Static PROC_MACRO_DERIVE_RESOLUTION_FALLBACK

pub static PROC_MACRO_DERIVE_RESOLUTION_FALLBACK: &'static Lint

The proc_macro_derive_resolution_fallback lint detects proc macro derives using inaccessible names from parent modules.

Example

// foo.rs
#![crate_type = "proc-macro"]

extern crate proc_macro;

use proc_macro::*;

#[proc_macro_derive(Foo)]
pub fn foo1(a: TokenStream) -> TokenStream {
    drop(a);
    "mod __bar { static mut BAR: Option<Something> = None; }".parse().unwrap()
}

// bar.rs
#[macro_use]
extern crate foo;

struct Something;

#[derive(Foo)]
struct Another;

fn main() {}

This will produce:

warning: cannot find type `Something` in this scope
 --> src/main.rs:8:10
  |
8 | #[derive(Foo)]
  |          ^^^ names from parent modules are not accessible without an explicit import
  |
  = note: `#[warn(proc_macro_derive_resolution_fallback)]` on by default
  = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
  = note: for more information, see issue #50504 <https://github.com/rust-lang/rust/issues/50504>

Explanation

If a proc-macro generates a module, the compiler unintentionally allowed items in that module to refer to items in the crate root without importing them. This is a future-incompatible lint to transition this to a hard error in the future. See issue #50504 for more details.