diff --git a/compiler/rustc_attr/src/builtin.rs b/compiler/rustc_attr/src/builtin.rs
index 6673d75d99df0..dcfbecedfe8ff 100644
--- a/compiler/rustc_attr/src/builtin.rs
+++ b/compiler/rustc_attr/src/builtin.rs
@@ -137,7 +137,7 @@ impl ConstStability {
 pub enum StabilityLevel {
     // Reason for the current stability level and the relevant rust-lang issue
     Unstable { reason: Option<Symbol>, issue: Option<NonZeroU32>, is_soft: bool },
-    Stable { since: Symbol },
+    Stable { since: Symbol, allowed_through_unstable_modules: bool },
 }
 
 impl StabilityLevel {
@@ -172,6 +172,7 @@ where
     let mut stab: Option<(Stability, Span)> = None;
     let mut const_stab: Option<(ConstStability, Span)> = None;
     let mut promotable = false;
+    let mut allowed_through_unstable_modules = false;
 
     let diagnostic = &sess.parse_sess.span_diagnostic;
 
@@ -182,6 +183,7 @@ where
             sym::unstable,
             sym::stable,
             sym::rustc_promotable,
+            sym::rustc_allowed_through_unstable_modules,
         ]
         .iter()
         .any(|&s| attr.has_name(s))
@@ -193,6 +195,8 @@ where
 
         if attr.has_name(sym::rustc_promotable) {
             promotable = true;
+        } else if attr.has_name(sym::rustc_allowed_through_unstable_modules) {
+            allowed_through_unstable_modules = true;
         }
         // attributes with data
         else if let Some(MetaItem { kind: MetaItemKind::List(ref metas), .. }) = meta {
@@ -406,7 +410,7 @@ where
 
                     match (feature, since) {
                         (Some(feature), Some(since)) => {
-                            let level = Stable { since };
+                            let level = Stable { since, allowed_through_unstable_modules: false };
                             if sym::stable == meta_name {
                                 stab = Some((Stability { level, feature }, attr.span));
                             } else {
@@ -447,6 +451,27 @@ where
         }
     }
 
+    if allowed_through_unstable_modules {
+        if let Some((
+            Stability {
+                level: StabilityLevel::Stable { ref mut allowed_through_unstable_modules, .. },
+                ..
+            },
+            _,
+        )) = stab
+        {
+            *allowed_through_unstable_modules = true;
+        } else {
+            struct_span_err!(
+                diagnostic,
+                item_sp,
+                E0789,
+                "`rustc_allowed_through_unstable_modules` attribute must be paired with a `stable` attribute"
+            )
+            .emit();
+        }
+    }
+
     (stab, const_stab)
 }
 
diff --git a/compiler/rustc_error_codes/src/error_codes.rs b/compiler/rustc_error_codes/src/error_codes.rs
index d507293ccb0d7..977318b85895e 100644
--- a/compiler/rustc_error_codes/src/error_codes.rs
+++ b/compiler/rustc_error_codes/src/error_codes.rs
@@ -644,4 +644,5 @@ E0788: include_str!("./error_codes/E0788.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
 }
diff --git a/compiler/rustc_feature/src/builtin_attrs.rs b/compiler/rustc_feature/src/builtin_attrs.rs
index 6fcdfe44d8f64..c806df8214586 100644
--- a/compiler/rustc_feature/src/builtin_attrs.rs
+++ b/compiler/rustc_feature/src/builtin_attrs.rs
@@ -512,6 +512,9 @@ pub const BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
         allow_internal_unsafe, Normal, template!(Word), WarnFollowing,
         "allow_internal_unsafe side-steps the unsafe_code lint",
     ),
+    rustc_attr!(rustc_allowed_through_unstable_modules, Normal, template!(Word), WarnFollowing,
+    "rustc_allowed_through_unstable_modules special cases accidental stabilizations of stable items \
+    through unstable paths"),
 
     // ==========================================================================
     // Internal attributes: Type system related:
diff --git a/compiler/rustc_middle/src/middle/stability.rs b/compiler/rustc_middle/src/middle/stability.rs
index 2de8d3180906b..96e068a36012a 100644
--- a/compiler/rustc_middle/src/middle/stability.rs
+++ b/compiler/rustc_middle/src/middle/stability.rs
@@ -471,13 +471,15 @@ impl<'tcx> TyCtxt<'tcx> {
     ///
     /// This function will also check if the item is deprecated.
     /// If so, and `id` is not `None`, a deprecated lint attached to `id` will be emitted.
+    ///
+    /// Returns `true` if item is allowed aka, stable or unstable under an enabled feature.
     pub fn check_stability(
         self,
         def_id: DefId,
         id: Option<HirId>,
         span: Span,
         method_span: Option<Span>,
-    ) {
+    ) -> bool {
         self.check_stability_allow_unstable(def_id, id, span, method_span, AllowUnstable::No)
     }
 
