lint ineffective #[unstable] annotations on re-exports

compiler/rustc_lint/src/levels.rs

@@ -969,14 +969,18 @@ where
                 let mut lint = Diag::new(dcx, level, msg!("unknown lint: `{$name}`"))
                     .with_arg("name", lint_id.lint.name_lower())
                     .with_note(msg!("the `{$name}` lint is unstable"));
-                rustc_session::diagnostics::add_feature_diagnostics_for_issue(
-                    &mut lint,
-                    sess,
-                    feature,
-                    GateIssue::Language,
-                    lint_from_cli,
-                    None,
-                );
+                // `staged_api` is only intended for the standard library, so don't
+                // suggest enabling it just to use this lint.
+                if feature != sym::staged_api {
+                    rustc_session::diagnostics::add_feature_diagnostics_for_issue(
+                        &mut lint,
+                        sess,
+                        feature,
+                        GateIssue::Language,
+                        lint_from_cli,
+                        None,
+                    );
+                }
                 lint
             }
         }

compiler/rustc_lint_defs/src/builtin.rs

@@ -54,6 +54,7 @@ pub mod hardwired {
             FUNCTION_ITEM_REFERENCES,
             HIDDEN_GLOB_REEXPORTS,
             ILL_FORMED_ATTRIBUTE_INPUT,
+            INCOMPATIBLE_REEXPORT_STABILITY,
             INCOMPLETE_INCLUDE,
             INEFFECTIVE_UNSTABLE_TRAIT_IMPL,
             INLINE_NO_SANITIZE,
@@ -2791,6 +2792,41 @@ declare_lint! {
     "detects deprecation attributes with no effect",
 }
 
+declare_lint! {
+    /// The `incompatible_reexport_stability` lint detects stability
+    /// annotations on re-exports that are incompatible with the stability
+    /// metadata of the re-exported item.
+    ///
+    /// ### Example
+    ///
+    /// ```rust,compile_fail
+    /// #![feature(staged_api)]
+    /// #![stable(feature = "test", since = "1.0.0")]
+    ///
+    /// #[stable(feature = "original", since = "1.0.0")]
+    /// pub struct S;
+    ///
+    /// #[stable(feature = "different", since = "1.0.0")]
+    /// pub use self::S as T;
+    ///
+    /// fn main() {}
+    /// ```
+    ///
+    /// {{produces}}
+    ///
+    /// ### Explanation
+    ///
+    /// Stability annotations on re-exports should be compatible with the
+    /// stability metadata of the item being re-exported. Stable metadata is
+    /// compared by feature and `since`, while unstable metadata is compared by
+    /// feature and issue. Stable re-exports of unstable definitions remain
+    /// handled by the existing stability machinery.
+    pub INCOMPATIBLE_REEXPORT_STABILITY,
+    Deny,
+    "detects incompatible stability annotations on re-exports",
+    @feature_gate = staged_api;
+}
+
 declare_lint! {
     /// The `ineffective_unstable_trait_impl` lint detects `#[unstable]` attributes which are not used.
     ///

compiler/rustc_passes/src/diagnostics.rs

@@ -964,6 +964,10 @@ pub(crate) struct UnnecessaryPartialStableFeature {
 #[note("see issue #55436 <https://github.com/rust-lang/rust/issues/55436> for more information")]
 pub(crate) struct IneffectiveUnstableImpl;
 
