I have this crate layout:

• Cargo.toml
• src/
  • main.rs
  • parent/
    • child.rs

where child.rs is an empty file and main.rs is:

mod parent { mod child; } fn main() {}

This compiles successfully and we end up with a module at crate::parent::child backed by the file src/parent/child.rs.

But if mod child; goes anywhere near a proc macro, it no longer works.

mod parent { repro::noop! { mod child; } } fn main() {}

where noop! is this macro:

#[proc_macro] pub fn noop(input: TokenStream) -> TokenStream { input }

The error is:

error[E0583]: file not found for module `child`  --> src/main.rs:2:5  | 2 | repro::noop! { mod child; }  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^  |  = help: name the file either child.rs or child/mod.rs inside the directory "src"

so it is looking for a file corresponding to mod child as though the mod child were not contained inside of mod parent. I hit this while working on https://github.com/dtolnay/automod.

The same problem does not occur if noop! is defined as a macro_rules macro.

Repro script

#!/bin/bash cargo new repro echo >>repro/Cargo.toml ' [lib] proc-macro = true ' echo >repro/src/lib.rs ' extern crate proc_macro; use proc_macro::TokenStream;  #[proc_macro] pub fn noop(input: TokenStream) -> TokenStream {  input } ' echo >repro/src/main.rs ' mod parent {  repro::noop! { mod child; } }  fn main() {} ' mkdir repro/src/parent touch repro/src/parent/child.rs cargo check --manifest-path repro/Cargo.toml

Mentioning @petrochenkov who may know what is going wrong or whether there is a simple rustc fix.