@@ -490,6 +492,8 @@ impl<'tcx> TyCtxt<'tcx> {
     /// If so, and `id` is not `None`, a deprecated lint attached to `id` will be emitted.
     ///
     /// Pass `AllowUnstable::Yes` to `allow_unstable` to force an unstable item to be allowed. Deprecation warnings will be emitted normally.
+    ///
+    /// Returns `true` if item is allowed aka, stable or unstable under an enabled feature.
     pub fn check_stability_allow_unstable(
         self,
         def_id: DefId,
@@ -497,7 +501,7 @@ impl<'tcx> TyCtxt<'tcx> {
         span: Span,
         method_span: Option<Span>,
         allow_unstable: AllowUnstable,
-    ) {
+    ) -> bool {
         self.check_optional_stability(
             def_id,
             id,
@@ -516,6 +520,8 @@ impl<'tcx> TyCtxt<'tcx> {
     /// missing stability attributes (not necessarily just emit a `bug!`). This is necessary
     /// for default generic parameters, which only have stability attributes if they were
     /// added after the type on which they're defined.
+    ///
+    /// Returns `true` if item is allowed aka, stable or unstable under an enabled feature.
     pub fn check_optional_stability(
         self,
         def_id: DefId,
@@ -524,13 +530,16 @@ impl<'tcx> TyCtxt<'tcx> {
         method_span: Option<Span>,
         allow_unstable: AllowUnstable,
         unmarked: impl FnOnce(Span, DefId),
-    ) {
+    ) -> bool {
         let soft_handler = |lint, span, msg: &_| {
             self.struct_span_lint_hir(lint, id.unwrap_or(hir::CRATE_HIR_ID), span, |lint| {
                 lint.build(msg).emit();
             })
         };
-        match self.eval_stability_allow_unstable(def_id, id, span, method_span, allow_unstable) {
+        let eval_result =
+            self.eval_stability_allow_unstable(def_id, id, span, method_span, allow_unstable);
+        let is_allowed = matches!(eval_result, EvalResult::Allow);
+        match eval_result {
             EvalResult::Allow => {}
             EvalResult::Deny { feature, reason, issue, suggestion, is_soft } => report_unstable(
                 self.sess,
@@ -544,6 +553,8 @@ impl<'tcx> TyCtxt<'tcx> {
             ),
             EvalResult::Unmarked => unmarked(span, def_id),
         }
+
+        is_allowed
     }
 
     pub fn lookup_deprecation(self, id: DefId) -> Option<Deprecation> {
diff --git a/compiler/rustc_passes/src/check_attr.rs b/compiler/rustc_passes/src/check_attr.rs
index f3d8a09a297ce..e626a1e4ed101 100644
--- a/compiler/rustc_passes/src/check_attr.rs
+++ b/compiler/rustc_passes/src/check_attr.rs
@@ -139,6 +139,7 @@ impl CheckAttrVisitor<'_> {
                 | sym::rustc_const_stable
                 | sym::unstable
                 | sym::stable
+                | sym::rustc_allowed_through_unstable_modules
                 | sym::rustc_promotable => self.check_stability_promotable(&attr, span, target),
                 _ => true,
             };
diff --git a/compiler/rustc_passes/src/stability.rs b/compiler/rustc_passes/src/stability.rs
index 77c8f7d2713b7..4e091c5b70d5c 100644
--- a/compiler/rustc_passes/src/stability.rs
+++ b/compiler/rustc_passes/src/stability.rs
@@ -1,6 +1,7 @@
 //! A pass that annotates every item and method with its stability level,
 //! propagating default levels lexically from parent to children ast nodes.
 
+use attr::StabilityLevel;
 use rustc_attr::{self as attr, ConstStability, Stability};
 use rustc_data_structures::fx::{FxHashSet, FxIndexMap};
 use rustc_errors::struct_span_err;
@@ -223,7 +224,7 @@ impl<'a, 'tcx> Annotator<'a, 'tcx> {
 
