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//! Traits for parsing the WebAssembly Text format
//!
//! This module contains the traits, abstractions, and utilities needed to
//! define custom parsers for WebAssembly text format items. This module exposes
//! a recursive descent parsing strategy and centers around the
//! [`Parse`](crate::parser::Parse) trait for defining new fragments of
//! WebAssembly text syntax.
//!
//! The top-level [`parse`](crate::parser::parse) function can be used to fully parse AST fragments:
//!
//! ```
//! use wast::Wat;
//! use wast::parser::{self, ParseBuffer};
//!
//! # fn foo() -> Result<(), wast::Error> {
//! let wat = "(module (func))";
//! let buf = ParseBuffer::new(wat)?;
//! let module = parser::parse::<Wat>(&buf)?;
//! # Ok(())
//! # }
//! ```
//!
//! and you can also define your own new syntax with the
//! [`Parse`](crate::parser::Parse) trait:
//!
//! ```
//! use wast::{kw, Import, Func};
//! use wast::parser::{Parser, Parse, Result};
//!
//! // Fields of a WebAssembly which only allow imports and functions, and all
//! // imports must come before all the functions
//! struct OnlyImportsAndFunctions<'a> {
//! imports: Vec<Import<'a>>,
//! functions: Vec<Func<'a>>,
//! }
//!
//! impl<'a> Parse<'a> for OnlyImportsAndFunctions<'a> {
//! fn parse(parser: Parser<'a>) -> Result<Self> {
//! // While the second token is `import` (the first is `(`, so we care
//! // about the second) we parse an `ast::ModuleImport` inside of
//! // parentheses. The `parens` function here ensures that what we
//! // parse inside of it is surrounded by `(` and `)`.
//! let mut imports = Vec::new();
//! while parser.peek2::<kw::import>() {
//! let import = parser.parens(|p| p.parse())?;
//! imports.push(import);
//! }
//!
//! // Afterwards we assume everything else is a function. Note that
//! // `parse` here is a generic function and type inference figures out
//! // that we're parsing functions here and imports above.
//! let mut functions = Vec::new();
//! while !parser.is_empty() {
//! let func = parser.parens(|p| p.parse())?;
//! functions.push(func);
//! }
//!
//! Ok(OnlyImportsAndFunctions { imports, functions })
//! }
//! }
//! ```
//!
//! This module is heavily inspired by [`syn`](https://docs.rs/syn) so you can
//! likely also draw inspiration from the excellent examples in the `syn` crate.
use crate::lexer::{Float, Integer, Lexer, Token};
use crate::{Error, Span};
use std::cell::{Cell, RefCell};
use std::collections::HashMap;
use std::fmt;
use std::usize;
/// A top-level convenience parseing function that parss a `T` from `buf` and
/// requires that all tokens in `buf` are consume.
///
/// This generic parsing function can be used to parse any `T` implementing the
/// [`Parse`] trait. It is not used from [`Parse`] trait implementations.
///
/// # Examples
///
/// ```
/// use wast::Wat;
/// use wast::parser::{self, ParseBuffer};
///
/// # fn foo() -> Result<(), wast::Error> {
/// let wat = "(module (func))";
/// let buf = ParseBuffer::new(wat)?;
/// let module = parser::parse::<Wat>(&buf)?;
/// # Ok(())
/// # }
/// ```
///
/// or parsing simply a fragment
///
/// ```
/// use wast::parser::{self, ParseBuffer};
///
/// # fn foo() -> Result<(), wast::Error> {
/// let wat = "12";
/// let buf = ParseBuffer::new(wat)?;
/// let val = parser::parse::<u32>(&buf)?;
/// assert_eq!(val, 12);
/// # Ok(())
/// # }
/// ```
pub fn parse<'a, T: Parse<'a>>(buf: &'a ParseBuffer<'a>) -> Result<T> {
let parser = buf.parser();
let result = parser.parse()?;
if parser.cursor().advance_token().is_none() {
Ok(result)
} else {
Err(parser.error("extra tokens remaining after parse"))
}
}
/// A trait for parsing a fragment of syntax in a recursive descent fashion.
///
/// The [`Parse`] trait is main abstraction you'll be working with when defining
/// custom parser or custom syntax for your WebAssembly text format (or when
/// using the official format items). Almost all items in the
/// [`ast`](crate::ast) module implement the [`Parse`] trait, and you'll
/// commonly use this with:
///
/// * The top-level [`parse`] function to parse an entire input.
/// * The intermediate [`Parser::parse`] function to parse an item out of an
/// input stream and then parse remaining items.
///
/// Implementation of [`Parse`] take a [`Parser`] as input and will mutate the
/// parser as they parse syntax. Once a token is consume it cannot be
/// "un-consumed". Utilities such as [`Parser::peek`] and [`Parser::lookahead1`]
/// can be used to determine what to parse next.
///
/// ## When to parse `(` and `)`?
///
/// Conventionally types are not responsible for parsing their own `(` and `)`
/// tokens which surround the type. For example WebAssembly imports look like:
///
/// ```text
/// (import "foo" "bar" (func (type 0)))
/// ```
///
/// but the [`Import`](crate::ast::Import) type parser looks like:
///
/// ```
/// # use wast::kw;
/// # use wast::parser::{Parser, Parse, Result};
/// # struct Import<'a>(&'a str);
/// impl<'a> Parse<'a> for Import<'a> {
/// fn parse(parser: Parser<'a>) -> Result<Self> {
/// parser.parse::<kw::import>()?;
/// // ...
