CODEWITHSUNDEEP
macrosrustinfinite-recursion
Anders
Anders

Reputation: 8578

Infinite recursion in Rust derive macro?

I have made the following attampt at a derive macro in Rust:

extern crate proc_macro;

use crate::proc_macro::TokenStream;
use quote::quote;
use syn;

#[proc_macro_derive(DeserializeConfigurable)]
pub fn deserialize_configurable_derive(input: TokenStream) -> TokenStream {
    let ast: syn::DeriveInput = syn::parse(input).unwrap();
    let name = &ast.ident;
    let gen = quote! {
        impl<'de> Deserialize<'de> for #name {
            fn deserialize<D>(deserializer: D) -> Result<#name, D::Error>
            where
                D: Deserializer<'de>,
            {
                let config = <#name as Configurable>::Config::deserialize(deserializer)?;
                Ok(#name::from_config(config))
            }
        }
    };
    gen.into()
}

The goal is to provide a specific blanket implementation for deserialize based on another trait, Configurable.

However, the compiler has the following complaint regarding my quote statement:

recursion limit reached while expanding the macro stringify

I don't see the recursion here. I am not even using stringify! What is calling what in infinity here?

Upvotes: 1

Views: 1868

Answers (1)

Ramon Snir
Ramon Snir

Reputation: 7560

Trying to compile this locally gives this full error message:

error: recursion limit reached while expanding the macro `stringify`
  --> src/lib.rs:13:15
   |
13 |       let gen = quote! {
   |  _______________^
14 | |         impl<'de> Deserialize<'de> for #name {
15 | |             fn deserialize<D>(deserializer: D) -> Result<#name, D::Error>
16 | |             where
...  |
22 | |         }
23 | |     };
   | |_____^
   |
   = help: consider adding a `#![recursion_limit="128"]` attribute to your crate
   = note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)                                                                                                                                 

error: aborting due to previous error

Putting the following as the first line of the crate makes the error go away. My guess is that the default recursion_limit is simply too low for a macro this complex (which IMO isn't that complex). I'm sure there's a good reason, and I'm glad the error message contained the solution:

#![recursion_limit="128"]

Upvotes: 2

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