             // Check if deprecated_since < stable_since. If it is,
             // this is *almost surely* an accident.
-            if let (&Some(dep_since), &attr::Stable { since: stab_since }) =
+            if let (&Some(dep_since), &attr::Stable { since: stab_since, .. }) =
                 (&depr.as_ref().and_then(|(d, _)| d.since), &stab.level)
             {
                 // Explicit version of iter::order::lt to handle parse errors properly
@@ -773,7 +774,7 @@ impl<'tcx> Visitor<'tcx> for Checker<'tcx> {
     fn visit_path(&mut self, path: &'tcx hir::Path<'tcx>, id: hir::HirId) {
         if let Some(def_id) = path.res.opt_def_id() {
             let method_span = path.segments.last().map(|s| s.ident.span);
-            self.tcx.check_stability_allow_unstable(
+            let item_is_allowed = self.tcx.check_stability_allow_unstable(
                 def_id,
                 Some(id),
                 path.span,
@@ -783,8 +784,52 @@ impl<'tcx> Visitor<'tcx> for Checker<'tcx> {
                 } else {
                     AllowUnstable::No
                 },
-            )
+            );
+
+            let is_allowed_through_unstable_modules = |def_id| {
+                self.tcx
+                    .lookup_stability(def_id)
+                    .map(|stab| match stab.level {
+                        StabilityLevel::Stable { allowed_through_unstable_modules, .. } => {
+                            allowed_through_unstable_modules
+                        }
+                        _ => false,
+                    })
+                    .unwrap_or(false)
+            };
+
+            if item_is_allowed && !is_allowed_through_unstable_modules(def_id) {
+                // Check parent modules stability as well if the item the path refers to is itself
+                // stable. We only emit warnings for unstable path segments if the item is stable
+                // or allowed because stability is often inherited, so the most common case is that
+                // both the segments and the item are unstable behind the same feature flag.
+                //
+                // We check here rather than in `visit_path_segment` to prevent visiting the last
+                // path segment twice
+                //
+                // We include special cases via #[rustc_allowed_through_unstable_modules] for items
+                // that were accidentally stabilized through unstable paths before this check was
+                // added, such as `core::intrinsics::transmute`
+                let parents = path.segments.iter().rev().skip(1);
+                for path_segment in parents {
+                    if let Some(def_id) = path_segment.res.as_ref().and_then(Res::opt_def_id) {
+                        // use `None` for id to prevent deprecation check
+                        self.tcx.check_stability_allow_unstable(
+                            def_id,
+                            None,
+                            path.span,
+                            None,
+                            if is_unstable_reexport(self.tcx, id) {
+                                AllowUnstable::Yes
+                            } else {
+                                AllowUnstable::No
+                            },
+                        );
+                    }
+                }
+            }
         }
+
         intravisit::walk_path(self, path)
     }
 }
diff --git a/compiler/rustc_span/src/symbol.rs b/compiler/rustc_span/src/symbol.rs
index dd0732387be11..0c271c047091f 100644
--- a/compiler/rustc_span/src/symbol.rs
+++ b/compiler/rustc_span/src/symbol.rs
@@ -1191,6 +1191,7 @@ symbols! {
         rustc_allocator_nounwind,
         rustc_allow_const_fn_unstable,
         rustc_allow_incoherent_impl,
+        rustc_allowed_through_unstable_modules,
         rustc_attrs,
         rustc_box,
         rustc_builtin_macro,
diff --git a/compiler/rustc_typeck/src/astconv/mod.rs b/compiler/rustc_typeck/src/astconv/mod.rs
index 1d4e64b6bfc30..699237446cf77 100644
--- a/compiler/rustc_typeck/src/astconv/mod.rs
+++ b/compiler/rustc_typeck/src/astconv/mod.rs
@@ -439,7 +439,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
                                 // as the rest of the type. As such, we ignore missing
                                 // stability attributes.
                             },
-                        )
+                        );
                     }
                     if let (hir::TyKind::Infer, false) = (&ty.kind, self.astconv.allow_ty_infer()) {
                         self.inferred_params.push(ty.span);
diff --git a/library/core/src/intrinsics.rs b/library/core/src/intrinsics.rs
index 2895c923adc13..9097ffc2cc5a7 100644
--- a/library/core/src/intrinsics.rs
+++ b/library/core/src/intrinsics.rs
@@ -1457,6 +1457,7 @@ extern "rust-intrinsic" {
     /// }
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
+    #[cfg_attr(not(bootstrap), rustc_allowed_through_unstable_modules)]
     #[rustc_const_stable(feature = "const_transmute", since = "1.56.0")]
     #[rustc_diagnostic_item = "transmute"]
     pub fn transmute<T, U>(e: T) -> U;
@@ -2649,6 +2650,7 @@ pub const unsafe fn write_bytes<T>(dst: *mut T, val: u8, count: usize) {
 /// Here is an example of how this could cause a problem:
 /// ```no_run
 /// #![feature(const_eval_select)]
+/// #![feature(core_intrinsics)]
 /// use std::hint::unreachable_unchecked;
 /// use std::intrinsics::const_eval_select;
 ///
diff --git a/library/core/src/macros/mod.rs b/library/core/src/macros/mod.rs
index 673a39c298f7d..bd62bc5c3056c 100644
--- a/library/core/src/macros/mod.rs
+++ b/library/core/src/macros/mod.rs
@@ -1537,7 +1537,7 @@ pub(crate) mod builtin {
     /// Unstable implementation detail of the `rustc` compiler, do not use.
     #[rustc_builtin_macro]
     #[stable(feature = "rust1", since = "1.0.0")]
-    #[allow_internal_unstable(core_intrinsics, libstd_sys_internals)]
+    #[allow_internal_unstable(core_intrinsics, libstd_sys_internals, rt)]
     #[deprecated(since = "1.52.0", note = "rustc-serialize is deprecated and no longer supported")]
     #[doc(hidden)] // While technically stable, using it is unstable, and deprecated. Hide it.
     pub macro RustcDecodable($item:item) {
@@ -1547,7 +1547,7 @@ pub(crate) mod builtin {
     /// Unstable implementation detail of the `rustc` compiler, do not use.
     #[rustc_builtin_macro]
     #[stable(feature = "rust1", since = "1.0.0")]
-    #[allow_internal_unstable(core_intrinsics)]
+    #[allow_internal_unstable(core_intrinsics, rt)]
     #[deprecated(since = "1.52.0", note = "rustc-serialize is deprecated and no longer supported")]
     #[doc(hidden)] // While technically stable, using it is unstable, and deprecated. Hide it.
     pub macro RustcEncodable($item:item) {
diff --git a/library/core/tests/unicode.rs b/library/core/tests/unicode.rs
index c28ea859115e1..bbace0ef66ca3 100644
--- a/library/core/tests/unicode.rs
+++ b/library/core/tests/unicode.rs
@@ -1,5 +1,5 @@
 #[test]
 pub fn version() {
-    let (major, _minor, _update) = core::unicode::UNICODE_VERSION;
+    let (major, _minor, _update) = core::char::UNICODE_VERSION;
     assert!(major >= 10);
 }
diff --git a/library/std/src/lib.rs b/library/std/src/lib.rs
index 244597950fa01..aa309065c88f2 100644
--- a/library/std/src/lib.rs
+++ b/library/std/src/lib.rs
@@ -214,7 +214,7 @@
 #![cfg_attr(not(bootstrap), deny(ffi_unwind_calls))]
 // std may use features in a platform-specific way
 #![allow(unused_features)]
-#![cfg_attr(test, feature(internal_output_capture, print_internals, update_panic_count))]
+#![cfg_attr(test, feature(internal_output_capture, print_internals, update_panic_count, rt))]
 #![cfg_attr(
     all(target_vendor = "fortanix", target_env = "sgx"),
     feature(slice_index_methods, coerce_unsized, sgx_platform)
@@ -297,6 +297,7 @@
 // Library features (alloc):
 #![feature(alloc_layout_extra)]
 #![feature(alloc_c_string)]
+#![feature(alloc_ffi)]
 #![feature(allocator_api)]
 #![feature(get_mut_unchecked)]
 #![feature(map_try_insert)]
diff --git a/library/std/src/panic.rs b/library/std/src/panic.rs
index ac16f476143ce..45bc56efb3b99 100644
--- a/library/std/src/panic.rs
+++ b/library/std/src/panic.rs
@@ -11,7 +11,7 @@ use crate::thread::Result;
 