/// # panic!()
/// }
/// }
/// ```
///
/// It is assumed here that the `(` and `)` tokens which surround an `import`
/// statement in the WebAssembly text format are parsed by the parent item
/// parsing `Import`.
///
/// Note that this is just a convention, so it's not necessarily required for
/// all types. It's recommended that your types stick to this convention where
/// possible to avoid nested calls to [`Parser::parens`] or accidentally trying
/// to parse too many parenthesis.
///
/// # Examples
///
/// Let's say you want to define your own WebAssembly text format which only
/// contains imports and functions. You also require all imports to be listed
/// before all functions. An example [`Parse`] implementation might look like:
///
/// ```
/// use wast::{Import, Func, kw};
/// use wast::parser::{Parser, Parse, Result};
///
/// // Fields of a WebAssembly which only allow imports and functions, and all
/// // imports must come before all the functions
/// struct OnlyImportsAndFunctions<'a> {
/// imports: Vec<Import<'a>>,
/// functions: Vec<Func<'a>>,
/// }
///
/// impl<'a> Parse<'a> for OnlyImportsAndFunctions<'a> {
/// fn parse(parser: Parser<'a>) -> Result<Self> {
/// // While the second token is `import` (the first is `(`, so we care
/// // about the second) we parse an `ast::ModuleImport` inside of
/// // parentheses. The `parens` function here ensures that what we
/// // parse inside of it is surrounded by `(` and `)`.
/// let mut imports = Vec::new();
/// while parser.peek2::<kw::import>() {
/// let import = parser.parens(|p| p.parse())?;
/// imports.push(import);
/// }
///
/// // Afterwards we assume everything else is a function. Note that
/// // `parse` here is a generic function and type inference figures out
/// // that we're parsing functions here and imports above.
/// let mut functions = Vec::new();
/// while !parser.is_empty() {
/// let func = parser.parens(|p| p.parse())?;
/// functions.push(func);
/// }
///
/// Ok(OnlyImportsAndFunctions { imports, functions })
/// }
/// }
/// ```
pub trait Parse<'a>: Sized {
/// Attempts to parse `Self` from `parser`, returning an error if it could
/// not be parsed.
///
/// This method will mutate the state of `parser` after attempting to parse
/// an instance of `Self`. If an error happens then it is likely fatal and
/// there is no guarantee of how many tokens have been consumed from
/// `parser`.
///
/// As recommended in the documentation of [`Parse`], implementations of
/// this function should not start out by parsing `(` and `)` tokens, but
/// rather parents calling recursive parsers should parse the `(` and `)`
/// tokens for their child item that's being parsed.
///
/// # Errors
///
/// This function will return an error if `Self` could not be parsed. Note
/// that creating an [`Error`] is not exactly a cheap operation, so
/// [`Error`] is typically fatal and propagated all the way back to the top
/// parse call site.
fn parse(parser: Parser<'a>) -> Result<Self>;
}
/// A trait for types which be used to "peek" to see if they're the next token
/// in an input stream of [`Parser`].
///
/// Often when implementing [`Parse`] you'll need to query what the next token
/// in the stream is to figure out what to parse next. This [`Peek`] trait
/// defines the set of types that can be tested whether they're the next token
/// in the input stream.
///
/// Implementations of [`Peek`] should only be present on types that consume
/// exactly one token (not zero, not more, exactly one). Types implementing
/// [`Peek`] should also typically implement [`Parse`] should also typically
/// implement [`Parse`].
///
/// See the documentation of [`Parser::peek`] for example usage.
pub trait Peek {
/// Tests to see whether this token is the first token within the [`Cursor`]
/// specified.
///
/// Returns `true` if [`Parse`] for this type is highly likely to succeed
/// failing no other error conditions happening (like an integer literal
/// being too big).
fn peek(cursor: Cursor<'_>) -> bool;
/// The same as `peek`, except it checks the token immediately following
/// the current token.
fn peek2(mut cursor: Cursor<'_>) -> bool {
if cursor.advance_token().is_some() {
Self::peek(cursor)
} else {
false
}
}
/// Returns a human-readable name of this token to display when generating
/// errors about this token missing.
fn display() -> &'static str;
}
/// A convenience type definition for `Result` where the error is hardwired to
/// [`Error`].
pub type Result<T> = std::result::Result<T, Error>;
/// A low-level buffer of tokens which represents a completely lexed file.
///
/// A `ParseBuffer` will immediately lex an entire file and then store all
/// tokens internally. A `ParseBuffer` only used to pass to the top-level
/// [`parse`] function.
pub struct ParseBuffer<'a> {
// list of tokens from the tokenized source (including whitespace and
// comments), and the second element is how to skip this token, if it can be
// skipped.
tokens: Box<[(Token<'a>, Cell<NextTokenAt>)]>,
input: &'a str,
cur: Cell<usize>,
known_annotations: RefCell<HashMap<String, usize>>,
depth: Cell<usize>,
}
#[derive(Copy, Clone, Debug)]
enum NextTokenAt {
/// Haven't computed where the next token is yet.
