#[cfg(feature = "parsing")]
use crate::buffer::Cursor;
use crate::thread::ThreadBound;
use proc_macro2::{
Delimiter, Group, Ident, LexError, Literal, Punct, Spacing, Span, TokenStream, TokenTree,
};
#[cfg(feature = "printing")]
use quote::ToTokens;
use std::fmt::{self, Debug, Display};
use std::slice;
use std::vec;
/// The result of a Syn parser.
pub type Result<T> = std::result::Result<T, Error>;
/// Error returned when a Syn parser cannot parse the input tokens.
///
/// # Error reporting in proc macros
///
/// The correct way to report errors back to the compiler from a procedural
/// macro is by emitting an appropriately spanned invocation of
/// [`compile_error!`] in the generated code. This produces a better diagnostic
/// message than simply panicking the macro.
///
/// [`compile_error!`]: std::compile_error!
///
/// When parsing macro input, the [`parse_macro_input!`] macro handles the
/// conversion to `compile_error!` automatically.
///
/// [`parse_macro_input!`]: crate::parse_macro_input!
///
/// ```
/// # extern crate proc_macro;
/// #
/// use proc_macro::TokenStream;
/// use syn::parse::{Parse, ParseStream, Result};
/// use syn::{parse_macro_input, ItemFn};
///
/// # const IGNORE: &str = stringify! {
/// #[proc_macro_attribute]
/// # };
/// pub fn my_attr(args: TokenStream, input: TokenStream) -> TokenStream {
/// let args = parse_macro_input!(args as MyAttrArgs);
/// let input = parse_macro_input!(input as ItemFn);
///
/// /* ... */
/// # TokenStream::new()
/// }
///
/// struct MyAttrArgs {
/// # _k: [(); { stringify! {
/// ...
/// # }; 0 }]
/// }
///
/// impl Parse for MyAttrArgs {
/// fn parse(input: ParseStream) -> Result<Self> {
/// # stringify! {
/// ...
/// # };
/// # unimplemented!()
/// }
/// }
/// ```
///
/// For errors that arise later than the initial parsing stage, the
/// [`.to_compile_error()`] or [`.into_compile_error()`] methods can be used to
/// perform an explicit conversion to `compile_error!`.
///
/// [`.to_compile_error()`]: Error::to_compile_error
/// [`.into_compile_error()`]: Error::into_compile_error
///
/// ```
/// # extern crate proc_macro;
/// #
/// # use proc_macro::TokenStream;
/// # use syn::{parse_macro_input, DeriveInput};
/// #
/// # const IGNORE: &str = stringify! {
/// #[proc_macro_derive(MyDerive)]
/// # };
/// pub fn my_derive(input: TokenStream) -> TokenStream {
/// let input = parse_macro_input!(input as DeriveInput);
///
/// // fn(DeriveInput) -> syn::Result<proc_macro2::TokenStream>
/// expand::my_derive(input)
/// .unwrap_or_else(syn::Error::into_compile_error)
/// .into()
/// }
/// #
/// # mod expand {
/// # use proc_macro2::TokenStream;
/// # use syn::{DeriveInput, Result};
/// #
/// # pub fn my_derive(input: DeriveInput) -> Result<TokenStream> {
/// # unimplemented!()
/// # }
/// # }
/// ```
pub struct Error {
messages: Vec<ErrorMessage>,
}
struct ErrorMessage {
// Span is implemented as an index into a thread-local interner to keep the
// size small. It is not safe to access from a different thread. We want
// errors to be Send and Sync to play nicely with ecosystem crates for error
// handling, so pin the span we're given to its original thread and assume
// it is Span::call_site if accessed from any other thread.
span: ThreadBound<SpanRange>,
message: String,
}
// Cannot use std::ops::Range<Span> because that does not implement Copy,
// whereas ThreadBound<T> requires a Copy impl as a way to ensure no Drop impls
// are involved.
struct SpanRange {
start: Span,
end: Span,
}
#[cfg(test)]
struct _Test
where
Error: Send + Sync;
impl Error {
/// Usually the [`ParseStream::error`] method will be used instead, which
/// automatically uses the correct span from the current position of the
/// parse stream.
///
/// Use `Error::new` when the error needs to be triggered on some span other
/// than where the parse stream is currently positioned.
