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

@@ -55,6 +55,7 @@ pub mod hardwired {
             HIDDEN_GLOB_REEXPORTS,
             ILL_FORMED_ATTRIBUTE_INPUT,
             INCOMPLETE_INCLUDE,
+            INEFFECTIVE_UNSTABLE_REEXPORTS,
             INEFFECTIVE_UNSTABLE_TRAIT_IMPL,
             INLINE_NO_SANITIZE,
             INVALID_DOC_ATTRIBUTES,
@@ -2791,6 +2792,38 @@ declare_lint! {
     "detects deprecation attributes with no effect",
 }
 
+declare_lint! {
+    /// The `ineffective_unstable_reexports` lint detects `#[unstable]`
+    /// attributes on re-exports where the attribute does not make the
+    /// re-exported path unstable.
+    ///
+    /// ### Example
+    ///
+    /// ```rust,compile_fail
+    /// #![feature(staged_api)]
+    /// #![stable(feature = "test", since = "1.0.0")]
+    ///
+    /// #[stable(feature = "test", since = "1.0.0")]
+    /// pub struct S;
+    ///
+    /// #[unstable(feature = "reexport", issue = "none")]
+    /// pub use self::S as T;
+    ///
+    /// fn main() {}
+    /// ```
+    ///
+    /// {{produces}}
+    ///
+    /// ### Explanation
+    ///
+    /// `#[unstable]` on a re-export does not currently make an otherwise
+    /// stable re-exported path unstable.
+    pub INEFFECTIVE_UNSTABLE_REEXPORTS,
+    Deny,
+    "detects ineffective `#[unstable]` attributes 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("`#[unstable]` does not make this re-exported path unstable")]
+pub(crate) struct IneffectiveUnstableReexport;
+
 // FIXME(jdonszelmann): move back to rustc_attr
 #[derive(Diagnostic)]
 #[diag(

compiler/rustc_passes/src/stability.rs

@@ -18,7 +18,8 @@ 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, INEFFECTIVE_UNSTABLE_REEXPORTS,
+    INEFFECTIVE_UNSTABLE_TRAIT_IMPL, STABLE_FEATURES,
 };
 use rustc_middle::hir::nested_filter;
 use rustc_middle::middle::lib_features::{FeatureStability, LibFeatures};
@@ -523,7 +524,9 @@ 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, unstable_reexports: FxIndexMap::default() };
+    tcx.hir_visit_item_likes_in_module(mod_id, &mut checker);
+    checker.emit_ineffective_unstable_reexports();
 
     let is_staged_api =
         tcx.sess.opts.unstable_opts.force_unstable_if_unmarked || tcx.features().staged_api();
@@ -553,8 +556,143 @@ pub(crate) fn provide(providers: &mut Providers) {
     };
 }
 