Unknown,
/// Previously computed the index of the next token.
Index(usize),
/// There is no next token, this is the last token.
Eof,
}
/// An in-progress parser for the tokens of a WebAssembly text file.
///
/// A `Parser` is argument to the [`Parse`] trait and is now the input stream is
/// interacted with to parse new items. Cloning [`Parser`] or copying a parser
/// refers to the same stream of tokens to parse, you cannot clone a [`Parser`]
/// and clone two items.
///
/// For more information about a [`Parser`] see its methods.
#[derive(Copy, Clone)]
pub struct Parser<'a> {
buf: &'a ParseBuffer<'a>,
}
/// A helpful structure to perform a lookahead of one token to determine what to
/// parse.
///
/// For more information see the [`Parser::lookahead1`] method.
pub struct Lookahead1<'a> {
parser: Parser<'a>,
attempts: Vec<&'static str>,
}
/// An immutable cursor into a list of tokens.
///
/// This cursor cannot be mutated but can be used to parse more tokens in a list
/// of tokens. Cursors are created from the [`Parser::step`] method. This is a
/// very low-level parsing structure and you likely won't use it much.
#[derive(Copy, Clone)]
pub struct Cursor<'a> {
parser: Parser<'a>,
cur: usize,
}
impl ParseBuffer<'_> {
/// Creates a new [`ParseBuffer`] by lexing the given `input` completely.
///
/// # Errors
///
/// Returns an error if `input` fails to lex.
pub fn new(input: &str) -> Result<ParseBuffer<'_>> {
ParseBuffer::new_with_lexer(Lexer::new(input))
}
/// Creates a new [`ParseBuffer`] by lexing the given `input` completely.
///
/// # Errors
///
/// Returns an error if `input` fails to lex.
pub fn new_with_lexer(lexer: Lexer<'_>) -> Result<ParseBuffer<'_>> {
let mut tokens = Vec::new();
let input = lexer.input();
for token in lexer {
tokens.push((token?, Cell::new(NextTokenAt::Unknown)));
}
let ret = ParseBuffer {
tokens: tokens.into_boxed_slice(),
cur: Cell::new(0),
depth: Cell::new(0),
input,
known_annotations: Default::default(),
};
ret.validate_annotations()?;
Ok(ret)
}
fn parser(&self) -> Parser<'_> {
Parser { buf: self }
}
// Validates that all annotations properly parse in that they have balanced
// delimiters. This is required since while parsing we generally skip
// annotations and there's no real opportunity to return a parse error.
fn validate_annotations(&self) -> Result<()> {
use crate::lexer::Token::*;
enum State {
None,
LParen,
Annotation { depth: usize, span: Span },
}
let mut state = State::None;
for token in self.tokens.iter() {
state = match (&token.0, state) {
// From nothing, a `(` starts the search for an annotation
(LParen(_), State::None) => State::LParen,
// ... otherwise in nothing we alwyas preserve that state.
(_, State::None) => State::None,
// If the previous state was an `LParen`, we may have an
// annotation if the next keyword is reserved
(Reserved(s), State::LParen) if s.starts_with("@") && s.len() > 0 => {
let offset = self.input_pos(s);
State::Annotation {
span: Span { offset },
depth: 1,
}
}
// ... otherwise anything after an `LParen` kills the lparen
// state.
(_, State::LParen) => State::None,
// Once we're in an annotation we need to balance parentheses,
// so handle the depth changes.
(LParen(_), State::Annotation { span, depth }) => State::Annotation {
span,
depth: depth + 1,
},
(RParen(_), State::Annotation { depth: 1, .. }) => State::None,
(RParen(_), State::Annotation { span, depth }) => State::Annotation {
span,
depth: depth - 1,
},
// ... and otherwise all tokens are allowed in annotations.
(_, s @ State::Annotation { .. }) => s,
};
}
if let State::Annotation { span, .. } = state {
return Err(Error::new(span, format!("unclosed annotation")));
}
Ok(())
}
fn input_pos(&self, src: &str) -> usize {
src.as_ptr() as usize - self.input.as_ptr() as usize
}
}
impl<'a> Parser<'a> {
/// Returns whether there are no more `Token` tokens to parse from this
/// [`Parser`].
///
/// This indicates that either we've reached the end of the input, or we're
/// a sub-[`Parser`] inside of a parenthesized expression and we've hit the
/// `)` token.
///
/// Note that if `false` is returned there *may* be more comments. Comments
/// and whitespace are not considered for whether this parser is empty.
pub fn is_empty(self) -> bool {
match self.cursor().advance_token() {
Some(Token::RParen(_)) | None => true,
Some(_) => false, // more tokens to parse!
}
}
pub(crate) fn has_meaningful_tokens(self) -> bool {
self.buf.tokens[self.cursor().cur..]
.iter()
.any(|(t, _)| match t {
Token::Whitespace(_) | Token::LineComment(_) | Token::BlockComment(_) => false,
_ => true,
})
}
/// Parses a `T` from this [`Parser`].
///
/// This method has a trivial definition (it simply calls
/// [`T::parse`](Parse::parse)) but is here for syntactic purposes. This is
/// what you'll call 99% of the time in a [`Parse`] implementation in order
/// to parse sub-items.