///
/// [`ParseStream::error`]: crate::parse::ParseBuffer::error
///
/// # Example
///
/// ```
/// use syn::{Error, Ident, LitStr, Result, Token};
/// use syn::parse::ParseStream;
///
/// // Parses input that looks like `name = "string"` where the key must be
/// // the identifier `name` and the value may be any string literal.
/// // Returns the string literal.
/// fn parse_name(input: ParseStream) -> Result<LitStr> {
/// let name_token: Ident = input.parse()?;
/// if name_token != "name" {
/// // Trigger an error not on the current position of the stream,
/// // but on the position of the unexpected identifier.
/// return Err(Error::new(name_token.span(), "expected `name`"));
/// }
/// input.parse::<Token![=]>()?;
/// let s: LitStr = input.parse()?;
/// Ok(s)
/// }
/// ```
pub fn new<T: Display>(span: Span, message: T) -> Self {
return new(span, message.to_string());
fn new(span: Span, message: String) -> Error {
Error {
messages: vec![ErrorMessage {
span: ThreadBound::new(SpanRange {
start: span,
end: span,
}),
message,
}],
}
}
}
/// Creates an error with the specified message spanning the given syntax
/// tree node.
///
/// Unlike the `Error::new` constructor, this constructor takes an argument
/// `tokens` which is a syntax tree node. This allows the resulting `Error`
/// to attempt to span all tokens inside of `tokens`. While you would
/// typically be able to use the `Spanned` trait with the above `Error::new`
/// constructor, implementation limitations today mean that
/// `Error::new_spanned` may provide a higher-quality error message on
/// stable Rust.
///
/// When in doubt it's recommended to stick to `Error::new` (or
/// `ParseStream::error`)!
#[cfg(feature = "printing")]
#[cfg_attr(docsrs, doc(cfg(feature = "printing")))]
pub fn new_spanned<T: ToTokens, U: Display>(tokens: T, message: U) -> Self {
return new_spanned(tokens.into_token_stream(), message.to_string());
fn new_spanned(tokens: TokenStream, message: String) -> Error {
let mut iter = tokens.into_iter();
let start = iter.next().map_or_else(Span::call_site, |t| t.span());
let end = iter.last().map_or(start, |t| t.span());
Error {
messages: vec![ErrorMessage {
span: ThreadBound::new(SpanRange { start, end }),
message,
}],
}
}
}
/// The source location of the error.
///
/// Spans are not thread-safe so this function returns `Span::call_site()`
/// if called from a different thread than the one on which the `Error` was
/// originally created.
pub fn span(&self) -> Span {
let SpanRange { start, end } = match self.messages[0].span.get() {
Some(span) => *span,
None => return Span::call_site(),
};
start.join(end).unwrap_or(start)
}
/// Render the error as an invocation of [`compile_error!`].
///
/// The [`parse_macro_input!`] macro provides a convenient way to invoke
/// this method correctly in a procedural macro.
///
/// [`compile_error!`]: std::compile_error!
/// [`parse_macro_input!`]: crate::parse_macro_input!
pub fn to_compile_error(&self) -> TokenStream {
self.messages
.iter()
.map(ErrorMessage::to_compile_error)
.collect()
}
/// Render the error as an invocation of [`compile_error!`].
///
/// [`compile_error!`]: std::compile_error!
///
/// # Example
///
/// ```
/// # extern crate proc_macro;
/// #
/// use proc_macro::TokenStream;
/// use syn::{parse_macro_input, DeriveInput, Error};
///
/// # const _: &str = stringify! {
/// #[proc_macro_derive(MyTrait)]
/// # };
/// pub fn derive_my_trait(input: TokenStream) -> TokenStream {
/// let input = parse_macro_input!(input as DeriveInput);
/// my_trait::expand(input)
/// .unwrap_or_else(Error::into_compile_error)
/// .into()
/// }
///
/// mod my_trait {
/// use proc_macro2::TokenStream;
/// use syn::{DeriveInput, Result};
///
/// pub(crate) fn expand(input: DeriveInput) -> Result<TokenStream> {
/// /* ... */
/// # unimplemented!()
/// }
/// }
/// ```
pub fn into_compile_error(self) -> TokenStream {
self.to_compile_error()
}
/// Add another error message to self such that when `to_compile_error()` is
/// called, both errors will be emitted together.
