Cannot use cfg_select! with EII

[sleepycatcoding]

I tried this code:

#![feature(extern_item_impls)]

cfg_select! {
    feature = "nightly" => {
        #[eii(do_something)]
        pub fn do_something();
    }
    _ => {
        // Fallback if not on nightly.
        pub fn do_something() {}
    }
}

I expected to see this compile.

Instead, I got a compile error, complaining about unsupported attributes on externally implementable items, I assumed this should work since it selects a branch to compile based on the environment and the attributes aren't directly applied to the function itself.

error: only a small subset of attributes are supported on externally implementable items
 --> src/main.rs:7:9
  |
7 |         pub fn do_something();
  |         ^^^^^^^^^^^^^^^^^^^^^^
  |
note: this attribute is not supported
 --> src/main.rs:5:5
  |
5 |     feature = "nightly" => {
  |     ^^^^^^^^^^^^^^^^^^^

Meta

rustc --version --verbose:

rustc 1.100.0-nightly (f248f4038 2026-09-05)
binary: rustc
commit-hash: f248f4038796913873f11ca65b1b901e311c8dae
commit-date: 2026-09-05
host: x86_64-unknown-linux-gnu
release: 1.100.0-nightly
LLVM version: 23.1.1

Backtrace

<backtrace>

[mejrs]

Not cfg_select specific, it also happens for

#![feature(extern_item_impls)]

#[cfg(true)]
#[eii(do_something)]
pub fn do_something();

it probably encounters the cfg trace attribute here.

[sleepycatcoding]

I know it used to work on an older nightly, but was probably changed by #157312. I might be wrong, but it seems a bit overzealous to not allow cfg attributes here?

[mejrs]

it seems a bit overzealous to not allow cfg attributes here?

The cfg attribute here is fine, but cfg leaves behind CfgTrace trace attributes (so that we can later tell it came from the expansion of cfg). That is what is producing the error here. See also https://doc.rust-lang.org/nightly/nightly-rustc/rustc_ast/ast/enum.SyntheticAttr.html

The list of allowed attributes is an allowlist to prevent accidentally allowing bad combinations, the trace attributes should be added to it. (alongside a test that it is correctly propagated during eii's expansion).

[maxdexh]

Highly idiomatic workaround:

#![feature(extern_item_impls)]

macro_rules! ident {
    ($($t:tt)*) => { $($t)* };
}

#[cfg(feature = "nightly")]
ident! {
    #[eii(do_something)]
    pub fn do_something();
}

mod implement {
    #[cfg(feature = "nightly")]
	#[super::do_something]
    fn do_something() {}
}

[sleepycatcoding]

The cfg attribute here is fine, but cfg leaves behind CfgTrace trace attributes (so that we can later tell it came from the expansion of cfg). That is what is producing the error here. See also https://doc.rust-lang.org/nightly/nightly-rustc/rustc_ast/ast/enum.SyntheticAttr.html

That makes more sense.

The list of allowed attributes is an allowlist to prevent accidentally allowing bad combinations, the trace attributes should be added to it. (alongside a test that it is correctly propagated during eii's expansion).

I might take a try at fixing this myself.