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// Copyright 2018 Syn Developers // // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your // option. This file may not be copied, modified, or distributed // except according to those terms. //! Parsing interface for parsing a token stream into a syntax tree node. //! //! Parsing in Syn is built on parser functions that take in a [`Cursor`] and //! produce a [`PResult<T>`] where `T` is some syntax tree node. `Cursor` is a //! cheaply copyable cursor over a range of tokens in a token stream, and //! `PResult` is a result that packages together a parsed syntax tree node `T` //! with a stream of remaining unparsed tokens after `T` represented as another //! `Cursor`, or a [`ParseError`] if parsing failed. //! //! [`Cursor`]: ../buffer/index.html //! [`PResult<T>`]: type.PResult.html //! [`ParseError`]: struct.ParseError.html //! //! This `Cursor`- and `PResult`-based interface is convenient for parser //! combinators and parser implementations, but not necessarily when you just //! have some tokens that you want to parse. For that we expose the following //! two entry points. //! //! ## The `syn::parse*` functions //! //! The [`syn::parse`], [`syn::parse2`], and [`syn::parse_str`] functions serve //! as an entry point for parsing syntax tree nodes that can be parsed in an //! obvious default way. These functions can return any syntax tree node that //! implements the [`Synom`] trait, which includes most types in Syn. //! //! [`syn::parse`]: ../fn.parse.html //! [`syn::parse2`]: ../fn.parse2.html //! [`syn::parse_str`]: ../fn.parse_str.html //! [`Synom`]: trait.Synom.html //! //! ``` //! use syn::Type; //! //! # fn run_parser() -> Result<(), syn::synom::ParseError> { //! let t: Type = syn::parse_str("std::collections::HashMap<String, Value>")?; //! # Ok(()) //! # } //! # //! # fn main() { //! # run_parser().unwrap(); //! # } //! ``` //! //! The [`parse_quote!`] macro also uses this approach. //! //! [`parse_quote!`]: ../macro.parse_quote.html //! //! ## The `Parser` trait //! //! Some types can be parsed in several ways depending on context. For example //! an [`Attribute`] can be either "outer" like `#[...]` or "inner" like //! `#![...]` and parsing the wrong one would be a bug. Similarly [`Punctuated`] //! may or may not allow trailing punctuation, and parsing it the wrong way //! would either reject valid input or accept invalid input. //! //! [`Attribute`]: ../struct.Attribute.html //! [`Punctuated`]: ../punctuated/index.html //! //! The `Synom` trait is not implemented in these cases because there is no good //! behavior to consider the default. //! //! ```ignore //! // Can't parse `Punctuated` without knowing whether trailing punctuation //! // should be allowed in this context. //! let path: Punctuated<PathSegment, Token![::]> = syn::parse(tokens)?; //! ``` //! //! In these cases the types provide a choice of parser functions rather than a //! single `Synom` implementation, and those parser functions can be invoked //! through the [`Parser`] trait. //! //! [`Parser`]: trait.Parser.html //! //! ``` //! # #[macro_use] //! # extern crate syn; //! # //! # extern crate proc_macro2; //! # use proc_macro2::TokenStream; //! # //! use syn::synom::Parser; //! use syn::punctuated::Punctuated; //! use syn::{PathSegment, Expr, Attribute}; //! //! # fn run_parsers() -> Result<(), syn::synom::ParseError> { //! # let tokens = TokenStream::new().into(); //! // Parse a nonempty sequence of path segments separated by `::` punctuation //! // with no trailing punctuation. //! let parser = Punctuated::<PathSegment, Token![::]>::parse_separated_nonempty; //! let path = parser.parse(tokens)?; //! //! # let tokens = TokenStream::new().into(); //! // Parse a possibly empty sequence of expressions terminated by commas with //! // an optional trailing punctuation. //! let parser = Punctuated::<Expr, Token![,]>::parse_terminated; //! let args = parser.parse(tokens)?; //! //! # let tokens = TokenStream::new().into(); //! // Parse zero or more outer attributes but not inner attributes. //! named!