`E0711` emitted even when `staged_api` is not enabled

[Ezrashaw]

I tried this code:

// #![feature(staged_api)] // note: `staged_api` not enabled

#![stable(feature = "foo", since = "1.0.0")]

// error: feature `foo` is declared unstable
#[unstable(feature = "foo", issue = "none")]
fn foo_unstable() {}

I expected to see this happen:

error[E0734]: stability attributes may not be used outside of the standard library
 --> src/lib.rs:6:1
  |
6 | #[unstable(feature = "foo", issue = "none")]
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error[E0734]: stability attributes may not be used outside of the standard library
 --> src/lib.rs:3:1
  |
3 | #![stable(feature = "foo", since = "1.0.0")]
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Two errors emitted about the use of stability attributes outside the standard library (without staged_api).

Instead, this happened:

error[E0734]: stability attributes may not be used outside of the standard library
 --> src/lib.rs:6:1
  |
6 | #[unstable(feature = "foo", issue = "none")]
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error[E0734]: stability attributes may not be used outside of the standard library
 --> src/lib.rs:3:1
  |
3 | #![stable(feature = "foo", since = "1.0.0")]
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

+error[E0711]: feature `foo` is declared unstable, but was previously declared stable
+ --> src/lib.rs:6:1
+  |
+6 | #[unstable(feature = "foo", issue = "none")]
+  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

An extra error relating to stability semantics which should not be emitted because stability attributes are not allowed here. Some permutations of this error exist: is staged_api enabled? are we on stable? etc.

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Occurs on latest stable and nightly.

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