Auto merge of #97177 - oli-obk:const-stability, r=davidtwco

Implement proper stability check for const impl Trait, fall back to unstable const when undeclared

Continuation of #93960

`@jhpratt` it looks to me like the test was simply not testing for the failure you were looking for? Your checks actually do the right thing for const traits?

Author: bors

compiler/rustc_attr/src/builtin.rs

@@ -101,6 +101,16 @@ pub struct Stability {
     pub feature: Symbol,
 }
 
+impl Stability {
+    pub fn is_unstable(&self) -> bool {
+        self.level.is_unstable()
+    }
+
+    pub fn is_stable(&self) -> bool {
+        self.level.is_stable()
+    }
+}
+
 /// Represents the `#[rustc_const_unstable]` and `#[rustc_const_stable]` attributes.
 #[derive(Encodable, Decodable, Copy, Clone, Debug, PartialEq, Eq, Hash)]
 #[derive(HashStable_Generic)]
@@ -111,6 +121,16 @@ pub struct ConstStability {
     pub promotable: bool,
 }
 
+impl ConstStability {
+    pub fn is_const_unstable(&self) -> bool {
+        self.level.is_unstable()
+    }
+
+    pub fn is_const_stable(&self) -> bool {
+        self.level.is_stable()
+    }
+}
+
 /// The available stability levels.
 #[derive(Encodable, Decodable, PartialEq, Copy, Clone, Debug, Eq, Hash)]
 #[derive(HashStable_Generic)]

compiler/rustc_const_eval/src/const_eval/fn_queries.rs

@@ -9,7 +9,7 @@ use rustc_span::symbol::Symbol;
 pub fn is_unstable_const_fn(tcx: TyCtxt<'_>, def_id: DefId) -> Option<Symbol> {
     if tcx.is_const_fn_raw(def_id) {
         let const_stab = tcx.lookup_const_stability(def_id)?;
-        if const_stab.level.is_unstable() { Some(const_stab.feature) } else { None }
+        if const_stab.is_const_unstable() { Some(const_stab.feature) } else { None }
     } else {
         None
     }

compiler/rustc_const_eval/src/transform/check_consts/check.rs

@@ -229,18 +229,6 @@ impl<'mir, 'tcx> Checker<'mir, 'tcx> {
 
         // The local type and predicate checks are not free and only relevant for `const fn`s.
         if self.const_kind() == hir::ConstContext::ConstFn {
-            // Prevent const trait methods from being annotated as `stable`.
-            // FIXME: Do this as part of stability checking.
-            if self.is_const_stable_const_fn() {
-                if crate::const_eval::is_parent_const_impl_raw(tcx, def_id) {
-                    self.ccx
-                        .tcx
-                        .sess
-                        .struct_span_err(self.span, "trait methods cannot be stable const fn")
-                        .emit();
-                }
-            }
-
             for (idx, local) in body.local_decls.iter_enumerated() {
                 // Handle the return place below.
                 if idx == RETURN_PLACE || local.internal {
@@ -944,7 +932,7 @@ impl<'tcx> Visitor<'tcx> for Checker<'_, 'tcx> {
                 // have no `rustc_const_stable` attributes to be const-unstable as well. This
                 // should be fixed later.
                 let callee_is_unstable_unmarked = tcx.lookup_const_stability(callee).is_none()
-                    && tcx.lookup_stability(callee).map_or(false, |s| s.level.is_unstable());
+                    && tcx.lookup_stability(callee).map_or(false, |s| s.is_unstable());
                 if callee_is_unstable_unmarked {
                     trace!("callee_is_unstable_unmarked");
                     // We do not use `const` modifiers for intrinsic "functions", as intrinsics are

compiler/rustc_const_eval/src/transform/check_consts/mod.rs

@@ -84,8 +84,6 @@ pub fn rustc_allow_const_fn_unstable(
 // functions are subject to more stringent restrictions than "const-unstable" functions: They
 // cannot use unstable features and can only call other "const-stable" functions.
 pub fn is_const_stable_const_fn(tcx: TyCtxt<'_>, def_id: DefId) -> bool {
-    use attr::{ConstStability, Stability, StabilityLevel};
-
     // A default body marked const is not const-stable because const
     // trait fns currently cannot be const-stable. We shouldn't
     // restrict default bodies to only call const-stable functions.
@@ -96,22 +94,39 @@ pub fn is_const_stable_const_fn(tcx: TyCtxt<'_>, def_id: DefId) -> bool {
     // Const-stability is only relevant for `const fn`.
     assert!(tcx.is_const_fn_raw(def_id));
 
