anchor/lang/attribute/error/src/lib.rs

extern crate proc_macro;

use anchor_syn::codegen::error as error_codegen;
use anchor_syn::parser::error as error_parser;
use syn::parse_macro_input;

/// Generates `Error` and `type Result<T> = Result<T, Error>` types to be
/// used as return types from Anchor instruction handlers. Importantly, the
/// attribute implements
/// [`From`](https://doc.rust-lang.org/std/convert/trait.From.html) on the
/// `ErrorCode` to support converting from the user defined error enum *into*
/// the generated `Error`.
///
/// # Example
///
/// ```ignore
/// use anchor_lang::prelude::*;
///
/// #[program]
/// mod errors {
///     use super::*;
///     pub fn hello(_ctx: Context<Hello>) -> Result<()> {
///         Err(MyError::Hello.into())
///     }
/// }
///
/// #[derive(Accounts)]
/// pub struct Hello {}
///
/// #[error]
/// pub enum MyError {
///     #[msg("This is an error message clients will automatically display")]
///     Hello,
/// }
/// ```
///
/// Note that we generate a new `Error` type so that we can return either the
/// user defined error enum *or* a
/// [`ProgramError`](../solana_program/enum.ProgramError.html), which is used
/// pervasively, throughout solana program crates. The generated `Error` type
/// should almost never be used directly, as the user defined error is
/// preferred. In the example above, `MyError::Hello.into()`.
///
/// # Msg
///
/// The `#[msg(..)]` attribute is inert, and is used only as a marker so that
/// parsers  and IDLs can map error codes to error messages.
#[proc_macro_attribute]
pub fn error(
    _args: proc_macro::TokenStream,
    input: proc_macro::TokenStream,
) -> proc_macro::TokenStream {
    let mut error_enum = parse_macro_input!(input as syn::ItemEnum);
    let error = error_codegen::generate(error_parser::parse(&mut error_enum));
    proc_macro::TokenStream::from(error)
}