 #[doc(hidden)]
 #[unstable(feature = "edition_panic", issue = "none", reason = "use panic!() instead")]
-#[allow_internal_unstable(libstd_sys_internals, const_format_args, core_panic)]
+#[allow_internal_unstable(libstd_sys_internals, const_format_args, core_panic, rt)]
 #[cfg_attr(not(test), rustc_diagnostic_item = "std_panic_2015_macro")]
 #[rustc_macro_transparency = "semitransparent"]
 pub macro panic_2015 {
diff --git a/src/librustdoc/html/render/mod.rs b/src/librustdoc/html/render/mod.rs
index 459b0fed6e872..c1fdece9ec6da 100644
--- a/src/librustdoc/html/render/mod.rs
+++ b/src/librustdoc/html/render/mod.rs
@@ -854,7 +854,7 @@ fn render_stability_since_raw(
     }
 
     let const_title_and_stability = match const_stability {
-        Some(ConstStability { level: StabilityLevel::Stable { since }, .. })
+        Some(ConstStability { level: StabilityLevel::Stable { since, .. }, .. })
             if Some(since) != containing_const_ver =>
         {
             Some((format!("const since {}", since), format!("const: {}", since)))
diff --git a/src/test/codegen/intrinsics/const_eval_select.rs b/src/test/codegen/intrinsics/const_eval_select.rs
index 34e653b4b9dd4..db8a04763d35d 100644
--- a/src/test/codegen/intrinsics/const_eval_select.rs
+++ b/src/test/codegen/intrinsics/const_eval_select.rs
@@ -2,6 +2,7 @@
 
 #![crate_type = "lib"]
 #![feature(const_eval_select)]
+#![feature(core_intrinsics)]
 
 use std::intrinsics::const_eval_select;
 
diff --git a/src/test/ui/intrinsics/const-eval-select-bad.rs b/src/test/ui/intrinsics/const-eval-select-bad.rs
index 7d924e2b7f366..52f4e594f1a0a 100644
--- a/src/test/ui/intrinsics/const-eval-select-bad.rs
+++ b/src/test/ui/intrinsics/const-eval-select-bad.rs
@@ -1,4 +1,5 @@
 #![feature(const_eval_select)]
+#![feature(core_intrinsics)]
 
 use std::intrinsics::const_eval_select;
 
