docs/test: add error-docs and UI test for `E0711`

compiler/rustc_error_codes/src/error_codes.rs

@@ -436,6 +436,7 @@ E0713: include_str!("./error_codes/E0713.md"),
 E0714: include_str!("./error_codes/E0714.md"),
 E0715: include_str!("./error_codes/E0715.md"),
 E0716: include_str!("./error_codes/E0716.md"),
+E0711: include_str!("./error_codes/E0711.md"),
 E0717: include_str!("./error_codes/E0717.md"),
 E0718: include_str!("./error_codes/E0718.md"),
 E0719: include_str!("./error_codes/E0719.md"),
@@ -640,7 +641,6 @@ E0791: include_str!("./error_codes/E0791.md"),
 //  E0702, // replaced with a generic attribute input check
 //  E0707, // multiple elided lifetimes used in arguments of `async fn`
 //  E0709, // multiple different lifetimes used in arguments of `async fn`
-    E0711, // a feature has been declared with conflicting stability attributes, internal error code
 //  E0721, // `await` keyword
 //  E0723, // unstable feature in `const` context
 //  E0738, // Removed; errored on `#[track_caller] fn`s in `extern "Rust" { ... }`.

compiler/rustc_error_codes/src/error_codes/E0711.md

@@ -0,0 +1,30 @@
+#### This error code is internal to the compiler and will not be emitted with normal Rust code.
+
+Feature declared with conflicting stability requirements.
+
+```compile_fail,E0711
+// NOTE: this attribute is perma-unstable and should *never* be used outside of
+//       stdlib and the compiler.
+#![feature(staged_api)]
+
+#![stable(feature = "...", since = "1.0.0")]
+
+#[stable(feature = "foo", since = "1.0.0")]
+fn foo_stable_1_0_0() {}
+
+// error: feature `foo` is declared stable since 1.29.0
+#[stable(feature = "foo", since = "1.29.0")]
+fn foo_stable_1_29_0() {}
+
+// error: feature `foo` is declared unstable
+#[unstable(feature = "foo", issue = "none")]
+fn foo_unstable() {}
+```
+
+In the above example, the `foo` feature is first defined to be stable since
+1.0.0, but is then re-declared stable since 1.29.0. This discrepancy in
+versions causes an error. Furthermore, `foo` is then re-declared as unstable,
+again the conflict causes an error.
+
+This error can be fixed by splitting the feature, this allows any
+stability requirements and removes any possibility of conflict.

src/test/ui/error-codes/E0711.rs

@@ -0,0 +1,18 @@
+// copied from: src/test/ui/feature-gates/stability-attribute-consistency.rs
+
+#![feature(staged_api)]
+
+#![stable(feature = "stable_test_feature", since = "1.0.0")]
+
+#[stable(feature = "foo", since = "1.0.0")]
+fn foo_stable_1_0_0() {}
+
+#[stable(feature = "foo", since = "1.29.0")]
+//~^ ERROR feature `foo` is declared stable since 1.29.0
+fn foo_stable_1_29_0() {}
+
+#[unstable(feature = "foo", issue = "none")]
+//~^ ERROR feature `foo` is declared unstable
+fn foo_unstable() {}
+
+fn main() {}

src/test/ui/error-codes/E0711.stderr

@@ -0,0 +1,15 @@
+error[E0711]: feature `foo` is declared stable since 1.29.0, but was previously declared stable since 1.0.0
+  --> $DIR/E0711.rs:10:1
+   |
+LL | #[stable(feature = "foo", since = "1.29.0")]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error[E0711]: feature `foo` is declared unstable, but was previously declared stable
+  --> $DIR/E0711.rs:14:1
+   |
+LL | #[unstable(feature = "foo", issue = "none")]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0711`.