Nightly compiler complains about a disabled nightly feature used in an impl block, but the block is cfg-gated by a disabled feature

[gendx]

I tried this code in the playground (using Rust nightly):

#![allow(unexpected_cfgs)]
#![cfg_attr(
    feature = "nightly",
    feature(associated_const_equality)
)]

pub trait NeedsDrop {
    const NEEDS_DROP: bool;
}

impl<T> NeedsDrop for T {
    const NEEDS_DROP: bool = std::mem::needs_drop::<T>();
}

pub trait Foo {}

#[cfg(feature = "nightly")]
impl<T> Foo for T where T: NeedsDrop<NEEDS_DROP = true> {}

I expected to see this happen:

• Given that the playground knows no feature called "nightly" (no Cargo.toml defined), the cfg_attr at the beginning doesn't apply i.e. the associated_const_equality feature isn't enabled.
• Additionally, I also expected the impl block at the last line (which depends on associated_const_equality) to be excluded from conditional compilation.

Instead, this happened: the compiler complained that the impl block uses a nightly feature that isn't enabled (rather than ignoring the impl block altogether).

   Compiling playground v0.0.1 (/playground)
error[E0658]: associated const equality is incomplete
  --> src/lib.rs:18:38
   |
18 | impl<T> Foo for T where T: NeedsDrop<NEEDS_DROP = true> {}
   |                                      ^^^^^^^^^^^^^^^^^
   |
   = note: see issue #92827 <https://github.com/rust-lang/rust/issues/92827> for more information
   = help: add `#![feature(associated_const_equality)]` to the crate attributes to enable
   = note: this compiler was built on 2025-05-12; consider upgrading it if it is out of date

For more information about this error, try `rustc --explain E0658`.
error: could not compile `playground` (lib) due to 1 previous error

Meta

Using the nightly compiler in the playground (https://play.rust-lang.org/?version=nightly&mode=debug&edition=2024).

From the error message:

   = note: this compiler was built on 2025-05-12; consider upgrading it if it is out of date

[fmease]

This works exactly as intended: All (modern) syntax feature gates apply pre-expansion. This is to prevent the situation where stable users start depending on crates that actually use experimental syntax features behind a cfg since the removal of or any modifications to the latter would still break stable users which would be incompatible with Rust's stability guarantees.

[fmease]

Consider feature auto_traits (auto trait Trait { … }) for example which uses a legacy post-expansion feature gate: This lead to crates like https://github.com/leptos-rs/leptos and https://github.com/PyO3/pyo3 using auto traits behind a "nightly" feature flag¹ while having tons of stable users! Modifying or removing the syntax of auto traits is no longer an option (at least not a straightforward one that can take effect immediately).

We strictly want to avoid this kind of impasse!

Footnotes

1. Disclaimer: Last time I checked out those crates was in 2023, so I don't know if that's still the case. ↩