted `pub`
778 /// visibilities.
779 ///
780 /// - `pub`
781 /// - `pub(crate)`
782 /// - `pub(self)`
783 /// - `pub(super)`
784 /// - `pub(in some::path)`
785 ///
786 /// To handle the case of visibilities inside of tuple structs, the parser
787 /// needs to distinguish parentheses that specify visibility restrictions
788 /// from parentheses that form part of a tuple type.
789 ///
790 /// ```
791 /// # struct A;
792 /// # struct B;
793 /// # struct C;
794 /// #
795 /// struct S(pub(crate) A, pub (B, C));
796 /// ```
797 ///
798 /// In this example input the first tuple struct element of `S` has
799 /// `pub(crate)` visibility while the second tuple struct element has `pub`
800 /// visibility; the parentheses around `(B, C)` are part of the type rather
801 /// than part of a visibility restriction.
802 ///
803 /// The parser uses a forked parse stream to check the first token inside of
804 /// parentheses after the `pub` keyword. This is a small bounded amount of
805 /// work performed against the forked parse stream.
806 ///
807 /// ```
808 /// use syn::{parenthesized, token, Ident, Path, Result, Token};
809 /// use syn::ext::IdentExt;
810 /// use syn::parse::{Parse, ParseStream};
811 ///
812 /// struct PubVisibility {
813 /// pub_token: Token![pub],
814 /// restricted: Option<Restricted>,
815 /// }
816 ///
817 /// struct Restricted {
818 /// paren_token: token::Paren,
819 /// in_token: Option<Token![in]>,
820 /// path: Path,
821 /// }
822 ///
823 /// impl Parse for PubVisibility {
824 /// fn parse(input: ParseStream) -> Result<Self> {
825 /// let pub_token: Token![pub] = input.parse()?;
826 ///
827 /// if input.peek(token::Paren) {
828 /// let ahead = input.fork();
829 /// let mut content;
830 /// parenthesized!(content in ahead);
831 ///
832 /// if content.peek(Token![crate])
833 /// || content.peek(Token![self])
834 /// || content.peek(Token![super])
835 /// {
836 /// return Ok(PubVisibility {
837 /// pub_token,
838 /// restricted: Some(Restricted {
839 /// paren_token: parenthesized!(content in input),
840 /// in_token: None,
841 /// path: Path::from(content.call(Ident::parse_any)?),
842 /// }),
843 /// });
844 /// } else if content.peek(Token![in]) {
845 /// return Ok(PubVisibility {
846 /// pub_token,
847 /// restricted: Some(Restricted {
848 /// paren_token: parenthesized!(content in input),
849 /// in_token: Some(content.parse()?),
850 /// path: content.call(Path::parse_mod_style)?,
851 /// }),
852 /// });
853 /// }
854 /// }
855 ///
856 /// Ok(PubVisibility {
857 /// pub_token,
858 /// restricted: None,
859 /// })
860 /// }
861 /// }
862 /// ```
fork(&self) -> Self863 pub fn fork(&self) -> Self {
864 ParseBuffer {
865 scope: self.scope,
866 cell: self.cell.clone(),
867 marker: PhantomData,
868 // Not the parent's unexpected. Nothing cares whether the clone
869 // parses all the way unless we `advance_to`.
870 unexpected: Cell::new(Some(Rc::new(Cell::new(Unexpected::None)))),
871 }
872 }
873
874 /// Triggers an error at the current position of the parse stream.
875 ///
876 /// # Example
877 ///
878 /// ```
879 /// use syn::{Expr, Result, Token};
880 /// use syn::parse::{Parse, ParseStream};
881 ///
882 /// // Some kind of loop: `while` or `for` or `loop`.
883 /// struct Loop {
884 /// expr: Expr,
885 /// }
886 ///
887 /// impl Parse for Loop {
888 /// fn parse(input: ParseStream) -> Result<Self> {
889 /// if input.peek(Token![while])
890 /// || input.peek(Token![for])
891 /// || input.peek(Token![loop])
892 /// {
893 /// Ok(Loop {
894 /// expr: input.parse()?,
895 /// })
896 /// } else {
897 /// Err(input.error("expected some kind of loop"))
898 /// }
899 /// }
900 /// }
901 /// ```
error<T: Display>(&self, message: T) -> Error902 pub fn error<T: Display>(&self, message: T) -> Error {
903 error::new_at(self.scope, self.cursor(), message)
904 }
905
906 /// Speculatively parses tokens from this parse stream, advancing the
907 /// position of this stream only if parsing succeeds.
