add UI test + docs for `E0789`

compiler/rustc_error_codes/src/error_codes.rs

@@ -506,6 +506,7 @@ E0785: include_str!("./error_codes/E0785.md"),
 E0786: include_str!("./error_codes/E0786.md"),
 E0787: include_str!("./error_codes/E0787.md"),
 E0788: include_str!("./error_codes/E0788.md"),
+E0789: include_str!("./error_codes/E0789.md"),
 E0790: include_str!("./error_codes/E0790.md"),
 E0791: include_str!("./error_codes/E0791.md"),
 E0792: include_str!("./error_codes/E0792.md"),
@@ -645,5 +646,4 @@ E0792: include_str!("./error_codes/E0792.md"),
 //  E0721, // `await` keyword
 //  E0723, // unstable feature in `const` context
 //  E0738, // Removed; errored on `#[track_caller] fn`s in `extern "Rust" { ... }`.
-    E0789, // rustc_allowed_through_unstable_modules without stability attribute
 }

compiler/rustc_error_codes/src/error_codes/E0789.md

@@ -0,0 +1,30 @@
+#### This error code is internal to the compiler and will not be emitted with normal Rust code.
+
+The internal `rustc_allowed_through_unstable_modules` attribute must be used
+on an item with a `stable` attribute.
+
+Erroneous code example:
+
+```compile_fail,E0789
+// NOTE: both of these attributes are perma-unstable and should *never* be
+//       used outside of the compiler and standard library.
+#![feature(rustc_attrs)]
+#![feature(staged_api)]
+
+#![unstable(feature = "foo_module", reason = "...", issue = "123")]
+
+#[rustc_allowed_through_unstable_modules]
+// #[stable(feature = "foo", since = "1.0")]
+struct Foo;
+// ^^^ error: `rustc_allowed_through_unstable_modules` attribute must be
+//            paired with a `stable` attribute
+```
+
+Typically when an item is marked with a `stable` attribute, the modules that
+enclose the item must also be marked with `stable` attributes, otherwise the
+item becomes *de facto* unstable. `#[rustc_allowed_through_unstable_modules]`
+is a workaround which allows an item to "escape" its unstable parent modules.
+This error occurs when an item is marked with
+`#[rustc_allowed_through_unstable_modules]` but no supplementary `stable`
+attribute exists. See [#99288](https://github.com/rust-lang/rust/pull/99288)
+for an example of `#[rustc_allowed_through_unstable_modules]` in use.

src/tools/tidy/src/error_codes.rs

@@ -31,7 +31,7 @@ const IGNORE_DOCTEST_CHECK: &[&str] = &["E0464", "E0570", "E0601", "E0602", "E06
 
 // Error codes that don't yet have a UI test. This list will eventually be removed.
 const IGNORE_UI_TEST_CHECK: &[&str] =
-    &["E0461", "E0465", "E0476", "E0514", "E0523", "E0554", "E0640", "E0717", "E0729", "E0789"];
+    &["E0461", "E0465", "E0476", "E0514", "E0523", "E0554", "E0640", "E0717", "E0729"];
 
 macro_rules! verbose_print {
     ($verbose:expr, $($fmt:tt)*) => {

tests/ui/error-codes/E0789.rs

@@ -0,0 +1,12 @@
+// compile-flags: --crate-type lib
+
+#![feature(rustc_attrs)]
+#![feature(staged_api)]
+#![unstable(feature = "foo_module", reason = "...", issue = "123")]
+
+#[rustc_allowed_through_unstable_modules]
+// #[stable(feature = "foo", since = "1.0")]
+struct Foo;
+//~^ ERROR `rustc_allowed_through_unstable_modules` attribute must be paired with a `stable` attribute
+//~^^ ERROR `rustc_allowed_through_unstable_modules` attribute must be paired with a `stable` attribute
+// FIXME: we shouldn't have two errors here, only occurs when using `-Zdeduplicate-diagnostics=no`

tests/ui/error-codes/E0789.stderr

@@ -0,0 +1,15 @@
+error[E0789]: `rustc_allowed_through_unstable_modules` attribute must be paired with a `stable` attribute
+  --> $DIR/E0789.rs:9:1
+   |
+LL | struct Foo;
+   | ^^^^^^^^^^^
+
+error[E0789]: `rustc_allowed_through_unstable_modules` attribute must be paired with a `stable` attribute
+  --> $DIR/E0789.rs:9:1
+   |
+LL | struct Foo;
+   | ^^^^^^^^^^^
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0789`.