-    // Functions with `#[rustc_const_unstable]` are const-unstable.
+    // A function is only const-stable if it has `#[rustc_const_stable]` or it the trait it belongs
+    // to is const-stable.
     match tcx.lookup_const_stability(def_id) {
-        Some(ConstStability { level: StabilityLevel::Unstable { .. }, .. }) => return false,
-        Some(ConstStability { level: StabilityLevel::Stable { .. }, .. }) => return true,
-        None => {}
+        Some(stab) => stab.is_const_stable(),
+        None if is_parent_const_stable_trait(tcx, def_id) => {
+            // Remove this when `#![feature(const_trait_impl)]` is stabilized,
+            // returning `true` unconditionally.
+            tcx.sess.delay_span_bug(
+                tcx.def_span(def_id),
+                "trait implementations cannot be const stable yet",
+            );
+            true
+        }
+        None => false, // By default, items are not const stable.
     }
+}
+
+fn is_parent_const_stable_trait(tcx: TyCtxt<'_>, def_id: DefId) -> bool {
+    let local_def_id = def_id.expect_local();
+    let hir_id = tcx.local_def_id_to_hir_id(local_def_id);
+
+    let Some(parent) = tcx.hir().find_parent_node(hir_id) else { return false };
+    let parent_def = tcx.hir().get(parent);
 
-    // Functions with `#[unstable]` are const-unstable.
-    //
-    // FIXME(ecstaticmorse): We should keep const-stability attributes wholly separate from normal stability
-    // attributes. `#[unstable]` should be irrelevant.
-    if let Some(Stability { level: StabilityLevel::Unstable { .. }, .. }) =
-        tcx.lookup_stability(def_id)
-    {
+    if !matches!(
+        parent_def,
+        hir::Node::Item(hir::Item {
+            kind: hir::ItemKind::Impl(hir::Impl { constness: hir::Constness::Const, .. }),
+            ..
+        })
+    ) {
         return false;
     }
 
-    true
+    tcx.lookup_const_stability(parent.owner).map_or(false, |stab| stab.is_const_stable())
 }

compiler/rustc_middle/src/ty/context.rs

@@ -2791,7 +2791,7 @@ impl<'tcx> TyCtxt<'tcx> {
     pub fn is_const_fn(self, def_id: DefId) -> bool {
         if self.is_const_fn_raw(def_id) {
             match self.lookup_const_stability(def_id) {
-                Some(stability) if stability.level.is_unstable() => {
+                Some(stability) if stability.is_const_unstable() => {
                     // has a `rustc_const_unstable` attribute, check whether the user enabled the
                     // corresponding feature gate.
                     self.features()
@@ -2808,6 +2808,21 @@ impl<'tcx> TyCtxt<'tcx> {
             false
         }
     }
+
+    /// Whether the trait impl is marked const. This does not consider stability or feature gates.
+    pub fn is_const_trait_impl_raw(self, def_id: DefId) -> bool {
+        let Some(local_def_id) = def_id.as_local() else { return false };
+        let hir_id = self.local_def_id_to_hir_id(local_def_id);
+        let node = self.hir().get(hir_id);
+
+        matches!(
+            node,
+            hir::Node::Item(hir::Item {
+                kind: hir::ItemKind::Impl(hir::Impl { constness: hir::Constness::Const, .. }),
+                ..
+            })
+        )
+    }
 }
 
 impl<'tcx> TyCtxtAt<'tcx> {

compiler/rustc_passes/src/stability.rs

@@ -147,7 +147,7 @@ impl<'a, 'tcx> Annotator<'a, 'tcx> {
             // Propagate unstability.  This can happen even for non-staged-api crates in case
             // -Zforce-unstable-if-unmarked is set.
             if let Some(stab) = self.parent_stab {
-                if inherit_deprecation.yes() && stab.level.is_unstable() {
+                if inherit_deprecation.yes() && stab.is_unstable() {
                     self.index.stab_map.insert(def_id, stab);
                 }
             }
@@ -190,7 +190,7 @@ impl<'a, 'tcx> Annotator<'a, 'tcx> {
         if const_stab.is_none() {
             debug!("annotate: const_stab not found, parent = {:?}", self.parent_const_stab);
             if let Some(parent) = self.parent_const_stab {
-                if parent.level.is_unstable() {
+                if parent.is_const_unstable() {
                     self.index.const_stab_map.insert(def_id, parent);
                 }
             }
@@ -272,9 +272,7 @@ impl<'a, 'tcx> Annotator<'a, 'tcx> {
         if stab.is_none() {
             debug!("annotate: stab not found, parent = {:?}", self.parent_stab);
             if let Some(stab) = self.parent_stab {
-                if inherit_deprecation.yes() && stab.level.is_unstable()
-                    || inherit_from_parent.yes()
-                {
+                if inherit_deprecation.yes() && stab.is_unstable() || inherit_from_parent.yes() {
                     self.index.stab_map.insert(def_id, stab);
                 }
             }
@@ -532,7 +530,8 @@ impl<'tcx> MissingStabilityAnnotations<'tcx> {
             return;
         }
 