+struct UnstableReexport {
+    hir_id: HirId,
+    span: Span,
+    has_target: bool,
+    all_targets_stable: bool,
+}
+
 struct Checker<'tcx> {
     tcx: TyCtxt<'tcx>,
+    mod_id: LocalModId,
+    unstable_reexports: FxIndexMap<Span, UnstableReexport>,
+}
+
+impl<'tcx> Checker<'tcx> {
+    fn unstable_reexport_span(&self, item: &'tcx hir::Item<'tcx>) -> Option<Span> {
+        let attrs = self.tcx.hir_attrs(item.hir_id());
+        let (stability, span) =
+            find_attr!(attrs, Stability { stability, span } => (*stability, *span))?;
+
+        stability.level.is_unstable().then_some(span)
+    }
+
+    fn classify_reexport_targets<Id>(
+        &self,
+        targets: impl IntoIterator<Item = Res<Id>>,
+    ) -> (bool, bool) {
+        let mut has_target = false;
+        let mut all_targets_stable = true;
+
+        for res in targets {
+            match res {
+                Res::Def(_, def_id) => {
+                    has_target = true;
+
+                    match self.tcx.lookup_stability(def_id) {
+                        Some(stability) if stability.level.is_unstable() => {
+                            all_targets_stable = false;
+                        }
+                        Some(_) => {}
+
+                        None => {
+                            // Items from crates without staged API metadata are
+                            // effectively stable. Unmarked items in staged API
+                            // crates are diagnosed by the existing stability checks.
+                            if self.tcx.lookup_stability(def_id.krate.as_def_id()).is_some() {
+                                all_targets_stable = false;
+                            }
+                        }
+                    }
+                }
+
+                // Primitives are stable and have no DefId.
+                Res::PrimTy(_) => {
+                    has_target = true;
+                }
+
+                // Do not lint if the target cannot be classified.
+                _ => {
+                    all_targets_stable = false;
+                }
+            }
+        }
+
+        (has_target, all_targets_stable)
+    }
+
+    fn record_unstable_reexport(
+        &mut self,
+        item: &'tcx hir::Item<'tcx>,
+        attr_span: Span,
+        span: Span,
+        has_target: bool,
+        all_targets_stable: bool,
+    ) {
+        let entry = self.unstable_reexports.entry(attr_span).or_insert(UnstableReexport {
+            hir_id: item.hir_id(),
+            span,
+            has_target: false,
+            all_targets_stable: true,
+        });
+
+        entry.has_target |= has_target;
+        entry.all_targets_stable &= all_targets_stable;
+    }
+
+    fn check_single_unstable_reexport(
+        &mut self,
+        item: &'tcx hir::Item<'tcx>,
+        path: &'tcx UsePath<'tcx>,
+    ) {
+        let Some(attr_span) = self.unstable_reexport_span(item) else {
+            return;
+        };
+
+        let (has_target, all_targets_stable) =
+            self.classify_reexport_targets(path.res.present_items());
+
+        self.record_unstable_reexport(item, attr_span, path.span, has_target, all_targets_stable);
+    }
+
+    fn check_glob_unstable_reexport(
+        &mut self,
+        item: &'tcx hir::Item<'tcx>,
+        path: &'tcx UsePath<'tcx>,
+    ) {
+        let Some(attr_span) = self.unstable_reexport_span(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| reexport.id() == Some(glob_def_id))
+            })
+            .map(|child| child.res);
+
+        let (has_target, all_targets_stable) = self.classify_reexport_targets(targets);
+
+        self.record_unstable_reexport(item, attr_span, path.span, has_target, all_targets_stable);
+    }
+
+    fn emit_ineffective_unstable_reexports(&self) {
+        for reexport in self.unstable_reexports.values() {
+            if reexport.has_target && reexport.all_targets_stable {
+                self.tcx.emit_node_span_lint(
+                    INEFFECTIVE_UNSTABLE_REEXPORTS,
+                    reexport.hir_id,
+                    reexport.span,
+                    diagnostics::IneffectiveUnstableReexport,
+                );
+            }
+        }
+    }
 }
 
 impl<'tcx> Visitor<'tcx> for Checker<'tcx> {
@@ -583,6 +721,20 @@ 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_unstable_reexport(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_unstable_reexport(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/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)]
+#[expect(ineffective_unstable_reexports, reason = "This re-export has its own stability")]
 #[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)]
+#[expect(ineffective_unstable_reexports, reason = "This re-export has its own stability")]
 #[unstable(feature = "alloc_io", issue = "154046")]
 pub use self::{
     buf_read::BufRead,

library/core/src/io/mod.rs

@@ -20,6 +20,8 @@ pub use self::error::RawOsError;
 pub use self::error::SimpleMessage;
 #[unstable(feature = "io_const_error", issue = "133448")]
 pub use self::error::const_error;
+#[allow(clippy::useless_attribute)]
+#[expect(ineffective_unstable_reexports, reason = "This re-export has its own stability")]
 #[unstable(feature = "core_io", issue = "154046")]
 pub use self::{
     cursor::Cursor,

library/core/src/lib.rs

@@ -227,6 +227,8 @@ pub mod offload;
 #[unstable(feature = "contracts", issue = "128044")]
 pub mod contracts;
 
+#[allow(clippy::useless_attribute)]
+#[expect(ineffective_unstable_reexports, reason = "This re-export has its own stability")]
 #[unstable(feature = "derive_macro_global_path", issue = "154645")]
 pub use crate::macros::builtin::derive;
 #[stable(feature = "cfg_select", since = "1.95.0")]

library/core/src/ops/mod.rs

@@ -156,7 +156,7 @@ mod unsize;
 pub use self::arith::{Add, Div, Mul, Neg, Rem, Sub};
 #[stable(feature = "op_assign_traits", since = "1.8.0")]
 pub use self::arith::{AddAssign, DivAssign, MulAssign, RemAssign, SubAssign};
-#[unstable(feature = "async_fn_traits", issue = "none")]
+#[stable(feature = "async_closure", since = "1.85.0")]
 pub use self::async_function::{AsyncFn, AsyncFnMut, AsyncFnOnce};
 #[stable(feature = "rust1", since = "1.0.0")]
 pub use self::bit::{BitAnd, BitOr, BitXor, Not, Shl, Shr};

