Rollup merge of #97301 - semicoleon:unstable-reexport, r=petrochenkov

Allow unstable items to be re-exported unstably without requiring the feature be enabled

Closes #94972

The diagnostic may need some work still, and I haven't added a test yet

src/test/ui/stability-attribute/allow-unstable-reexport.rs

@@ -0,0 +1,30 @@
+// Allow an unstable re-export without requiring a feature gate.
+// #94972
+
+// aux-build:lint-stability.rs
+// aux-build:lint-stability-reexport.rs
+#![feature(staged_api)]
+#![stable(feature = "lint_stability", since = "1.0.0")]
+
+extern crate lint_stability;
+extern crate lint_stability_reexport;
+
+#[unstable(feature = "unstable_test_feature", issue = "none")]
+pub use lint_stability::unstable;
+
+// We want to confirm that using a re-export through another crate behaves
+// the same way as using an item directly
+#[unstable(feature = "unstable_test_feature", issue = "none")]
+pub use lint_stability_reexport::unstable_text;
+
+// Ensure items which aren't marked as unstable can't re-export unstable items
+#[stable(feature = "lint_stability", since = "1.0.0")]
+pub use lint_stability::unstable as unstable2;
+//~^ ERROR use of unstable library feature 'unstable_test_feature'
+
+fn main() {
+    // Since we didn't enable the feature in this crate, we still can't
+    // use these items, even though they're in scope from the `use`s which are now allowed.
+    unstable(); //~ ERROR use of unstable library feature 'unstable_test_feature'
+    unstable_text(); //~ ERROR use of unstable library feature 'unstable_test_feature'
+}

src/test/ui/stability-attribute/allow-unstable-reexport.stderr

@@ -0,0 +1,27 @@
+error[E0658]: use of unstable library feature 'unstable_test_feature'
+  --> $DIR/allow-unstable-reexport.rs:22:9
+   |
+LL | pub use lint_stability::unstable as unstable2;
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: add `#![feature(unstable_test_feature)]` to the crate attributes to enable
+
+error[E0658]: use of unstable library feature 'unstable_test_feature'
+  --> $DIR/allow-unstable-reexport.rs:28:5
+   |
+LL |     unstable();
+   |     ^^^^^^^^
+   |
+   = help: add `#![feature(unstable_test_feature)]` to the crate attributes to enable
+
+error[E0658]: use of unstable library feature 'unstable_test_feature': text
+  --> $DIR/allow-unstable-reexport.rs:29:5
+   |
+LL |     unstable_text();
+   |     ^^^^^^^^^^^^^
+   |
+   = help: add `#![feature(unstable_test_feature)]` to the crate attributes to enable
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0658`.

src/test/ui/stability-attribute/auxiliary/lint-stability-reexport.rs

@@ -0,0 +1,9 @@
+#![crate_type = "lib"]
+#![feature(staged_api)]
+#![stable(feature = "lint_stability", since = "1.0.0")]
+
+extern crate lint_stability;
+
+// Re-exporting without enabling the feature "unstable_test_feature" in this crate
+#[unstable(feature = "unstable_test_feature", issue = "none")]
+pub use lint_stability::unstable_text;