diff --git a/src/test/ui/intrinsics/const-eval-select-bad.stderr b/src/test/ui/intrinsics/const-eval-select-bad.stderr
index 1d3bff3a724ba..6103d6c6e3a6f 100644
--- a/src/test/ui/intrinsics/const-eval-select-bad.stderr
+++ b/src/test/ui/intrinsics/const-eval-select-bad.stderr
@@ -1,18 +1,18 @@
-error[E0277]: the trait bound `[closure@$DIR/const-eval-select-bad.rs:6:27: 6:29]: ~const FnOnce<()>` is not satisfied
-  --> $DIR/const-eval-select-bad.rs:6:27
+error[E0277]: the trait bound `[closure@$DIR/const-eval-select-bad.rs:7:27: 7:29]: ~const FnOnce<()>` is not satisfied
+  --> $DIR/const-eval-select-bad.rs:7:27
    |
 LL |     const_eval_select((), || {}, || {});
-   |     -----------------     ^^^^^ expected an `FnOnce<()>` closure, found `[closure@$DIR/const-eval-select-bad.rs:6:27: 6:29]`
+   |     -----------------     ^^^^^ expected an `FnOnce<()>` closure, found `[closure@$DIR/const-eval-select-bad.rs:7:27: 7:29]`
    |     |
    |     required by a bound introduced by this call
    |
-   = help: the trait `~const FnOnce<()>` is not implemented for `[closure@$DIR/const-eval-select-bad.rs:6:27: 6:29]`
-note: the trait `FnOnce<()>` is implemented for `[closure@$DIR/const-eval-select-bad.rs:6:27: 6:29]`, but that implementation is not `const`
-  --> $DIR/const-eval-select-bad.rs:6:27
+   = help: the trait `~const FnOnce<()>` is not implemented for `[closure@$DIR/const-eval-select-bad.rs:7:27: 7:29]`
+note: the trait `FnOnce<()>` is implemented for `[closure@$DIR/const-eval-select-bad.rs:7:27: 7:29]`, but that implementation is not `const`
+  --> $DIR/const-eval-select-bad.rs:7:27
    |
 LL |     const_eval_select((), || {}, || {});
    |                           ^^^^^
-   = note: wrap the `[closure@$DIR/const-eval-select-bad.rs:6:27: 6:29]` in a closure with no arguments: `|| { /* code */ }`
+   = note: wrap the `[closure@$DIR/const-eval-select-bad.rs:7:27: 7:29]` in a closure with no arguments: `|| { /* code */ }`
 note: required by a bound in `const_eval_select`
   --> $SRC_DIR/core/src/intrinsics.rs:LL:COL
    |
@@ -20,7 +20,7 @@ LL |     F: ~const FnOnce<ARG, Output = RET>,
    |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `const_eval_select`
 
 error[E0277]: the trait bound `{integer}: ~const FnOnce<()>` is not satisfied
-  --> $DIR/const-eval-select-bad.rs:8:27
+  --> $DIR/const-eval-select-bad.rs:9:27
    |
 LL |     const_eval_select((), 42, 0xDEADBEEF);
    |     -----------------     ^^ expected an `FnOnce<()>` closure, found `{integer}`
@@ -36,7 +36,7 @@ LL |     F: ~const FnOnce<ARG, Output = RET>,
    |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `const_eval_select`
 
 error[E0277]: expected a `FnOnce<()>` closure, found `{integer}`
-  --> $DIR/const-eval-select-bad.rs:8:31
+  --> $DIR/const-eval-select-bad.rs:9:31
    |
 LL |     const_eval_select((), 42, 0xDEADBEEF);
    |     -----------------         ^^^^^^^^^^ expected an `FnOnce<()>` closure, found `{integer}`
@@ -52,7 +52,7 @@ LL |     G: FnOnce<ARG, Output = RET> + ~const Destruct,
    |        ^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `const_eval_select`
 
 error[E0271]: type mismatch resolving `<fn(i32) -> bool {bar} as FnOnce<(i32,)>>::Output == i32`
-  --> $DIR/const-eval-select-bad.rs:28:5
+  --> $DIR/const-eval-select-bad.rs:29:5
    |
 LL |     const_eval_select((1,), foo, bar);
    |     ^^^^^^^^^^^^^^^^^ expected `i32`, found `bool`
@@ -64,7 +64,7 @@ LL |     G: FnOnce<ARG, Output = RET> + ~const Destruct,
    |                    ^^^^^^^^^^^^ required by this bound in `const_eval_select`
 
 error[E0631]: type mismatch in function arguments
-  --> $DIR/const-eval-select-bad.rs:33:32
+  --> $DIR/const-eval-select-bad.rs:34:32
    |
 LL | const fn foo(n: i32) -> i32 {
    | --------------------------- found signature of `fn(i32) -> _`
diff --git a/src/test/ui/intrinsics/const-eval-select-stability.rs b/src/test/ui/intrinsics/const-eval-select-stability.rs
index db2462aee5922..f9554decec16b 100644
--- a/src/test/ui/intrinsics/const-eval-select-stability.rs
+++ b/src/test/ui/intrinsics/const-eval-select-stability.rs
@@ -1,5 +1,6 @@
 #![feature(staged_api)]
 #![feature(const_eval_select)]
+#![feature(core_intrinsics)]
 #![stable(since = "1.0", feature = "ui_test")]
 
 use std::intrinsics::const_eval_select;
diff --git a/src/test/ui/intrinsics/const-eval-select-stability.stderr b/src/test/ui/intrinsics/const-eval-select-stability.stderr
index 79641bbb46abe..65b507b887b46 100644
--- a/src/test/ui/intrinsics/const-eval-select-stability.stderr
+++ b/src/test/ui/intrinsics/const-eval-select-stability.stderr
@@ -1,5 +1,5 @@
 error: `const_eval_select` is not yet stable as a const fn
-  --> $DIR/const-eval-select-stability.rs:16:5
+  --> $DIR/const-eval-select-stability.rs:17:5
    |
 LL |     const_eval_select((), nothing, log);
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
diff --git a/src/test/ui/intrinsics/const-eval-select-x86_64.rs b/src/test/ui/intrinsics/const-eval-select-x86_64.rs
index afec8e054bb76..f3924acf0fa8b 100644
--- a/src/test/ui/intrinsics/const-eval-select-x86_64.rs
+++ b/src/test/ui/intrinsics/const-eval-select-x86_64.rs
@@ -2,6 +2,7 @@
 // only-x86_64
 
