Infinite recursion in Rust derive macro? - macros

Question

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?

Answer 1

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"]