908 ///
909 /// This is a powerful low-level API used for defining the `Parse` impls of
910 /// the basic built-in token types. It is not something that will be used
911 /// widely outside of the Syn codebase.
912 ///
913 /// # Example
914 ///
915 /// ```
916 /// use proc_macro2::TokenTree;
917 /// use syn::Result;
918 /// use syn::parse::ParseStream;
919 ///
920 /// // This function advances the stream past the next occurrence of `@`. If
921 /// // no `@` is present in the stream, the stream position is unchanged and
922 /// // an error is returned.
923 /// fn skip_past_next_at(input: ParseStream) -> Result<()> {
924 /// input.step(|cursor| {
925 /// let mut rest = *cursor;
926 /// while let Some((tt, next)) = rest.token_tree() {
927 /// match &tt {
928 /// TokenTree::Punct(punct) if punct.as_char() == '@' => {
929 /// return Ok(((), next));
930 /// }
931 /// _ => rest = next,
932 /// }
933 /// }
934 /// Err(cursor.error("no `@` was found after this point"))
935 /// })
936 /// }
937 /// #
938 /// # fn remainder_after_skipping_past_next_at(
939 /// # input: ParseStream,
940 /// # ) -> Result<proc_macro2::TokenStream> {
941 /// # skip_past_next_at(input)?;
942 /// # input.parse()
943 /// # }
944 /// #
945 /// # use syn::parse::Parser;
946 /// # let remainder = remainder_after_skipping_past_next_at
947 /// # .parse_str("a @ b c")
948 /// # .unwrap();
949 /// # assert_eq!(remainder.to_string(), "b c");
950 /// ```
step<F, R>(&self, function: F) -> Result<R> where F: for<'c> FnOnce(StepCursor<'c, 'a>) -> Result<(R, Cursor<'c>)>,951 pub fn step<F, R>(&self, function: F) -> Result<R>
952 where
953 F: for<'c> FnOnce(StepCursor<'c, 'a>) -> Result<(R, Cursor<'c>)>,
954 {
955 // Since the user's function is required to work for any 'c, we know
956 // that the Cursor<'c> they return is either derived from the input
957 // StepCursor<'c, 'a> or from a Cursor<'static>.
958 //
959 // It would not be legal to write this function without the invariant
960 // lifetime 'c in StepCursor<'c, 'a>. If this function were written only
961 // in terms of 'a, the user could take our ParseBuffer<'a>, upcast it to
962 // a ParseBuffer<'short> which some shorter lifetime than 'a, invoke
963 // `step` on their ParseBuffer<'short> with a closure that returns
964 // Cursor<'short>, and we would wrongly write that Cursor<'short> into
965 // the Cell intended to hold Cursor<'a>.
966 //
967 // In some cases it may be necessary for R to contain a Cursor<'a>.
968 // Within Syn we solve this using `advance_step_cursor` which uses the
969 // existence of a StepCursor<'c, 'a> as proof that it is safe to cast
970 // from Cursor<'c> to Cursor<'a>. If needed outside of Syn, it would be
971 // safe to expose that API as a method on StepCursor.
972 let (node, rest) = function(StepCursor {
973 scope: self.scope,
974 cursor: self.cell.get(),
975 marker: PhantomData,
976 })?;
977 self.cell.set(rest);
978 Ok(node)
979 }
980
981 /// Returns the `Span` of the next token in the parse stream, or
982 /// `Span::call_site()` if this parse stream has completely exhausted its
983 /// input `TokenStream`.
span(&self) -> Span984 pub fn span(&self) -> Span {
985 let cursor = self.cursor();
986 if cursor.eof() {
987 self.scope
988 } else {
989 crate::buffer::open_span_of_group(cursor)
990 }
991 }
992
993 /// Provides low-level access to the token representation underlying this
994 /// parse stream.
995 ///
996 /// Cursors are immutable so no operations you perform against the cursor
997 /// will affect the state of this parse stream.
