pub struct Error { /* private fields */ }
Available on crate feature parsing only.
Expand description

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.

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

#[proc_macro_attribute]
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() or .into_compile_error() methods can be used to perform an explicit conversion to compile_error!.

#[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()
}

Implementations

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)
}

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)!

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.

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.

Render the error as an invocation of compile_error!.

Example
use proc_macro::TokenStream;
use syn::{parse_macro_input, DeriveInput, Error};

#[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> {
        /* ... */
    }
}

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

Trait Implementations

Returns a copy of the value. Read more
Performs copy-assignment from source. Read more
Formats the value using the given formatter. Read more
Formats the value using the given formatter. Read more
The lower-level source of this error, if any. Read more
👎Deprecated since 1.42.0: use the Display impl or to_string()
👎Deprecated since 1.33.0: replaced by Error::source, which can support downcasting
🔬This is a nightly-only experimental API. (error_generic_member_access)
Provides type based access to context intended for error reports. Read more
Extends a collection with the contents of an iterator. Read more
🔬This is a nightly-only experimental API. (extend_one)
Extends a collection with exactly one element.
🔬This is a nightly-only experimental API. (extend_one)
Reserves capacity in a collection for the given number of additional elements. Read more
Converts to this type from the input type.
The type of the elements being iterated over.
Which kind of iterator are we turning this into?
Creates an iterator from a value. Read more
The type of the elements being iterated over.
Which kind of iterator are we turning this into?
Creates an iterator from a value. Read more

Auto Trait Implementations

Blanket Implementations

Gets the TypeId of self. Read more
Immutably borrows from an owned value. Read more
Mutably borrows from an owned value. Read more

Returns the argument unchanged.

Calls U::from(self).

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

🔬This is a nightly-only experimental API. (provide_any)
Data providers should implement this method to provide all values they are able to provide by using demand. Read more
The resulting type after obtaining ownership.
Creates owned data from borrowed data, usually by cloning. Read more
Uses borrowed data to replace owned data, usually by cloning. Read more
Converts the given value to a String. Read more
The type returned in the event of a conversion error.
Performs the conversion.
The type returned in the event of a conversion error.
Performs the conversion.