///
/// Typically you always want to use `?` with the result of this method, you
/// should not handle errors and decide what else to parse. To handle
/// branches in parsing, use [`Parser::peek`].
///
/// # Examples
///
/// A good example of using `parse` is to see how the [`TableType`] type is
/// parsed in this crate. A [`TableType`] is defined in the official
/// specification as [`tabletype`][spec] and is defined as:
///
/// [spec]: https://webassembly.github.io/spec/core/text/types.html#table-types
///
/// ```text
/// tabletype ::= lim:limits et:reftype
/// ```
///
/// so to parse a [`TableType`] we recursively need to parse a [`Limits`]
/// and a [`RefType`]
///
/// ```
/// # use wast::*;
/// # use wast::parser::*;
/// struct TableType<'a> {
/// limits: Limits,
/// elem: RefType<'a>,
/// }
///
/// impl<'a> Parse<'a> for TableType<'a> {
/// fn parse(parser: Parser<'a>) -> Result<Self> {
/// // parse the `lim` then `et` in sequence
/// Ok(TableType {
/// limits: parser.parse()?,
/// elem: parser.parse()?,
/// })
/// }
/// }
/// ```
///
/// [`Limits`]: crate::ast::Limits
/// [`TableType`]: crate::ast::TableType
/// [`RefType`]: crate::ast::RefType
pub fn parse<T: Parse<'a>>(self) -> Result<T> {
T::parse(self)
}
/// Performs a cheap test to see whether the current token in this stream is
/// `T`.
///
/// This method can be used to efficiently determine what next to parse. The
/// [`Peek`] trait is defined for types which can be used to test if they're
/// the next item in the input stream.
///
/// Nothing is actually parsed in this method, nor does this mutate the
/// state of this [`Parser`]. Instead, this simply performs a check.
///
/// This method is frequently combined with the [`Parser::lookahead1`]
/// method to automatically produce nice error messages if some tokens
/// aren't found.
///
/// # Examples
///
/// For an example of using the `peek` method let's take a look at parsing
/// the [`Limits`] type. This is [defined in the official spec][spec] as:
///
/// ```text
/// limits ::= n:u32
/// | n:u32 m:u32
/// ```
///
/// which means that it's either one `u32` token or two, so we need to know
/// whether to consume two tokens or one:
///
/// ```
/// # use wast::parser::*;
/// struct Limits {
/// min: u32,
/// max: Option<u32>,
/// }
///
/// impl<'a> Parse<'a> for Limits {
/// fn parse(parser: Parser<'a>) -> Result<Self> {
/// // Always parse the first number...
/// let min = parser.parse()?;
///
/// // ... and then test if there's a second number before parsing
/// let max = if parser.peek::<u32>() {
/// Some(parser.parse()?)
/// } else {
/// None
/// };
///
/// Ok(Limits { min, max })
/// }
/// }
/// ```
///
/// [spec]: https://webassembly.github.io/spec/core/text/types.html#limits
/// [`Limits`]: crate::ast::Limits
pub fn peek<T: Peek>(self) -> bool {
T::peek(self.cursor())
}
/// Same as the [`Parser::peek`] method, except checks the next token, not
/// the current token.
pub fn peek2<T: Peek>(self) -> bool {
let mut cursor = self.cursor();
if cursor.advance_token().is_some() {
T::peek(cursor)
} else {
false
}
}
/// A helper structure to perform a sequence of `peek` operations and if
/// they all fail produce a nice error message.
///
/// This method purely exists for conveniently producing error messages and
/// provides no functionality that [`Parser::peek`] doesn't already give.
/// The [`Lookahead1`] structure has one main method [`Lookahead1::peek`],
/// which is the same method as [`Parser::peek`]. The difference is that the
/// [`Lookahead1::error`] method needs no arguments.
///
/// # Examples
///
/// Let's look at the parsing of [`Index`]. This type is either a `u32` or
/// an [`Id`] and is used in name resolution primarily. The [official
/// grammar for an index][spec] is:
///
/// ```text
/// idx ::= x:u32
/// | v:id
/// ```
///
/// Which is to say that an index is either a `u32` or an [`Id`]. When
/// parsing an [`Index`] we can do:
///
/// ```
/// # use wast::*;
/// # use wast::parser::*;
/// enum Index<'a> {
/// Num(u32),
/// Id(Id<'a>),
/// }
///
/// impl<'a> Parse<'a> for Index<'a> {
/// fn parse(parser: Parser<'a>) -> Result<Self> {
/// let mut l = parser.lookahead1();
/// if l.peek::<Id>() {
/// Ok(Index::Id(parser.parse()?))
/// } else if l.peek::<u32>() {
/// Ok(Index::Num(parser.parse()?))
/// } else {
/// // produces error message of `expected identifier or u32`
/// Err(l.error())
/// }
/// }
/// }
/// ```
///
/// [spec]: https://webassembly.github.io/spec/core/text/modules.html#indices
/// [`Index`]: crate::ast::Index
/// [`Id`]: crate::ast::Id
pub fn lookahead1(self) -> Lookahead1<'a> {
Lookahead1 {
attempts: Vec::new(),
parser: self,
}
}
/// Parse an item surrounded by parentheses.
