impl Error

fn new(span: Span, message: impl Display) -> 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)
}

fn new_spanned(tokens: impl ToTokens, message: impl Display) -> 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)!

fn new_range<'a>(span: impl Into<Range<Cursor<'a>>>, message: impl Display) -> Self

Creates an error spanning the given cursor range.

Example

This parses a sequence of '+'-separated lifetimes and types like Vec<u8> + dyn Display + 'static, at least one of which must be a type.

use syn::{Lifetime, Token, Type};
use syn::parse::{Error, ParseStream, Result};

enum Thing {
    Lifetime(Lifetime),
    Type(Type),
}

fn parse_things(input: ParseStream) -> Result<Vec<Thing>> {
    let things_begin = input.cursor();
    let things_end;
    let mut things = Vec::new();
    let mut has_type = false;
    loop {
        if input.peek(Lifetime) {
            let lifetime: Lifetime = input.parse()?;
            things.push(Thing::Lifetime(lifetime));
        } else {
            let ty = input.call(Type::without_plus)?;
            things.push(Thing::Type(ty));
            has_type = true;
        }
        let plus_token: Option<Token![+]> = input.parse()?;
        if plus_token.is_none() {
            things_end = input.cursor();
            break;
        }
    }

    if has_type {
        Ok(things)
    } else {
        let msg = "things must not be all lifetimes, at least one type is required";
        Err(Error::new_range(things_begin..things_end, msg))
    }
}

error: things must not be all lifetimes, at least one type is required
 --> example.rs:4:23
  |
4 |     example!(THINGS = 'a + 'b, ...);
  |                       ^^^^^^^

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.

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.

fn into_compile_error(self) -> TokenStream

Render the error as an invocation of 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!()
    }
}

fn combine(&mut self, another: Error)

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