cursor(&self) -> Cursor<'a>998 pub fn cursor(&self) -> Cursor<'a> {
999 self.cell.get()
1000 }
1001
check_unexpected(&self) -> Result<()>1002 fn check_unexpected(&self) -> Result<()> {
1003 match inner_unexpected(self).1 {
1004 Some(span) => Err(Error::new(span, "unexpected token")),
1005 None => Ok(()),
1006 }
1007 }
1008 }
1009
1010 impl<T: Parse> Parse for Box<T> {
parse(input: ParseStream) -> Result<Self>1011 fn parse(input: ParseStream) -> Result<Self> {
1012 input.parse().map(Box::new)
1013 }
1014 }
1015
1016 impl<T: Parse + Token> Parse for Option<T> {
parse(input: ParseStream) -> Result<Self>1017 fn parse(input: ParseStream) -> Result<Self> {
1018 if T::peek(input.cursor()) {
1019 Ok(Some(input.parse()?))
1020 } else {
1021 Ok(None)
1022 }
1023 }
1024 }
1025
1026 impl Parse for TokenStream {
parse(input: ParseStream) -> Result<Self>1027 fn parse(input: ParseStream) -> Result<Self> {
1028 input.step(|cursor| Ok((cursor.token_stream(), Cursor::empty())))
1029 }
1030 }
1031
1032 impl Parse for TokenTree {
parse(input: ParseStream) -> Result<Self>1033 fn parse(input: ParseStream) -> Result<Self> {
1034 input.step(|cursor| match cursor.token_tree() {
1035 Some((tt, rest)) => Ok((tt, rest)),
1036 None => Err(cursor.error("expected token tree")),
1037 })
1038 }
1039 }
1040
1041 impl Parse for Group {
parse(input: ParseStream) -> Result<Self>1042 fn parse(input: ParseStream) -> Result<Self> {
1043 input.step(|cursor| {
1044 for delim in &[Delimiter::Parenthesis, Delimiter::Brace, Delimiter::Bracket] {
1045 if let Some((inside, span, rest)) = cursor.group(*delim) {
1046 let mut group = Group::new(*delim, inside.token_stream());
1047 group.set_span(span);
1048 return Ok((group, rest));
1049 }
1050 }
1051 Err(cursor.error("expected group token"))
1052 })
1053 }
1054 }
1055
1056 impl Parse for Punct {
parse(input: ParseStream) -> Result<Self>1057 fn parse(input: ParseStream) -> Result<Self> {
1058 input.step(|cursor| match cursor.punct() {
1059 Some((punct, rest)) => Ok((punct, rest)),
1060 None => Err(cursor.error("expected punctuation token")),
1061 })
1062 }
1063 }
1064
1065 impl Parse for Literal {
parse(input: ParseStream) -> Result<Self>1066 fn parse(input: ParseStream) -> Result<Self> {
1067 input.step(|cursor| match cursor.literal() {
1068 Some((literal, rest)) => Ok((literal, rest)),
1069 None => Err(cursor.error("expected literal token")),
1070 })
1071 }
1072 }
1073
1074 /// Parser that can parse Rust tokens into a particular syntax tree node.
1075 ///
1076 /// Refer to the [module documentation] for details about parsing in Syn.
1077 ///
1078 /// [module documentation]: self
1079 ///
1080 /// *This trait is available if Syn is built with the `"parsing"` feature.*
1081 pub trait Parser: Sized {
1082 type Output;
1083
1084 /// Parse a proc-macro2 token stream into the chosen syntax tree node.
1085 ///
1086 /// This function will check that the input is fully parsed. If there are
1087 /// any unparsed tokens at the end of the stream, an error is returned.
parse2(self, tokens: TokenStream) -> Result<Self::Output>1088 fn parse2(self, tokens: TokenStream) -> Result<Self::Output>;
1089
1090 /// Parse tokens of source code into the chosen syntax tree node.
1091 ///
1092 /// This function will check that the input is fully parsed. If there are
1093 /// any unparsed tokens at the end of the stream, an error is returned.
1094 ///
1095 /// *This method is available if Syn is built with both the `"parsing"` and
1096 /// `"proc-macro"` features.*
1097 #[cfg(all(
1098 not(all(target_arch = "wasm32", any(target_os = "unknown", target_os = "wasi"))),
1099 feature = "proc-macro"
1100 ))]
parse(self, tokens: proc_macro::TokenStream) -> Result<Self::Output>1101 fn parse(self, tokens: proc_macro::TokenStream) -> Result<Self::Output> {
1102 self.parse2(proc_macro2::TokenStream::from(tokens))
1103 }
1104
1105 /// Parse a string of Rust code into the chosen syntax tree node.
1106 ///
1107 /// This function will check that the input is fully parsed. If there are
1108 /// any unparsed tokens at the end of the string, an error is returned.
1109 ///
1110 /// # Hygiene
1111 ///
1112 /// Every span in the resulting syntax tree will be set to resolve at the
1113 /// macro call site.
parse_str(self, s: &str) -> Result<Self::Output>1114 fn parse_str(self, s: &str) -> Result<Self::Output> {
1115 self.parse2(proc_macro2::TokenStream::from_str(s)?)
1116 }
1117
1118 // Not public API.