///
/// WebAssembly's text format is all based on s-expressions, so naturally
/// you're going to want to parse a lot of parenthesized things! As noted in
/// the documentation of [`Parse`] you typically don't parse your own
/// surrounding `(` and `)` tokens, but the parser above you parsed them for
/// you. This is method method the parser above you uses.
///
/// This method will parse a `(` token, and then call `f` on a sub-parser
/// which when finished asserts that a `)` token is the next token. This
/// requires that `f` consumes all tokens leading up to the paired `)`.
///
/// Usage will often simply be `parser.parens(|p| p.parse())?` to
/// automatically parse a type within parentheses, but you can, as always,
/// go crazy and do whatever you'd like too.
///
/// # Examples
///
/// A good example of this is to see how a `Module` is parsed. This isn't
/// the exact definition, but it's close enough!
///
/// ```
/// # use wast::*;
/// # use wast::parser::*;
/// struct Module<'a> {
/// fields: Vec<ModuleField<'a>>,
/// }
///
/// impl<'a> Parse<'a> for Module<'a> {
/// fn parse(parser: Parser<'a>) -> Result<Self> {
/// // Modules start out with a `module` keyword
/// parser.parse::<kw::module>()?;
///
/// // And then everything else is `(field ...)`, so while we've got
/// // items left we continuously parse parenthesized items.
/// let mut fields = Vec::new();
/// while !parser.is_empty() {
/// fields.push(parser.parens(|p| p.parse())?);
/// }
/// Ok(Module { fields })
/// }
/// }
/// ```
pub fn parens<T>(self, f: impl FnOnce(Parser<'a>) -> Result<T>) -> Result<T> {
self.buf.depth.set(self.buf.depth.get() + 1);
let before = self.buf.cur.get();
let res = self.step(|cursor| {
let mut cursor = match cursor.lparen() {
Some(rest) => rest,
None => return Err(cursor.error("expected `(`")),
};
cursor.parser.buf.cur.set(cursor.cur);
let result = f(cursor.parser)?;
cursor.cur = cursor.parser.buf.cur.get();
match cursor.rparen() {
Some(rest) => Ok((result, rest)),
None => Err(cursor.error("expected `)`")),
}
});
self.buf.depth.set(self.buf.depth.get() - 1);
if res.is_err() {
self.buf.cur.set(before);
}
return res;
}
/// Return the depth of nested parens we've parsed so far.
///
/// This is a low-level method that is only useful for implementing
/// recursion limits in custom parsers.
pub fn parens_depth(&self) -> usize {
self.buf.depth.get()
}
fn cursor(self) -> Cursor<'a> {
Cursor {
parser: self,
cur: self.buf.cur.get(),
}
}
/// A low-level parsing method you probably won't use.
///
/// This is used to implement parsing of the most primitive types in the
/// [`ast`](crate::ast) module. You probably don't want to use this, but
/// probably want to use something like [`Parser::parse`] or
/// [`Parser::parens`].
pub fn step<F, T>(self, f: F) -> Result<T>
where
F: FnOnce(Cursor<'a>) -> Result<(T, Cursor<'a>)>,
{
let (result, cursor) = f(self.cursor())?;
self.buf.cur.set(cursor.cur);
Ok(result)
}
/// Creates an error whose line/column information is pointing at the
/// current token.
///
/// This is used to produce human-readable error messages which point to the
/// right location in the input stream, and the `msg` here is arbitrary text
/// used to associate with the error and indicate why it was generated.
pub fn error(self, msg: impl fmt::Display) -> Error {
self.error_at(self.cursor().cur_span(), &msg)
}
fn error_at(self, span: Span, msg: &dyn fmt::Display) -> Error {
Error::parse(span, self.buf.input, msg.to_string())
}
/// Returns the span of the current token
pub fn cur_span(&self) -> Span {
self.cursor().cur_span()
}
/// Returns the span of the previous token
pub fn prev_span(&self) -> Span {
self.cursor().prev_span().unwrap_or(Span::from_offset(0))
}
/// Registers a new known annotation with this parser to allow parsing
/// annotations with this name.
///
/// [WebAssembly annotations][annotation] are a proposal for the text format
/// which allows decorating the text format with custom structured
/// information. By default all annotations are ignored when parsing, but
/// the whole purpose of them is to sometimes parse them!
///
/// To support parsing text annotations this method is used to allow
/// annotations and their tokens to *not* be skipped. Once an annotation is
/// registered with this method, then while the return value has not been
/// dropped (e.g. the scope of where this function is called) annotations
/// with the name `annotation` will be parse of the token stream and not
/// implicitly skipped.
///
/// # Skipping annotations
///
/// The behavior of skipping unknown/unregistered annotations can be
/// somewhat subtle and surprising, so if you're interested in parsing
/// annotations it's important to point out the importance of this method
/// and where to call it.
///
/// Generally when parsing tokens you'll be bottoming out in various
/// `Cursor` methods. These are all documented as advancing the stream as
/// much as possible to the next token, skipping "irrelevant stuff" like
/// comments, whitespace, etc. The `Cursor` methods will also skip unknown
/// annotations. This means that if you parse *any* token, it will skip over
/// any number of annotations that are unknown at all times.
