[][src]Struct syn::Error

pub struct Error { /* fields omitted */ }

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.

When parsing macro input, the parse_macro_input! macro handles the conversion to compile_error! automatically.

extern crate proc_macro;

use proc_macro::TokenStream;
use syn::{parse_macro_input, AttributeArgs, ItemFn};

pub fn my_attr(args: TokenStream, input: TokenStream) -> TokenStream {
    let args = parse_macro_input!(args as AttributeArgs);
    let input = parse_macro_input!(input as ItemFn);

    /* ... */

For errors that arise later than the initial parsing stage, the .to_compile_error() method can be used to perform an explicit conversion to compile_error!.

pub fn my_derive(input: TokenStream) -> TokenStream {
    let input = parse_macro_input!(input as DeriveInput);

    // fn(DeriveInput) -> syn::Result<proc_macro2::TokenStream>
        .unwrap_or_else(|err| err.to_compile_error())


impl Error[src]

pub fn new<T: Display>(span: Span, message: T) -> Self[src]

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.


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`"));
    let s: LitStr = input.parse()?;

pub fn new_spanned<T: ToTokens, U: Display>(tokens: T, message: U) -> Self[src]

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)!

pub fn span(&self) -> Span[src]

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 to_compile_error(&self) -> TokenStream[src]

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.

pub fn combine(&mut self, another: Error)[src]

Add another error message to self such that when to_compile_error() is called, both errors will be emitted together.

Trait Implementations

impl Clone for Error[src]

impl Debug for Error[src]

impl Display for Error[src]

impl Error for Error[src]

impl From<LexError> for Error[src]

impl IntoIterator for Error[src]

type Item = Error

The type of the elements being iterated over.

type IntoIter = IntoIter

Which kind of iterator are we turning this into?

impl<'a> IntoIterator for &'a Error[src]

type Item = Error

The type of the elements being iterated over.

type IntoIter = Iter<'a>

Which kind of iterator are we turning this into?

Auto Trait Implementations

impl !RefUnwindSafe for Error

impl Send for Error

impl Sync for Error

impl Unpin for Error

impl UnwindSafe for Error

Blanket Implementations

impl<T> Any for T where
    T: 'static + ?Sized

impl<T> Borrow<T> for T where
    T: ?Sized

impl<T> BorrowMut<T> for T where
    T: ?Sized

impl<T> From<T> for T[src]

impl<T, U> Into<U> for T where
    U: From<T>, 

impl<I> IntoIterator for I where
    I: Iterator

type Item = <I as Iterator>::Item

The type of the elements being iterated over.

type IntoIter = I

Which kind of iterator are we turning this into?

impl<T> ToOwned for T where
    T: Clone

type Owned = T

The resulting type after obtaining ownership.

impl<T> ToString for T where
    T: Display + ?Sized

impl<T, U> TryFrom<U> for T where
    U: Into<T>, 

type Error = Infallible

The type returned in the event of a conversion error.

impl<T, U> TryInto<U> for T where
    U: TryFrom<T>, 

type Error = <U as TryFrom<T>>::Error

The type returned in the event of a conversion error.