Struct `Error`

struct 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.

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::{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!.

# 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!()
#     }
# }

Implementations

`impl Error`

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)
}

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)!

fn span(self: &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: &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: 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(self: &mut Self, another: Error)

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

`impl Clone for Error`

fn clone(self: &Self) -> Self

`impl Debug for Error`

fn fmt(self: &Self, formatter: &mut Formatter<'_>) -> Result

`impl Display for Error`

fn fmt(self: &Self, formatter: &mut Formatter<'_>) -> Result

`impl Error for Error`

`impl Extend for Error`

fn extend<T: IntoIterator<Item = Error>>(self: &mut Self, iter: T)

`impl Freeze for Error`

`impl From for Error`

fn from(err: LexError) -> Self

`impl IntoIterator for Error`

fn into_iter(self: Self) -> <Self as >::IntoIter

`impl RefUnwindSafe for Error`

`impl Send for Error`

`impl Sync for Error`

`impl Unpin for Error`

`impl UnsafeUnpin for Error`

`impl UnwindSafe for Error`

`impl<T> Any for Error`

fn type_id(self: &Self) -> TypeId

`impl<T> Borrow for Error`

fn borrow(self: &Self) -> &T

`impl<T> BorrowMut for Error`

fn borrow_mut(self: &mut Self) -> &mut T

`impl<T> CloneToUninit for Error`

unsafe fn clone_to_uninit(self: &Self, dest: *mut u8)

`impl<T> From for Error`

fn from(t: T) -> T: Returns the argument unchanged.

`impl<T> ToOwned for Error`

fn to_owned(self: &Self) -> T
fn clone_into(self: &Self, target: &mut T)

`impl<T> ToString for Error`

fn to_string(self: &Self) -> String

`impl<T, U> Into for Error`

fn into(self: Self) -> U

Calls U::from(self).

That is, this conversion is whatever the implementation of [From]<T> for U chooses to do.

`impl<T, U> TryFrom for Error`

fn try_from(value: U) -> Result<T, <T as TryFrom<U>>::Error>

`impl<T, U> TryInto for Error`

fn try_into(self: Self) -> Result<U, <U as TryFrom<T>>::Error>