1119 #[doc(hidden)]
__parse_scoped(self, scope: Span, tokens: TokenStream) -> Result<Self::Output>1120 fn __parse_scoped(self, scope: Span, tokens: TokenStream) -> Result<Self::Output> {
1121 let _ = scope;
1122 self.parse2(tokens)
1123 }
1124
1125 // Not public API.
1126 #[doc(hidden)]
__parse_stream(self, input: ParseStream) -> Result<Self::Output>1127 fn __parse_stream(self, input: ParseStream) -> Result<Self::Output> {
1128 input.parse().and_then(|tokens| self.parse2(tokens))
1129 }
1130 }
1131
tokens_to_parse_buffer(tokens: &TokenBuffer) -> ParseBuffer1132 fn tokens_to_parse_buffer(tokens: &TokenBuffer) -> ParseBuffer {
1133 let scope = Span::call_site();
1134 let cursor = tokens.begin();
1135 let unexpected = Rc::new(Cell::new(Unexpected::None));
1136 new_parse_buffer(scope, cursor, unexpected)
1137 }
1138
1139 impl<F, T> Parser for F
1140 where
1141 F: FnOnce(ParseStream) -> Result<T>,
1142 {
1143 type Output = T;
1144
parse2(self, tokens: TokenStream) -> Result<T>1145 fn parse2(self, tokens: TokenStream) -> Result<T> {
1146 let buf = TokenBuffer::new2(tokens);
1147 let state = tokens_to_parse_buffer(&buf);
1148 let node = self(&state)?;
1149 state.check_unexpected()?;
1150 if state.is_empty() {
1151 Ok(node)
1152 } else {
1153 Err(state.error("unexpected token"))
1154 }
1155 }
1156
1157 #[doc(hidden)]
__parse_scoped(self, scope: Span, tokens: TokenStream) -> Result<Self::Output>1158 fn __parse_scoped(self, scope: Span, tokens: TokenStream) -> Result<Self::Output> {
1159 let buf = TokenBuffer::new2(tokens);
1160 let cursor = buf.begin();
1161 let unexpected = Rc::new(Cell::new(Unexpected::None));
1162 let state = new_parse_buffer(scope, cursor, unexpected);
1163 let node = self(&state)?;
1164 state.check_unexpected()?;
1165 if state.is_empty() {
1166 Ok(node)
1167 } else {
1168 Err(state.error("unexpected token"))
1169 }
1170 }
1171
1172 #[doc(hidden)]
__parse_stream(self, input: ParseStream) -> Result<Self::Output>1173 fn __parse_stream(self, input: ParseStream) -> Result<Self::Output> {
1174 self(input)
1175 }
1176 }
1177
parse_scoped<F: Parser>(f: F, scope: Span, tokens: TokenStream) -> Result<F::Output>1178 pub(crate) fn parse_scoped<F: Parser>(f: F, scope: Span, tokens: TokenStream) -> Result<F::Output> {
1179 f.__parse_scoped(scope, tokens)
1180 }
1181
parse_stream<F: Parser>(f: F, input: ParseStream) -> Result<F::Output>1182 pub(crate) fn parse_stream<F: Parser>(f: F, input: ParseStream) -> Result<F::Output> {
1183 f.__parse_stream(input)
1184 }
1185
1186 /// An empty syntax tree node that consumes no tokens when parsed.
1187 ///
1188 /// This is useful for attribute macros that want to ensure they are not
1189 /// provided any attribute args.
1190 ///
1191 /// ```
1192 /// extern crate proc_macro;
1193 ///
1194 /// use proc_macro::TokenStream;
1195 /// use syn::parse_macro_input;
1196 /// use syn::parse::Nothing;
1197 ///
1198 /// # const IGNORE: &str = stringify! {
1199 /// #[proc_macro_attribute]
1200 /// # };
1201 /// pub fn my_attr(args: TokenStream, input: TokenStream) -> TokenStream {
1202 /// parse_macro_input!(args as Nothing);
1203 ///
1204 /// /* ... */
1205 /// # "".parse().unwrap()
1206 /// }
1207 /// ```
1208 ///
1209 /// ```text
1210 /// error: unexpected token
1211 /// --> src/main.rs:3:19
1212 /// |
1213 /// 3 | #[my_attr(asdf)]
1214 /// | ^^^^
1215 /// ```
1216 pub struct Nothing;
1217
1218 impl Parse for Nothing {
parse(_input: ParseStream) -> Result<Self>1219 fn parse(_input: ParseStream) -> Result<Self> {
1220 Ok(Nothing)
1221 }
1222 }
1223