(outer_attrs -> Vec<Attribute>, many0!(Attribute::parse_outer)); //! let attrs = outer_attrs.parse(tokens)?; //! # //! # Ok(()) //! # } //! # //! # fn main() {} //! ``` //! //! # Implementing a parser function //! //! Parser functions are usually implemented using the [`nom`]-style parser //! combinator macros provided by Syn, but may also be implemented without //! macros be using the low-level [`Cursor`] API directly. //! //! [`nom`]: https://github.com/Geal/nom //! //! The following parser combinator macros are available and a `Synom` parsing //! example is provided for each one. //! //! - [`alt!`](../macro.alt.html) //! - [`braces!`](../macro.braces.html) //! - [`brackets!`](../macro.brackets.html) //! - [`call!`](../macro.call.html) //! - [`cond!`](../macro.cond.html) //! - [`cond_reduce!`](../macro.cond_reduce.html) //! - [`custom_keyword!`](../macro.custom_keyword.html) //! - [`do_parse!`](../macro.do_parse.html) //! - [`epsilon!`](../macro.epsilon.html) //! - [`input_end!`](../macro.input_end.html) //! - [`keyword!`](../macro.keyword.html) //! - [`many0!`](../macro.many0.html) //! - [`map!`](../macro.map.html) //! - [`not!`](../macro.not.html) //! - [`option!`](../macro.option.html) //! - [`parens!`](../macro.parens.html) //! - [`punct!`](../macro.punct.html) //! - [`reject!`](../macro.reject.html) //! - [`switch!`](../macro.switch.html) //! - [`syn!`](../macro.syn.html) //! - [`tuple!`](../macro.tuple.html) //! - [`value!`](../macro.value.html) //! //! *This module is available if Syn is built with the `"parsing"` feature.* #[cfg(all( not(all(target_arch = "wasm32", target_os = "unknown")), feature = "proc-macro" ))] use proc_macro; use proc_macro2::{Delimiter, Group, Ident, Literal, Punct, TokenStream, TokenTree}; use error::parse_error; pub use error::{PResult, ParseError}; use buffer::{Cursor, TokenBuffer}; /// Parsing interface implemented by all types that can be parsed in a default /// way from a token stream. /// /// Refer to the [module documentation] for details about parsing in Syn. /// /// [module documentation]: index.html /// /// *This trait is available if Syn is built with the `"parsing"` feature.* pub trait Synom: Sized { fn parse(input: Cursor) -> PResult<Self>; /// A short name of the type being parsed. /// /// The description should only be used for a simple name. It should not /// contain newlines or sentence-ending punctuation, to facilitate embedding in /// larger user-facing strings. Syn will use this description when building /// error messages about parse failures. /// /// # Examples /// /// ``` /// # use syn::buffer::Cursor; /// # use syn::synom::{Synom, PResult}; /// # /// struct ExprMacro { /// // ... /// } /// /// impl Synom for ExprMacro { /// # fn parse(input: Cursor) -> PResult<Self> { unimplemented!() } /// // fn parse(...) -> ... { ... } /// /// fn description() -> Option<&'static str> { /// // Will result in messages like /// // /// // "failed to parse macro invocation expression: $reason" /// Some("macro invocation expression") /// } /// } /// ``` fn description() -> Option<&'static str> { None } } impl Synom for TokenStream { fn parse(input: Cursor) -> PResult<Self> { Ok((input.token_stream(), Cursor::empty())) } fn description() -> Option<&'static str> { Some("arbitrary token stream") } } impl Synom for TokenTree { fn parse(input: Cursor) -> PResult<Self> { match input.token_tree() { Some((tt, rest)) => Ok((tt, rest)), None => parse_error(), } } fn description() -> Option<&'static str> { Some("token tree") } } impl Synom for Group { fn parse(input: Cursor) -> PResult<Self> { for delim in &[Delimiter::Parenthesis, Delimiter::Brace, Delimiter::Bracket] { if let Some((inside, span, rest)) = input.group(*delim) { let mut group = Group::new(*delim, inside.token_stream()); group.set_span(span); return Ok((group, rest)); } } parse_error() } fn description() -> Option<&'static str> { Some("group token") } } impl Synom for Ident { fn parse(input: Cursor) -> PResult<Self> { let (ident, rest) = match input.ident() { Some(ident) => ident, _ => return parse_error(), }; match &ident.to_string()[..] { "_" // Based on https://doc.rust-lang.org/grammar.html#keywords // and https://github.com/rust-lang/rfcs/blob/master/text/2421-unreservations-2018.md | "abstract" | "as" | "become" | "box" | "break" | "const" | "continue" | "crate" | "do" | "else" | "enum" | "extern" | "false" | "final" | "fn" | "for" | "if" | "impl" | "in" | "let" | "loop" | "macro" | "match" | "mod" | "move" | "mut" | "override" | "priv" | "proc" | "pub" | "ref" | "return" | "Self" | "self" | "static" | "struct" | "super" | "trait" | "true" | "type" | "typeof" | "unsafe" | "unsized" | "use" | "virtual" | "where" | "while" | "yield" => return parse_error(), _ => {} } Ok((ident, rest)) } fn description() -> Option<&'static str> { Some("identifier") } } impl Synom for Punct { fn parse(input: Cursor) -> PResult<Self> { match input.punct() { Some((punct, rest)) => Ok((punct, rest)), None => parse_error(), } } fn description() -> Option<&'static str> { Some("punctuation token") } } impl Synom for Literal { fn parse(input: Cursor) -> PResult<Self> { match input.literal() { Some((literal, rest)) => Ok((literal, rest)), None => parse_error(), } } fn description() -> Option<&'static str> { Some("literal token") } } /// Parser that can parse Rust tokens into a particular syntax tree node. /// /// Refer to the [module documentation] for details about parsing in Syn. /// /// [module documentation]: index.html /// /// *This trait is available if Syn is built with the `"parsing"` feature.* pub trait Parser: Sized { type Output; /// Parse a proc-macro2 token stream into the chosen syntax tree node. fn parse2(self, tokens: TokenStream) -> Result<Self::Output, ParseError>; /// Parse tokens of source code into the chosen syntax tree node. /// /// *This method is available if Syn is built with both the `"parsing"` and /// `"proc-macro"` features.* #[cfg(all( not(all(target_arch = "wasm32", target_os = "unknown")), feature = "proc-macro" ))] fn parse(self, tokens: proc_macro::TokenStream) -> Result<Self::Output, ParseError> { self.parse2(tokens.into()) } /// Parse a string of Rust code into the chosen syntax tree node. /// /// # Hygiene /// /// Every span in the resulting syntax tree will be set to resolve at the /// macro call site. fn parse_str(self, s: &str) -> Result<Self::Output, ParseError> { match s.parse() { Ok(tts) => self.parse2(tts), Err(_) => Err(ParseError::new("error while lexing input string")), } } } impl<F, T> Parser for F where F: FnOnce(Cursor) -> PResult<T>, { type Output = T; fn parse2(self, tokens: TokenStream) -> Result<T, ParseError> { let buf = TokenBuffer::new2(tokens); let (t, rest) = self(buf.begin())?; if rest.eof() { Ok(t) } else if rest == buf.begin() { // parsed nothing Err(ParseError::new("failed to parse anything")) } else { Err(ParseError::new("failed to parse all tokens")) } } } /// Extension traits that are made available within the `call!` parser. /// /// *This module is available if Syn is built with the `"parsing"` feature.* pub mod ext { use super::*; use proc_macro2::Ident; /// Additional parsing methods for `Ident`. /// /// This trait is sealed and cannot be implemented for types outside of Syn. /// /// *This trait is available if Syn is built with the `"parsing"` feature.* pub trait IdentExt: Sized + private::Sealed { /// Parses any identifier including keywords. /// /// This is useful when parsing a DSL which allows Rust keywords as /// identifiers. /// /// ```rust /// #[macro_use] /// extern crate syn; /// /// use syn::Ident; /// /// // Parses input that looks like `name = NAME` where `NAME` can be /// // any identifier. /// // /// // Examples: /// // /// // name = anything /// // name = impl /// named!(parse_dsl -> Ident, do_parse!( /// custom_keyword!(name) >> /// punct!(=) >> /// name: call!(Ident::parse_any) >> /// (name) /// )); /// # /// # fn main() {} /// ``` fn parse_any(input: Cursor) -> PResult<Self>; } impl IdentExt for Ident { fn parse_any(input: Cursor) -> PResult<Self> { input.ident().map_or_else(parse_error, Ok) } } mod private { use proc_macro2::Ident; pub trait Sealed {} impl Sealed for Ident {} } }