pub fn combine(&mut self, another: Error) {
self.messages.extend(another.messages);
}
}
impl ErrorMessage {
fn to_compile_error(&self) -> TokenStream {
let (start, end) = match self.span.get() {
Some(range) => (range.start, range.end),
None => (Span::call_site(), Span::call_site()),
};
// ::core::compile_error!($message)
TokenStream::from_iter([
TokenTree::Punct({
let mut punct = Punct::new(':', Spacing::Joint);
punct.set_span(start);
punct
}),
TokenTree::Punct({
let mut punct = Punct::new(':', Spacing::Alone);
punct.set_span(start);
punct
}),
TokenTree::Ident(Ident::new("core", start)),
TokenTree::Punct({
let mut punct = Punct::new(':', Spacing::Joint);
punct.set_span(start);
punct
}),
TokenTree::Punct({
let mut punct = Punct::new(':', Spacing::Alone);
punct.set_span(start);
punct
}),
TokenTree::Ident(Ident::new("compile_error", start)),
TokenTree::Punct({
let mut punct = Punct::new('!', Spacing::Alone);
punct.set_span(start);
punct
}),
TokenTree::Group({
let mut group = Group::new(Delimiter::Brace, {
TokenStream::from_iter([TokenTree::Literal({
let mut string = Literal::string(&self.message);
string.set_span(end);
string
})])
});
group.set_span(end);
group
}),
])
}
}
#[cfg(feature = "parsing")]
pub(crate) fn new_at<T: Display>(scope: Span, cursor: Cursor, message: T) -> Error {
if cursor.eof() {
Error::new(scope, format!("unexpected end of input, {}", message))
} else {
let span = crate::buffer::open_span_of_group(cursor);
Error::new(span, message)
}
}
#[cfg(all(feature = "parsing", any(feature = "full", feature = "derive")))]
pub(crate) fn new2<T: Display>(start: Span, end: Span, message: T) -> Error {
return new2(start, end, message.to_string());
fn new2(start: Span, end: Span, message: String) -> Error {
Error {
messages: vec![ErrorMessage {
span: ThreadBound::new(SpanRange { start, end }),
message,
}],
}
}
}
impl Debug for Error {
fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
if self.messages.len() == 1 {
formatter
.debug_tuple("Error")
.field(&self.messages[0])
.finish()
} else {
formatter
.debug_tuple("Error")
.field(&self.messages)
.finish()
}
}
}
impl Debug for ErrorMessage {
fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
Debug::fmt(&self.message, formatter)
}
}
impl Display for Error {
fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
formatter.write_str(&self.messages[0].message)
}
}
impl Clone for Error {
fn clone(&self) -> Self {
Error {
messages: self.messages.clone(),
}
}
}
impl Clone for ErrorMessage {
fn clone(&self) -> Self {
ErrorMessage {
span: self.span,
message: self.message.clone(),
}
}
}
impl Clone for SpanRange {
fn clone(&self) -> Self {
*self
}
}
impl Copy for SpanRange {}
impl std::error::Error for Error {}
impl From<LexError> for Error {
fn from(err: LexError) -> Self {
Error::new(err.span(), err)
}
}
impl IntoIterator for Error {
type Item = Error;
type IntoIter = IntoIter;
fn into_iter(self) -> Self::IntoIter {
IntoIter {
messages: self.messages.into_iter(),
}
}
}
pub struct IntoIter {
messages: vec::IntoIter<ErrorMessage>,
}
impl Iterator for IntoIter {
type Item = Error;
fn next(&mut self) -> Option<Self::Item> {
Some(Error {
messages: vec![self.messages.next()?],
})
}
}
impl<'a> IntoIterator for &'a Error {
type Item = Error;
type IntoIter = Iter<'a>;
fn into_iter(self) -> Self::IntoIter {
Iter {
messages: self.messages.iter(),
}
}
}
pub struct Iter<'a> {
messages: slice::Iter<'a, ErrorMessage>,
}
impl<'a> Iterator for Iter<'a> {
type Item = Error;
fn next(&mut self) -> Option<Self::Item> {
Some(Error {
messages: vec![self.messages.next()?.clone()],
})
}
}
impl Extend<Error> for Error {
fn extend<T: IntoIterator<Item = Error>>(&mut self, iter: T) {
for err in iter {
self.combine(err);
}
}
}