+#[derive(Diagnostic)]
+#[diag("stability annotation on this re-export does not match the re-exported item")]
+pub(crate) struct IncompatibleReexportStability;
+
 // FIXME(jdonszelmann): move back to rustc_attr
 #[derive(Diagnostic)]
 #[diag(

compiler/rustc_passes/src/stability.rs

@@ -18,9 +18,11 @@ use rustc_hir::{
 };
 use rustc_lint_defs as lint;
 use rustc_lint_defs::builtin::{
-    DEPRECATED, DUPLICATE_FEATURES, INEFFECTIVE_UNSTABLE_TRAIT_IMPL, STABLE_FEATURES,
+    DEPRECATED, DUPLICATE_FEATURES, INCOMPATIBLE_REEXPORT_STABILITY,
+    INEFFECTIVE_UNSTABLE_TRAIT_IMPL, STABLE_FEATURES,
 };
 use rustc_middle::hir::nested_filter;
+use rustc_middle::metadata::Reexport;
 use rustc_middle::middle::lib_features::{FeatureStability, LibFeatures};
 use rustc_middle::middle::privacy::EffectiveVisibilities;
 use rustc_middle::middle::stability::{AllowUnstable, Deprecated, DeprecationEntry, EvalResult};
@@ -523,7 +525,9 @@ impl<'tcx> Visitor<'tcx> for MissingStabilityAnnotations<'tcx> {
 /// Cross-references the feature names of unstable APIs with enabled
 /// features and possibly prints errors.
 fn check_mod_unstable_api_usage(tcx: TyCtxt<'_>, mod_id: LocalModId) {
-    tcx.hir_visit_item_likes_in_module(mod_id, &mut Checker { tcx });
+    let mut checker = Checker { tcx, mod_id, reexport_stability: FxIndexMap::default() };
+    tcx.hir_visit_item_likes_in_module(mod_id, &mut checker);
+    checker.emit_incompatible_reexport_stability();
 
     let is_staged_api =
         tcx.sess.opts.unstable_opts.force_unstable_if_unmarked || tcx.features().staged_api();
@@ -553,8 +557,183 @@ pub(crate) fn provide(providers: &mut Providers) {
     };
 }
 
