add opt in attribute for stable-in-unstable items

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,
+                E0788,
+                "`rustc_allowed_through_unstable_modules` attribute must be paired with a `stable` attribute"
+            )
+            .emit();
+        }
+    }
+
     (stab, const_stab)
 }
 

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" { ... }`.
+    E0788, // rustc_allowed_through_unstable_modules without stability attribute
 }

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:

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,
             };

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;
@@ -224,7 +225,7 @@ 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
@@ -819,7 +820,19 @@ impl<'tcx> Visitor<'tcx> for Checker<'tcx> {
                 },
             );
 
-            if item_is_allowed {
+            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
@@ -827,6 +840,10 @@ impl<'tcx> Visitor<'tcx> for Checker<'tcx> {
                 //
                 // 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) {

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,

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;

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)))

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

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`.

src/test/ui/stability-attribute/allow-through-unstable-misuse.rs

@@ -0,0 +1,8 @@
+#![crate_type = "lib"]
+#![feature(staged_api)]
+#![feature(rustc_attrs)]
+#![stable(feature = "foo", since = "1.0.0")]
+
+#[unstable(feature = "bar", issue = "none")]
+#[rustc_allowed_through_unstable_modules]
+pub struct UnstableType(());

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'

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:10:5
+   |
+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`.

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 {}
+}

src/tools/clippy/clippy_utils/src/qualify_min_const_fn.rs

@@ -353,7 +353,7 @@ 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.