 #![feature(const_eval_select)]
+#![feature(core_intrinsics)]
 use std::intrinsics::const_eval_select;
 use std::arch::x86_64::*;
 use std::mem::transmute;
diff --git a/src/test/ui/intrinsics/const-eval-select.rs b/src/test/ui/intrinsics/const-eval-select.rs
index 744db2f15b056..9ff20d3fbdd9e 100644
--- a/src/test/ui/intrinsics/const-eval-select.rs
+++ b/src/test/ui/intrinsics/const-eval-select.rs
@@ -1,6 +1,7 @@
 // run-pass
 
 #![feature(const_eval_select)]
+#![feature(core_intrinsics)]
 
 use std::intrinsics::const_eval_select;
 
diff --git a/src/test/ui/lint/lint-stability.rs b/src/test/ui/lint/lint-stability.rs
index 464b32c5f43ed..d0f0e9f807123 100644
--- a/src/test/ui/lint/lint-stability.rs
+++ b/src/test/ui/lint/lint-stability.rs
@@ -191,11 +191,11 @@ mod inheritance {
         stable_mod::unstable(); //~ ERROR use of unstable library feature
         stable_mod::stable();
 
-        unstable_mod::deprecated();
+        unstable_mod::deprecated(); //~ ERROR use of unstable library feature
         unstable_mod::unstable(); //~ ERROR use of unstable library feature
 
         let _ = Unstable::UnstableVariant; //~ ERROR use of unstable library feature
-        let _ = Unstable::StableVariant;
+        let _ = Unstable::StableVariant; //~ ERROR use of unstable library feature
 
         let x: usize = 0;
         x.stable();
diff --git a/src/test/ui/lint/lint-stability.stderr b/src/test/ui/lint/lint-stability.stderr
index 167140ef92b46..bd1a57dc4cc50 100644
--- a/src/test/ui/lint/lint-stability.stderr
+++ b/src/test/ui/lint/lint-stability.stderr
@@ -294,6 +294,14 @@ LL |         stable_mod::unstable();
    |
    = help: add `#![feature(unstable_test_feature)]` to the crate attributes to enable
 
+error[E0658]: use of unstable library feature 'unstable_test_feature'
+  --> $DIR/lint-stability.rs:194:9
+   |
+LL |         unstable_mod::deprecated();
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: add `#![feature(unstable_test_feature)]` to the crate attributes to enable
+
 error[E0658]: use of unstable library feature 'unstable_test_feature'
   --> $DIR/lint-stability.rs:195:9
    |
@@ -310,6 +318,14 @@ LL |         let _ = Unstable::UnstableVariant;
    |
    = help: add `#![feature(unstable_test_feature)]` to the crate attributes to enable
 
+error[E0658]: use of unstable library feature 'unstable_test_feature'
+  --> $DIR/lint-stability.rs:198:17
+   |
+LL |         let _ = Unstable::StableVariant;
+   |                 ^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: add `#![feature(unstable_test_feature)]` to the crate attributes to enable
+
 error[E0658]: use of unstable library feature 'unstable_test_feature'
   --> $DIR/lint-stability.rs:88:48
    |
@@ -326,6 +342,6 @@ LL |             TypeUnstable = u8,
    |
    = help: add `#![feature(unstable_test_feature)]` to the crate attributes to enable
 
-error: aborting due to 41 previous errors
+error: aborting due to 43 previous errors
 
