Custom attribute used on thing disallowing custom attributes should say that

[Havvy]

crate: custom_attr

extern crate proc_macro;
use proc_macro::TokenStream;

#[proc_macro_attribute]
pub fn custom(_attr: TokenStream, item: TokenStream) -> TokenStream {
    item
}

---

crate: not the previous one

extern crate custom_attr;
use custom_attr::custom;

fn main() {
   match () {
        #[custom]
        () => {}
    }
}

---

Current Error

error[E0658]: The attribute `custom` is currently unknown to the compiler and may have meaning added to it in the future (see issue #29642)
  --> src/main.rs:LL:L
   |
L |         #[custom]
   |         ^^^^^^^^
   |
   = help: add #![feature(custom_attribute)] to the crate attributes to enable

---

Wanted Error

error[E0NNN]: Custom attributes can only be applied to items
  --> src/main.rs:LL:L
   |
L |         #[custom]
   |         ^^^^^^^^
   |

(I'm assuming they can only be applied to items in nightly without using feature flags in Rust today. It'd be nice to get that assumption confirmed as well)

[eddyb]

cc @petrochenkov

[petrochenkov]

This is item 4 in #50911 (comment) more or less.