///
/// To parse an annotation you must, before parsing any token of the
/// annotation, register the annotation via this method. This includes the
/// beginning `(` token, which is otherwise skipped if the annotation isn't
/// marked as registered. Typically parser parse the *contents* of an
/// s-expression, so this means that the outer parser of an s-expression
/// must register the custom annotation name, rather than the inner parser.
///
/// # Return
///
/// This function returns an RAII guard which, when dropped, will unregister
/// the `annotation` given. Parsing `annotation` is only supported while the
/// returned value is still alive, and once dropped the parser will go back
/// to skipping annotations with the name `annotation`.
///
/// # Example
///
/// Let's see an example of how the `@name` annotation is parsed for modules
/// to get an idea of how this works:
///
/// ```
/// # use wast::*;
/// # use wast::parser::*;
/// struct Module<'a> {
/// name: Option<NameAnnotation<'a>>,
/// }
///
/// impl<'a> Parse<'a> for Module<'a> {
/// fn parse(parser: Parser<'a>) -> Result<Self> {
/// // Modules start out with a `module` keyword
/// parser.parse::<kw::module>()?;
///
/// // Next may be `(@name "foo")`. Typically this annotation would
/// // skipped, but we don't want it skipped, so we register it.
/// // Note that the parse implementation of
/// // `Option<NameAnnotation>` is the one that consumes the
/// // parentheses here.
/// let _r = parser.register_annotation("name");
/// let name = parser.parse()?;
///
/// // ... and normally you'd otherwise parse module fields here ...
///
/// Ok(Module { name })
/// }
/// }
/// ```
///
/// Another example is how we parse the `@custom` annotation. Note that this
/// is parsed as part of `ModuleField`, so note how the annotation is
/// registered *before* we parse the parentheses of the annotation.
///
/// ```
/// # use wast::*;
/// # use wast::parser::*;
/// struct Module<'a> {
/// fields: Vec<ModuleField<'a>>,
/// }
///
/// impl<'a> Parse<'a> for Module<'a> {
/// fn parse(parser: Parser<'a>) -> Result<Self> {
/// // Modules start out with a `module` keyword
/// parser.parse::<kw::module>()?;
///
/// // register the `@custom` annotation *first* before we start
/// // parsing fields, because each field is contained in
/// // parentheses and to parse the parentheses of an annotation we
/// // have to known to not skip it.
/// let _r = parser.register_annotation("custom");
///
/// let mut fields = Vec::new();
/// while !parser.is_empty() {
/// fields.push(parser.parens(|p| p.parse())?);
/// }
/// Ok(Module { fields })
/// }
/// }
///
/// enum ModuleField<'a> {
/// Custom(Custom<'a>),
/// // ...
/// }
///
/// impl<'a> Parse<'a> for ModuleField<'a> {
/// fn parse(parser: Parser<'a>) -> Result<Self> {
/// // Note that because we have previously registered the `@custom`
/// // annotation with the parser we known that `peek` methods like
/// // this, working on the annotation token, are enabled to ever
/// // return `true`.
/// if parser.peek::<annotation::custom>() {
/// return Ok(ModuleField::Custom(parser.parse()?));
/// }
///
/// // .. typically we'd parse other module fields here...
///
/// Err(parser.error("unknown module field"))
/// }
/// }
/// ```
///
/// [annotation]: https://github.com/WebAssembly/annotations
pub fn register_annotation<'b>(self, annotation: &'b str) -> impl Drop + 'b
where
'a: 'b,
{
let mut annotations = self.buf.known_annotations.borrow_mut();
if !annotations.contains_key(annotation) {
annotations.insert(annotation.to_string(), 0);
}
*annotations.get_mut(annotation).unwrap() += 1;
return RemoveOnDrop(self, annotation);
struct RemoveOnDrop<'a>(Parser<'a>, &'a str);
impl Drop for RemoveOnDrop<'_> {
fn drop(&mut self) {
let mut annotations = self.0.buf.known_annotations.borrow_mut();
let slot = annotations.get_mut(self.1).unwrap();
*slot -= 1;
}
}
}
}
impl<'a> Cursor<'a> {
/// Returns the span of the next `Token` token.
///
/// Does not take into account whitespace or comments.
pub fn cur_span(&self) -> Span {
let offset = match self.clone().advance_token() {
Some(t) => self.parser.buf.input_pos(t.src()),
None => self.parser.buf.input.len(),
};
Span { offset }
}
/// Returns the span of the previous `Token` token.
///
/// Does not take into account whitespace or comments.
pub(crate) fn prev_span(&self) -> Option<Span> {
let (token, _) = self.parser.buf.tokens.get(self.cur.checked_sub(1)?)?;
Some(Span {
offset: self.parser.buf.input_pos(token.src()),
})
}
/// Same as [`Parser::error`], but works with the current token in this
/// [`Cursor`] instead.
pub fn error(&self, msg: impl fmt::Display) -> Error {
self.parser.error_at(self.cur_span(), &msg)
}
/// Attempts to advance this cursor if the current token is a `(`.
///
/// If the current token is `(`, returns a new [`Cursor`] pointing at the
/// rest of the tokens in the stream. Otherwise returns `None`.
///
/// This function will automatically skip over any comments, whitespace, or
/// unknown annotations.
pub fn lparen(mut self) -> Option<Self> {
match self.advance_token()? {
Token::LParen(_) => Some(self),
_ => None,
}
}
/// Attempts to advance this cursor if the current token is a `)`.
