PROC_MACRO_DERIVE_RESOLUTION_FALLBACK in rustc_lint::builtin - Rust
Description: The `proc_macro_derive_resolution_fallback` lint detects proc macro derives using inaccessible names from parent modules.

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## PROC_MACRO_DERIVE_RESOLUTION_FALLBACK

# Static PROC_MACRO_DERIVE_RESOLUTION_FALLBACK

Source

    pub static PROC_MACRO_DERIVE_RESOLUTION_FALLBACK: &'static Lint

Expand description

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.