Spurious legacy_derive_helpers lint when unrelated attribute fails to resolve

[Aaron1011]

The following code:

// src/lib.rs
use proc_macro::{TokenStream};

#[proc_macro_derive(MyDerive, attributes(my_helper))]
pub fn my_derive(_target: TokenStream) -> TokenStream {
    TokenStream::new()
}


// src/main.rs

#![dummy]

use derive_helper::*;

#[derive(MyDerive)]
#[my_helper]
struct Foo {}

fn main() {}

produces the following errors:

error: cannot find attribute `dummy` in this scope
 --> src/main.rs:1:4
  |
1 | #![dummy]
  |    ^^^^^

error: cannot determine resolution for the attribute macro `derive`
 --> src/main.rs:5:3
  |
5 | #[derive(MyDerive)]
  |   ^^^^^^
  |
  = note: import resolution is stuck, try simplifying macro imports

error: cannot determine resolution for the derive macro `MyDerive`
 --> src/main.rs:5:10
  |
5 | #[derive(MyDerive)]
  |          ^^^^^^^^
  |
  = note: import resolution is stuck, try simplifying macro imports

error: cannot determine resolution for the attribute macro `my_helper`
 --> src/main.rs:6:3
  |
6 | #[my_helper]
  |   ^^^^^^^^^
  |
  = note: import resolution is stuck, try simplifying macro imports

warning: derive helper attribute `my_helper` is used before it is introduced
 --> src/main.rs:6:3
  |
5 | #[derive(MyDerive)]
  |          -------- the attribute is introduced here
6 | #[my_helper]
  |   ^^^^^^^^^
  |
  = note: `#[warn(legacy_derive_helpers)]` 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 #79202 <https://github.com/rust-lang/rust/issues/79202>

error: aborting due to 4 previous errors; 1 warning emitted

The legacy_derive_helpers warning is spurious and will disappear if #![dummy] is removed.

[Aaron1011]

Caused by #79078
cc @petrochenkov

[petrochenkov]

This looks like a consequence of #67839.

Initial resolution of derive fails, so it's recovered as an unresolved inert attribute, but due to the linked issue the order of attributes swaps during the "inert attribute expansion".
So we have this code before resolving my_helper:

#[my_helper]
#[derive(MyDerive)]
struct Foo {}

and my_derive gets resolved as a legacy helper.

A fix for #67839 is one of direct follow ups to #79078, so it is in my queue and pretty close to the top.

[petrochenkov]

Copypasting a future test for the ui test suite:

// aux-build:test-macros.rs

#![dummy] //~ ERROR cannot find attribute `dummy` in this scope

#[macro_use]
extern crate test_macros;

#[derive(Empty)] //~ ERROR cannot determine resolution for the attribute macro `derive`
#[empty_helper] //~ WARN derive helper attribute is used before it is introduced
                //~| WARN this was previously accepted
struct Foo {}

fn main() {}

[petrochenkov]

Fixed in #82419.