///
/// If the current token is `)`, returns a new [`Cursor`] pointing at the
/// rest of the tokens in the stream. Otherwise returns `None`.
///
/// This function will automatically skip over any comments, whitespace, or
/// unknown annotations.
pub fn rparen(mut self) -> Option<Self> {
match self.advance_token()? {
Token::RParen(_) => Some(self),
_ => None,
}
}
/// Attempts to advance this cursor if the current token is a
/// [`Token::Id`](crate::lexer::Token)
///
/// If the current token is `Id`, returns the identifier minus the leading
/// `$` character as well as a new [`Cursor`] pointing at the rest of the
/// tokens in the stream. Otherwise returns `None`.
///
/// This function will automatically skip over any comments, whitespace, or
/// unknown annotations.
pub fn id(mut self) -> Option<(&'a str, Self)> {
match self.advance_token()? {
Token::Id(id) => Some((&id[1..], self)),
_ => None,
}
}
/// Attempts to advance this cursor if the current token is a
/// [`Token::Keyword`](crate::lexer::Token)
///
/// If the current token is `Keyword`, returns the keyword as well as a new
/// [`Cursor`] pointing at the rest of the tokens in the stream. Otherwise
/// returns `None`.
///
/// This function will automatically skip over any comments, whitespace, or
/// unknown annotations.
pub fn keyword(mut self) -> Option<(&'a str, Self)> {
match self.advance_token()? {
Token::Keyword(id) => Some((id, self)),
_ => None,
}
}
/// Attempts to advance this cursor if the current token is a
/// [`Token::Reserved`](crate::lexer::Token)
///
/// If the current token is `Reserved`, returns the reserved token as well
/// as a new [`Cursor`] pointing at the rest of the tokens in the stream.
/// Otherwise returns `None`.
///
/// This function will automatically skip over any comments, whitespace, or
/// unknown annotations.
pub fn reserved(mut self) -> Option<(&'a str, Self)> {
match self.advance_token()? {
Token::Reserved(id) => Some((id, self)),
_ => None,
}
}
/// Attempts to advance this cursor if the current token is a
/// [`Token::Integer`](crate::lexer::Token)
///
/// If the current token is `Integer`, returns the integer as well as a new
/// [`Cursor`] pointing at the rest of the tokens in the stream. Otherwise
/// returns `None`.
///
/// This function will automatically skip over any comments, whitespace, or
/// unknown annotations.
pub fn integer(mut self) -> Option<(&'a Integer<'a>, Self)> {
match self.advance_token()? {
Token::Integer(i) => Some((i, self)),
_ => None,
}
}
/// Attempts to advance this cursor if the current token is a
/// [`Token::Float`](crate::lexer::Token)
///
/// If the current token is `Float`, returns the float as well as a new
/// [`Cursor`] pointing at the rest of the tokens in the stream. Otherwise
/// returns `None`.
///
/// This function will automatically skip over any comments, whitespace, or
/// unknown annotations.
pub fn float(mut self) -> Option<(&'a Float<'a>, Self)> {
match self.advance_token()? {
Token::Float(f) => Some((f, self)),
_ => None,
}
}
/// Attempts to advance this cursor if the current token is a
/// [`Token::String`](crate::lexer::Token)
///
/// If the current token is `String`, returns the byte value of the string
/// as well as a new [`Cursor`] pointing at the rest of the tokens in the
/// stream. Otherwise returns `None`.
///
/// This function will automatically skip over any comments, whitespace, or
/// unknown annotations.
pub fn string(mut self) -> Option<(&'a [u8], Self)> {
match self.advance_token()? {
Token::String(s) => Some((s.val(), self)),
_ => None,
}
}
/// Attempts to advance this cursor if the current token is a
/// [`Token::Reserved`](crate::lexer::Token) and looks like the start of an
/// annotation.
///
/// [Annotations][annotation] are a WebAssembly proposal for the text format
/// which allows placing structured text inside of a text file, for example
/// to specify the name section or other custom sections.
///
/// This function will attempt to see if the current token is the `@foo`
/// part of the annotation. This requires the previous token to be `(` and
/// the current token is `Reserved` which starts with `@` and has a nonzero
/// length for the following name.
///
/// Note that this will skip *unknown* annoations. Only pre-registered
/// annotations will be returned here.
///
/// This function will automatically skip over any comments, whitespace, or
/// unknown annotations.