+struct ReexportStability {
+    hir_id: HirId,
+    span: Span,
+    has_target: bool,
+    all_targets_compatible: bool,
+}
+
 struct Checker<'tcx> {
     tcx: TyCtxt<'tcx>,
+    mod_id: LocalModId,
+    reexport_stability: FxIndexMap<Span, ReexportStability>,
+}
+
+impl<'tcx> Checker<'tcx> {
+    fn reexport_stability_attr(&self, item: &'tcx hir::Item<'tcx>) -> Option<(Stability, Span)> {
+        let attrs = self.tcx.hir_attrs(item.hir_id());
+        find_attr!(attrs, Stability { stability, span } => (*stability, *span))
+    }
+
+    fn stability_is_compatible(reexport: &Stability, target: &Stability) -> bool {
+        match (&reexport.level, &target.level) {
+            (
+                StabilityLevel::Stable { since: reexport_since, .. },
+                StabilityLevel::Stable { since: target_since, .. },
+            ) => {
+                // Avoid another error for an invalid `since`.
+                matches!(
+                    (*reexport_since, *target_since),
+                    (StableSince::Err(_), _) | (_, StableSince::Err(_))
+                ) || (reexport.feature == target.feature && reexport_since == target_since)
+            }
+
+            (
+                StabilityLevel::Unstable { issue: reexport_issue, .. },
+                StabilityLevel::Unstable { issue: target_issue, .. },
+            ) => reexport.feature == target.feature && reexport_issue == target_issue,
+
+            // An unstable re-export cannot make a stable item unstable.
+            (StabilityLevel::Unstable { .. }, StabilityLevel::Stable { .. }) => false,
+
+            // Stable re-exports of unstable items are handled elsewhere.
+            (StabilityLevel::Stable { .. }, StabilityLevel::Unstable { .. }) => true,
+        }
+    }
+
+    fn classify_reexport_targets<Id>(
+        &self,
+        own_stability: &Stability,
+        targets: impl IntoIterator<Item = Res<Id>>,
+    ) -> (bool, bool) {
+        let mut has_target = false;
+        let mut all_targets_compatible = true;
+
+        for res in targets {
+            match res {
+                Res::Def(_, def_id) => {
+                    has_target = true;
+
+                    if let Some(target_stability) = self.tcx.lookup_stability(def_id)
+                        && !Self::stability_is_compatible(own_stability, &target_stability)
+                    {
+                        all_targets_compatible = false;
+                    }
+                }
+
+                Res::PrimTy(_) => {
+                    has_target = true;
+
+                    // Primitives are stable and have no DefId.
+                    if own_stability.level.is_unstable() {
+                        all_targets_compatible = false;
+                    }
+                }
+
+                // No stability metadata to compare.
+                _ => {}
+            }
+        }
+
+        (has_target, all_targets_compatible)
+    }
+
+    fn record_reexport_stability(
+        &mut self,
+        item: &'tcx hir::Item<'tcx>,
+        attr_span: Span,
+        span: Span,
+        has_target: bool,
+        all_targets_compatible: bool,
+    ) {
+        let entry = self.reexport_stability.entry(attr_span).or_insert(ReexportStability {
+            hir_id: item.hir_id(),
+            span,
+            has_target: false,
+            all_targets_compatible: true,
+        });
+
+        entry.has_target |= has_target;
+
+        // Keep the first bad path for the diagnostic.
+        if entry.all_targets_compatible && !all_targets_compatible {
+            entry.span = span;
+        }
+
+        entry.all_targets_compatible &= all_targets_compatible;
+    }
+
+    fn check_single_reexport_stability(
+        &mut self,
+        item: &'tcx hir::Item<'tcx>,
+        path: &'tcx UsePath<'tcx>,
+    ) {
+        let Some((own_stability, attr_span)) = self.reexport_stability_attr(item) else {
+            return;
+        };
+
+        let (has_target, all_targets_compatible) =
+            self.classify_reexport_targets(&own_stability, path.res.present_items());
+
+        self.record_reexport_stability(
+            item,
+            attr_span,
+            path.span,
+            has_target,
+            all_targets_compatible,
+        );
+    }
+
+    fn check_glob_reexport_stability(
+        &mut self,
+        item: &'tcx hir::Item<'tcx>,
+        path: &'tcx UsePath<'tcx>,
+    ) {
+        let Some((own_stability, attr_span)) = self.reexport_stability_attr(item) else {
+            return;
+        };
+
+        let glob_def_id = item.owner_id.def_id.to_def_id();
+
+        let targets = self
+            .tcx
+            .module_children_local(self.mod_id.to_local_def_id())
+            .iter()
+            .filter(|child| {
+                child.reexport_chain.iter().any(|reexport| {
+                    matches!(
+                        *reexport,
+                        Reexport::Glob(def_id) if def_id == glob_def_id
+                    )
+                })
+            })
+            .map(|child| child.res);
+
+        let (has_target, all_targets_compatible) =
+            self.classify_reexport_targets(&own_stability, targets);
+
+        self.record_reexport_stability(
+            item,
+            attr_span,
+            path.span,
+            has_target,
+            all_targets_compatible,
+        );
+    }
+
+    fn emit_incompatible_reexport_stability(&self) {
+        for reexport in self.reexport_stability.values() {
+            if reexport.has_target && !reexport.all_targets_compatible {
+                self.tcx.emit_node_span_lint(
+                    INCOMPATIBLE_REEXPORT_STABILITY,
+                    reexport.hir_id,
+                    reexport.span,
+                    diagnostics::IncompatibleReexportStability,
+                );
+            }
+        }
+    }
 }
 
 impl<'tcx> Visitor<'tcx> for Checker<'tcx> {
@@ -583,6 +762,20 @@ impl<'tcx> Visitor<'tcx> for Checker<'tcx> {
                 self.tcx.check_stability(def_id, Some(item.hir_id()), item.span, None);
             }
 