 For more information about this error, try `rustc --explain E0658`.
diff --git a/src/test/ui/stability-attribute/accidental-stable-in-unstable.rs b/src/test/ui/stability-attribute/accidental-stable-in-unstable.rs
new file mode 100644
index 0000000000000..f8bbe90cfc53b
--- /dev/null
+++ b/src/test/ui/stability-attribute/accidental-stable-in-unstable.rs
@@ -0,0 +1,10 @@
+#![crate_type = "lib"]
+extern crate core;
+
+// Known accidental stabilizations with no known users, slated for un-stabilization
+// fully stable @ core::char::UNICODE_VERSION
+use core::unicode::UNICODE_VERSION; //~ ERROR use of unstable library feature 'unicode_internals'
+
+// Known accidental stabilizations with known users
+// fully stable @ core::mem::transmute
+use core::intrinsics::transmute; // depended upon by rand_core
diff --git a/src/test/ui/stability-attribute/accidental-stable-in-unstable.stderr b/src/test/ui/stability-attribute/accidental-stable-in-unstable.stderr
new file mode 100644
index 0000000000000..ff733822cab98
--- /dev/null
+++ b/src/test/ui/stability-attribute/accidental-stable-in-unstable.stderr
@@ -0,0 +1,11 @@
+error[E0658]: use of unstable library feature 'unicode_internals'
+  --> $DIR/accidental-stable-in-unstable.rs:6:5
+   |
+LL | use core::unicode::UNICODE_VERSION;
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: add `#![feature(unicode_internals)]` to the crate attributes to enable
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0658`.
diff --git a/src/test/ui/stability-attribute/allowed-through-unstable.rs b/src/test/ui/stability-attribute/allowed-through-unstable.rs
new file mode 100644
index 0000000000000..ff0228e4da6a8
--- /dev/null
+++ b/src/test/ui/stability-attribute/allowed-through-unstable.rs
@@ -0,0 +1,9 @@
+// Test for new `#[rustc_allowed_through_unstable_modules]` attribute
+//
+// aux-build:allowed-through-unstable-core.rs
+#![crate_type = "lib"]
+
+extern crate allowed_through_unstable_core;
+
+use allowed_through_unstable_core::unstable_module::OldStableTraitAllowedThoughUnstable;
+use allowed_through_unstable_core::unstable_module::NewStableTraitNotAllowedThroughUnstable; //~ ERROR use of unstable library feature 'unstable_test_feature'
diff --git a/src/test/ui/stability-attribute/allowed-through-unstable.stderr b/src/test/ui/stability-attribute/allowed-through-unstable.stderr
new file mode 100644
index 0000000000000..132c00b89b229
--- /dev/null
+++ b/src/test/ui/stability-attribute/allowed-through-unstable.stderr
@@ -0,0 +1,12 @@
+error[E0658]: use of unstable library feature 'unstable_test_feature'
+  --> $DIR/allowed-through-unstable.rs:9:5
+   |
+LL | use allowed_through_unstable_core::unstable_module::NewStableTraitNotAllowedThroughUnstable;
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #1 <https://github.com/rust-lang/rust/issues/1> for more information
+   = help: add `#![feature(unstable_test_feature)]` to the crate attributes to enable
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0658`.
diff --git a/src/test/ui/stability-attribute/auxiliary/allowed-through-unstable-core.rs b/src/test/ui/stability-attribute/auxiliary/allowed-through-unstable-core.rs
new file mode 100644
index 0000000000000..b597009a309c9
--- /dev/null
+++ b/src/test/ui/stability-attribute/auxiliary/allowed-through-unstable-core.rs
@@ -0,0 +1,14 @@
+#![crate_type = "lib"]
+#![feature(staged_api)]
+#![feature(rustc_attrs)]
+#![stable(feature = "stable_test_feature", since = "1.2.0")]
+
+#[unstable(feature = "unstable_test_feature", issue = "1")]
+pub mod unstable_module {
+    #[stable(feature = "stable_test_feature", since = "1.2.0")]
+    #[rustc_allowed_through_unstable_modules]
+    pub trait OldStableTraitAllowedThoughUnstable {}
+
+    #[stable(feature = "stable_test_feature", since = "1.2.0")]
+    pub trait NewStableTraitNotAllowedThroughUnstable {}
+}
diff --git a/src/test/ui/stability-attribute/auxiliary/stable-in-unstable-core.rs b/src/test/ui/stability-attribute/auxiliary/stable-in-unstable-core.rs
new file mode 100644
index 0000000000000..e45b00f994af4
--- /dev/null
+++ b/src/test/ui/stability-attribute/auxiliary/stable-in-unstable-core.rs
@@ -0,0 +1,8 @@
+#![feature(staged_api)]
+#![stable(feature = "stable_test_feature", since = "1.2.0")]
+
+#[unstable(feature = "unstable_test_feature", issue = "1")]
+pub mod new_unstable_module {
+    #[stable(feature = "stable_test_feature", since = "1.2.0")]
+    pub trait OldTrait {}
+}
diff --git a/src/test/ui/stability-attribute/auxiliary/stable-in-unstable-std.rs b/src/test/ui/stability-attribute/auxiliary/stable-in-unstable-std.rs
new file mode 100644
index 0000000000000..28ad8c28da158
--- /dev/null
+++ b/src/test/ui/stability-attribute/auxiliary/stable-in-unstable-std.rs
@@ -0,0 +1,11 @@
+#![feature(staged_api)]
+#![feature(unstable_test_feature)]
+#![stable(feature = "stable_test_feature", since = "1.2.0")]
+
+extern crate stable_in_unstable_core;
+
+#[stable(feature = "stable_test_feature", since = "1.2.0")]
+pub mod old_stable_module {
+    #[stable(feature = "stable_test_feature", since = "1.2.0")]
+    pub use stable_in_unstable_core::new_unstable_module::OldTrait;
+}
diff --git a/src/test/ui/stability-attribute/stable-in-unstable.rs b/src/test/ui/stability-attribute/stable-in-unstable.rs
new file mode 100644
index 0000000000000..272a1a972340c
--- /dev/null