///
/// [annotation]: https://github.com/WebAssembly/annotations
pub fn annotation(self) -> Option<(&'a str, Self)> {
let (token, cursor) = self.reserved()?;
if !token.starts_with("@") || token.len() <= 1 {
return None;
}
match &self.parser.buf.tokens.get(self.cur.wrapping_sub(1))?.0 {
Token::LParen(_) => Some((&token[1..], cursor)),
_ => None,
}
}
/// Attempts to advance this cursor if the current token is a
/// [`Token::LineComment`](crate::lexer::Token) or a
/// [`Token::BlockComment`](crate::lexer::Token)
///
/// This function will only skip whitespace, no other tokens.
pub fn comment(mut self) -> Option<(&'a str, Self)> {
let comment = loop {
match &self.parser.buf.tokens.get(self.cur)?.0 {
Token::LineComment(c) | Token::BlockComment(c) => {
self.cur += 1;
break c;
}
Token::Whitespace(_) => {
self.cur += 1;
}
_ => return None,
}
};
Some((comment, self))
}
fn advance_token(&mut self) -> Option<&'a Token<'a>> {
let known_annotations = self.parser.buf.known_annotations.borrow();
let is_known_annotation = |name: &str| match known_annotations.get(name) {
Some(0) | None => false,
Some(_) => true,
};
loop {
let (token, next) = self.parser.buf.tokens.get(self.cur)?;
// If we're currently pointing at a token, and it's not the start
// of an annotation, then we return that token and advance
// ourselves to just after that token.
match token {
Token::Whitespace(_) | Token::LineComment(_) | Token::BlockComment(_) => {}
_ => match self.annotation_start() {
Some(n) if !is_known_annotation(n) => {}
_ => {
self.cur += 1;
return Some(token);
}
},
}
// ... otherwise we need to skip the current token, and possibly
// more. Here we're skipping whitespace, comments, annotations, etc.
// Basically stuff that's intended to not be that relevant to the
// text format. This is a pretty common operation, though, and we
// may do it multiple times through peeks and such. As a result
// this is somewhat cached.
//
// The `next` field, if "unknown", means we haven't calculated the
// next token. Otherwise it's an index of where to resume searching
// for the next token.
//
// Note that this entire operation happens in a loop (hence the
// "somewhat cached") because the set of known annotations is
// dynamic and we can't cache which annotations are skipped. What we
// can do though is cache the number of tokens in the annotation so
// we know how to skip ahead of it.
match next.get() {
NextTokenAt::Unknown => match self.find_next() {
Some(i) => {
next.set(NextTokenAt::Index(i));
self.cur = i;
}
None => {
next.set(NextTokenAt::Eof);
return None;
}
},
NextTokenAt::Eof => return None,
NextTokenAt::Index(i) => self.cur = i,
}
}
}
fn annotation_start(&self) -> Option<&'a str> {
match self.parser.buf.tokens.get(self.cur).map(|p| &p.0) {
Some(Token::LParen(_)) => {}
_ => return None,
}
let reserved = match self.parser.buf.tokens.get(self.cur + 1).map(|p| &p.0) {
Some(Token::Reserved(n)) => n,
_ => return None,
};
if reserved.starts_with("@") && reserved.len() > 1 {
Some(&reserved[1..])
} else {
None
}
}
/// Finds the next "real" token from the current position onwards.
///
/// This is a somewhat expensive operation to call quite a lot, so it's
/// cached in the token list. See the comment above in `advance_token` for
/// how this works.
///
/// Returns the index of the next relevant token to parse
fn find_next(mut self) -> Option<usize> {
// If we're pointing to the start of annotation we need to skip it
// in its entirety, so match the parentheses and figure out where
// the annotation ends.
if self.annotation_start().is_some() {
let mut depth = 1;
self.cur += 1;
while depth > 0 {
match &self.parser.buf.tokens.get(self.cur)?.0 {
Token::LParen(_) => depth += 1,
Token::RParen(_) => depth -= 1,
_ => {}
}
self.cur += 1;
}
return Some(self.cur);
}
// ... otherwise we're pointing at whitespace/comments, so we need to
// figure out how many of them we can skip.
loop {
let (token, _) = self.parser.buf.tokens.get(self.cur)?;
// and otherwise we skip all comments/whitespace and otherwise
// get real intersted once a normal `Token` pops up.
match token {
Token::Whitespace(_) | Token::LineComment(_) | Token::BlockComment(_) => {
self.cur += 1
}
_ => return Some(self.cur),
}
}
}
}
impl Lookahead1<'_> {
/// Attempts to see if `T` is the next token in the [`Parser`] this
/// [`Lookahead1`] references.
///
/// For more information see [`Parser::lookahead1`] and [`Parser::peek`]
pub fn peek<T: Peek>(&mut self) -> bool {
if self.parser.peek::<T>() {
true
} else {
self.attempts.push(T::display());
false
}
}
/// Generates an error message saying that one of the tokens passed to
/// [`Lookahead1::peek`] method was expected.
///
/// Before calling this method you should call [`Lookahead1::peek`] for all
/// possible tokens you'd like to parse.
pub fn error(self) -> Error {
match self.attempts.len() {
0 => {
if self.parser.is_empty() {
self.parser.error("unexpected end of input")
} else {
self.parser.error("unexpected token")
}
}
1 => {
let message = format!("unexpected token, expected {}", self.attempts[0]);
self.parser.error(&message)
}
2 => {
let message = format!(
"unexpected token, expected {} or {}",
self.attempts[0], self.attempts[1]
);
self.parser.error(&message)
}
_ => {
let join = self.attempts.join(", ");
let message = format!("unexpected token, expected one of: {}", join);
self.parser.error(&message)
}
}
}
}
impl<'a, T: Peek + Parse<'a>> Parse<'a> for Option<T> {
fn parse(parser: Parser<'a>) -> Result<Option<T>> {
if parser.peek::<T>() {
Ok(Some(parser.parse()?))
} else {
Ok(None)
}
}
}