library/std/src/lib.rs

@@ -712,8 +712,12 @@ pub mod arch {
     pub use std_detect::is_aarch64_feature_detected;
     #[unstable(feature = "stdarch_arm_feature_detection", issue = "111190")]
     pub use std_detect::is_arm_feature_detected;
+    #[allow(clippy::useless_attribute)]
+    #[expect(ineffective_unstable_reexports, reason = "This re-export has its own stability")]
     #[unstable(feature = "is_loongarch_feature_detected", issue = "117425")]
     pub use std_detect::is_loongarch_feature_detected;
+    #[allow(clippy::useless_attribute)]
+    #[expect(ineffective_unstable_reexports, reason = "This re-export has its own stability")]
     #[unstable(feature = "is_riscv_feature_detected", issue = "111192")]
     pub use std_detect::is_riscv_feature_detected;
     #[stable(feature = "stdarch_s390x_feature_detection", since = "1.93.0")]
@@ -750,6 +754,8 @@ pub use core::cfg_select;
     reason = "`concat_bytes` is not stable enough for use and is subject to change"
 )]
 pub use core::concat_bytes;
+#[allow(clippy::useless_attribute)]
+#[expect(ineffective_unstable_reexports, reason = "This re-export has its own stability")]
 #[unstable(feature = "derive_macro_global_path", issue = "154645")]
 pub use core::derive;
 #[stable(feature = "matches_macro", since = "1.42.0")]

library/std/src/prelude/mod.rs

@@ -187,6 +187,8 @@ pub mod rust_future {
 
     // There are two different panic macros, one in `core` and one in `std`. They are slightly
     // different. For `std` we explicitly want the one defined in `std`.
+    #[allow(clippy::useless_attribute)]
+    #[expect(ineffective_unstable_reexports, reason = "This re-export has its own stability")]
     #[unstable(feature = "prelude_next", issue = "none")]
     pub use super::v1::panic;
 }

tests/ui/feature-gates/feature-gate-ineffective_unstable_reexports.rs

@@ -0,0 +1,7 @@
+//@ check-pass
+//@ normalize-stderr: "(\n)\n$" -> "$1"
+// This lint is only available with `staged_api`.
+#![allow(ineffective_unstable_reexports)]
+//~^ WARNING unknown lint: `ineffective_unstable_reexports`
+
+fn main() {}

tests/ui/feature-gates/feature-gate-ineffective_unstable_reexports.stderr

@@ -0,0 +1,10 @@
+warning: unknown lint: `ineffective_unstable_reexports`
+  --> $DIR/feature-gate-ineffective_unstable_reexports.rs:4:10
+   |
+LL | #![allow(ineffective_unstable_reexports)]
+   |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: the `ineffective_unstable_reexports` lint is unstable
+   = note: `#[warn(unknown_lints)]` on by default
+
+warning: 1 warning emitted

tests/ui/stability-attribute/auxiliary/non-staged-reexport-source.rs

@@ -0,0 +1,4 @@
+#![crate_type = "lib"]
+#![crate_name = "non_staged_reexport_source"]
+
+pub fn stable() {}

tests/ui/stability-attribute/auxiliary/stable-glob-source.rs

@@ -0,0 +1,10 @@
+#![crate_type = "lib"]
+#![crate_name = "stable_glob_source"]
+#![feature(staged_api)]
+#![stable(feature = "stable_glob_source", since = "1.0.0")]
+
+#[stable(feature = "stable_glob_source", since = "1.0.0")]
+pub fn stable_a() {}
+
+#[stable(feature = "stable_glob_source", since = "1.0.0")]
+pub fn stable_b() {}

tests/ui/stability-attribute/auxiliary/unstable-glob-source.rs

@@ -0,0 +1,13 @@
+#![crate_type = "lib"]
+#![feature(staged_api)]
+#![stable(feature = "unstable_glob_source_crate", since = "1.0.0")]
+
+#[unstable(feature = "unstable_glob_source", issue = "none")]
+pub fn unstable_a() {}
+
+#[unstable(
+    feature = "unstable_glob_source",
+    reason = "different reason",
+    issue = "none"
+)]
+pub fn unstable_b() {}