Struct syn::Error

pub struct Error { /* fields omitted */ }

Error returned when a Syn parser cannot parse the input tokens.

Refer to the module documentation for details about parsing in Syn.

This type is available if Syn is built with the "parsing" feature.

Methods

impl Error

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

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.

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_spanned<T: ToTokens, U: Display>(tokens: T, message: U) -> Self

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

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

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.

Trait Implementations

impl Clone for Error

fn clone(&self) -> Self

Returns a copy of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl From<LexError> for Error

fn from(err: LexError) -> Self

Performs the conversion.

impl Display for Error

fn fmt(&self, formatter: &mut Formatter) -> Result

Formats the value using the given formatter.

impl Debug for Error

fn fmt(&self, formatter: &mut Formatter) -> Result

Formats the value using the given formatter.

impl Error for Error

fn description(&self) -> &str

This method is soft-deprecated.

fn cause(&self) -> Option<&dyn Error>

Deprecated since 1.33.0:

replaced by Error::source, which can support downcasting

The lower-level cause of this error, if any.

fn source(&self) -> Option<&(dyn Error + 'static)>

The lower-level source of this error, if any.