+++ b/src/test/ui/stability-attribute/stable-in-unstable.rs
@@ -0,0 +1,46 @@
+// This test is meant to test that we can have a stable item in an unstable module, and that
+// calling that item through the unstable module is unstable, but that re-exporting it from another
+// crate in a stable module is fine.
+//
+// This is necessary to support moving items from `std` into `core` or `alloc` unstably while still
+// exporting the original stable interface in `std`, such as moving `Error` into `core`.
+//
+// aux-build:stable-in-unstable-core.rs
+// aux-build:stable-in-unstable-std.rs
+#![crate_type = "lib"]
+
+extern crate stable_in_unstable_core;
+extern crate stable_in_unstable_std;
+
+mod isolated1 {
+    use stable_in_unstable_core::new_unstable_module; //~ ERROR use of unstable library feature 'unstable_test_feature'
+    use stable_in_unstable_core::new_unstable_module::OldTrait; //~ ERROR use of unstable library feature 'unstable_test_feature'
+}
+
+mod isolated2 {
+    use stable_in_unstable_std::old_stable_module::OldTrait;
+
+    struct LocalType;
+
+    impl OldTrait for LocalType {}
+}
+
+mod isolated3 {
+    use stable_in_unstable_core::new_unstable_module::OldTrait; //~ ERROR use of unstable library feature 'unstable_test_feature'
+
+    struct LocalType;
+
+    impl OldTrait for LocalType {}
+}
+
+mod isolated4 {
+    struct LocalType;
+
+    impl stable_in_unstable_core::new_unstable_module::OldTrait for LocalType {} //~ ERROR use of unstable library feature 'unstable_test_feature'
+}
+
+mod isolated5 {
+    struct LocalType;
+
+    impl stable_in_unstable_std::old_stable_module::OldTrait for LocalType {}
+}
diff --git a/src/test/ui/stability-attribute/stable-in-unstable.stderr b/src/test/ui/stability-attribute/stable-in-unstable.stderr
new file mode 100644
index 0000000000000..e123d83584c81
--- /dev/null
+++ b/src/test/ui/stability-attribute/stable-in-unstable.stderr
@@ -0,0 +1,39 @@
+error[E0658]: use of unstable library feature 'unstable_test_feature'
+  --> $DIR/stable-in-unstable.rs:16:9
+   |
+LL |     use stable_in_unstable_core::new_unstable_module;
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #1 <https://github.com/rust-lang/rust/issues/1> for more information
+   = help: add `#![feature(unstable_test_feature)]` to the crate attributes to enable
+
+error[E0658]: use of unstable library feature 'unstable_test_feature'
+  --> $DIR/stable-in-unstable.rs:17:9
+   |
+LL |     use stable_in_unstable_core::new_unstable_module::OldTrait;
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #1 <https://github.com/rust-lang/rust/issues/1> for more information
+   = help: add `#![feature(unstable_test_feature)]` to the crate attributes to enable
+
+error[E0658]: use of unstable library feature 'unstable_test_feature'
+  --> $DIR/stable-in-unstable.rs:29:9
+   |
+LL |     use stable_in_unstable_core::new_unstable_module::OldTrait;
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #1 <https://github.com/rust-lang/rust/issues/1> for more information
+   = help: add `#![feature(unstable_test_feature)]` to the crate attributes to enable
+
+error[E0658]: use of unstable library feature 'unstable_test_feature'
+  --> $DIR/stable-in-unstable.rs:39:10
+   |
+LL |     impl stable_in_unstable_core::new_unstable_module::OldTrait for LocalType {}
+   |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #1 <https://github.com/rust-lang/rust/issues/1> for more information
+   = help: add `#![feature(unstable_test_feature)]` to the crate attributes to enable
+
+error: aborting due to 4 previous errors
+
+For more information about this error, try `rustc --explain E0658`.
diff --git a/src/tools/clippy/clippy_utils/src/qualify_min_const_fn.rs b/src/tools/clippy/clippy_utils/src/qualify_min_const_fn.rs
index f3283588c7320..3bf75bcbee83e 100644
--- a/src/tools/clippy/clippy_utils/src/qualify_min_const_fn.rs
+++ b/src/tools/clippy/clippy_utils/src/qualify_min_const_fn.rs
@@ -354,7 +354,7 @@ fn check_terminator<'a, 'tcx>(
 fn is_const_fn(tcx: TyCtxt<'_>, def_id: DefId, msrv: Option<RustcVersion>) -> bool {
     tcx.is_const_fn(def_id)
         && tcx.lookup_const_stability(def_id).map_or(true, |const_stab| {
-            if let rustc_attr::StabilityLevel::Stable { since } = const_stab.level {
+            if let rustc_attr::StabilityLevel::Stable { since, .. } = const_stab.level {
                 // Checking MSRV is manually necessary because `rustc` has no such concept. This entire
                 // function could be removed if `rustc` provided a MSRV-aware version of `is_const_fn`.
                 // as a part of an unimplemented MSRV check https://github.com/rust-lang/rust/issues/65262.
diff --git a/src/tools/tidy/src/error_codes_check.rs b/src/tools/tidy/src/error_codes_check.rs
index e56ce3329cc47..4ffa1fa8b28d3 100644
--- a/src/tools/tidy/src/error_codes_check.rs
+++ b/src/tools/tidy/src/error_codes_check.rs
@@ -11,7 +11,7 @@ use regex::Regex;
 // A few of those error codes can't be tested but all the others can and *should* be tested!
 const EXEMPTED_FROM_TEST: &[&str] = &[
     "E0279", "E0313", "E0377", "E0461", "E0462", "E0465", "E0476", "E0490", "E0514", "E0519",
-    "E0523", "E0554", "E0640", "E0717", "E0729",
+    "E0523", "E0554", "E0640", "E0717", "E0729", "E0789",
 ];
 
 // Some error codes don't have any tests apparently...