+            hir::ItemKind::Use(path, hir::UseKind::Single(_))
+                if self.tcx.features().staged_api()
+                    && self.tcx.local_visibility(item.owner_id.def_id).is_public() =>
+            {
+                self.check_single_reexport_stability(item, path);
+            }
+
+            hir::ItemKind::Use(path, hir::UseKind::Glob)
+                if self.tcx.features().staged_api()
+                    && self.tcx.local_visibility(item.owner_id.def_id).is_public() =>
+            {
+                self.check_glob_reexport_stability(item, path);
+            }
+
             // For implementations of traits, check the stability of each item
             // individually as it's possible to have a stable trait with unstable
             // items.

library/alloc/src/alloc.rs

@@ -4,6 +4,8 @@
 
 #[stable(feature = "alloc_module", since = "1.28.0")]
 #[doc(inline)]
+#[allow(clippy::useless_attribute)]
+#[allow(incompatible_reexport_stability)] // This facade has its own path stability.
 pub use core::alloc::*;
 use core::mem::Alignment;
 use core::ptr::{self, NonNull};

library/alloc/src/collections/mod.rs

@@ -20,6 +20,8 @@ pub mod btree_map {
     //! An ordered map based on a B-Tree.
     #[stable(feature = "rust1", since = "1.0.0")]
     #[cfg(not(test))]
+    #[allow(clippy::useless_attribute)]
+    #[allow(incompatible_reexport_stability)] // Public facade over internal B-tree items.
     pub use super::btree::map::*;
 }
 
@@ -29,6 +31,8 @@ pub mod btree_set {
     //! An ordered set based on a B-Tree.
     #[stable(feature = "rust1", since = "1.0.0")]
     #[cfg(not(test))]
+    #[allow(clippy::useless_attribute)]
+    #[allow(incompatible_reexport_stability)] // Public facade over internal B-tree items.
     pub use super::btree::set::*;
 }
 

library/alloc/src/fmt.rs

@@ -600,10 +600,14 @@ pub use core::fmt::{Arguments, write};
 #[stable(feature = "rust1", since = "1.0.0")]
 pub use core::fmt::{Binary, Octal};
 #[stable(feature = "rust1", since = "1.0.0")]
+#[allow(clippy::useless_attribute)]
+#[allow(incompatible_reexport_stability)] // This re-export has its own stability.
 pub use core::fmt::{Debug, Display};
 #[unstable(feature = "formatting_options", issue = "118117")]
 pub use core::fmt::{DebugAsHex, FormattingOptions, Sign};
 #[stable(feature = "rust1", since = "1.0.0")]
+#[allow(clippy::useless_attribute)]
+#[allow(incompatible_reexport_stability)] // This re-export has its own stability.
 pub use core::fmt::{DebugList, DebugMap, DebugSet, DebugStruct, DebugTuple};
 #[stable(feature = "rust1", since = "1.0.0")]
 pub use core::fmt::{Formatter, Result, Write};

library/alloc/src/io/mod.rs

@@ -194,6 +194,8 @@ pub use core::io::SimpleMessage;
 pub use core::io::const_error;
 #[unstable(feature = "core_io_borrowed_buf", issue = "117693")]
 pub use core::io::{BorrowedBuf, BorrowedCursor};
+#[allow(clippy::useless_attribute)]
+#[allow(incompatible_reexport_stability)] // FIXME(#161153)
 #[unstable(feature = "alloc_io", issue = "154046")]
 pub use core::io::{
     Chain, Cursor, Empty, Error, ErrorKind, IoSlice, IoSliceMut, Repeat, Result, Seek, SeekFrom,
@@ -209,6 +211,8 @@ use core::io::{
 
 use self::read::{append_to_string, default_read_buf_exact, default_read_exact};
 use self::util::{bytes, lines, split, uninlined_slow_read_byte};
+#[allow(clippy::useless_attribute)]
+#[allow(incompatible_reexport_stability)] // FIXME(#161153)
 #[unstable(feature = "alloc_io", issue = "154046")]
 pub use self::{
     buf_read::BufRead,