-        let is_const = self.tcx.is_const_fn(def_id.to_def_id());
+        let is_const = self.tcx.is_const_fn(def_id.to_def_id())
+            || self.tcx.is_const_trait_impl_raw(def_id.to_def_id());
         let is_stable = self
             .tcx
             .lookup_stability(def_id)
@@ -710,16 +709,23 @@ impl<'tcx> Visitor<'tcx> for Checker<'tcx> {
             // For implementations of traits, check the stability of each item
             // individually as it's possible to have a stable trait with unstable
             // items.
-            hir::ItemKind::Impl(hir::Impl { of_trait: Some(ref t), self_ty, items, .. }) => {
-                if self.tcx.features().staged_api {
+            hir::ItemKind::Impl(hir::Impl {
+                of_trait: Some(ref t),
+                self_ty,
+                items,
+                constness,
+                ..
+            }) => {
+                let features = self.tcx.features();
+                if features.staged_api {
+                    let attrs = self.tcx.hir().attrs(item.hir_id());
+                    let (stab, const_stab) = attr::find_stability(&self.tcx.sess, attrs, item.span);
+
                     // If this impl block has an #[unstable] attribute, give an
                     // error if all involved types and traits are stable, because
                     // it will have no effect.
                     // See: https://github.com/rust-lang/rust/issues/55436
-                    let attrs = self.tcx.hir().attrs(item.hir_id());
-                    if let (Some((Stability { level: attr::Unstable { .. }, .. }, span)), _) =
-                        attr::find_stability(&self.tcx.sess, attrs, item.span)
-                    {
+                    if let Some((Stability { level: attr::Unstable { .. }, .. }, span)) = stab {
                         let mut c = CheckTraitImplStable { tcx: self.tcx, fully_stable: true };
                         c.visit_ty(self_ty);
                         c.visit_trait_ref(t);
@@ -735,6 +741,19 @@ impl<'tcx> Visitor<'tcx> for Checker<'tcx> {
                             );
                         }
                     }
+
+                    // `#![feature(const_trait_impl)]` is unstable, so any impl declared stable
+                    // needs to have an error emitted.
+                    if features.const_trait_impl
+                        && *constness == hir::Constness::Const
+                        && const_stab.map_or(false, |(stab, _)| stab.is_const_stable())
+                    {
+                        self.tcx
+                            .sess
+                            .struct_span_err(item.span, "trait implementations cannot be const stable yet")
+                            .note("see issue #67792 <https://github.com/rust-lang/rust/issues/67792> for more information")
+                            .emit();
+                    }
                 }
 
                 for impl_item_ref in *items {

src/librustdoc/clean/inline.rs

@@ -344,7 +344,7 @@ pub(crate) fn build_impl(
             }
 
             if let Some(stab) = tcx.lookup_stability(did) {
-                if stab.level.is_unstable() && stab.feature == sym::rustc_private {
+                if stab.is_unstable() && stab.feature == sym::rustc_private {
                     return;
                 }
             }
@@ -373,7 +373,7 @@ pub(crate) fn build_impl(
             }
 
             if let Some(stab) = tcx.lookup_stability(did) {
-                if stab.level.is_unstable() && stab.feature == sym::rustc_private {
+                if stab.is_unstable() && stab.feature == sym::rustc_private {
                     return;
                 }
             }

src/librustdoc/clean/types.rs

@@ -632,7 +632,7 @@ impl Item {
         self.stability(tcx).as_ref().and_then(|s| {
             let mut classes = Vec::with_capacity(2);
 
-            if s.level.is_unstable() {
+            if s.is_unstable() {
                 classes.push("unstable");
             }
 

src/librustdoc/html/render/print_item.rs

@@ -445,10 +445,7 @@ fn extra_info_tags(item: &clean::Item, parent: &clean::Item, tcx: TyCtxt<'_>) ->
 
     // The "rustc_private" crates are permanently unstable so it makes no sense
     // to render "unstable" everywhere.
-    if item
-        .stability(tcx)
-        .as_ref()
-        .map(|s| s.level.is_unstable() && s.feature != sym::rustc_private)
+    if item.stability(tcx).as_ref().map(|s| s.is_unstable() && s.feature != sym::rustc_private)
         == Some(true)
     {
         tags += &tag_html("unstable", "", "Experimental");

src/test/ui/consts/rustc-impl-const-stability.rs

@@ -1,19 +1,18 @@
-// build-pass
+// check-pass
 
 #![crate_type = "lib"]
 #![feature(staged_api)]
 #![feature(const_trait_impl)]
 #![stable(feature = "foo", since = "1.0.0")]
 
-
 #[stable(feature = "potato", since = "1.27.0")]
 pub struct Data {
-    _data: u128
+    _data: u128,
 }
 
 #[stable(feature = "potato", since = "1.27.0")]
+#[rustc_const_unstable(feature = "data_foo", issue = "none")]
 impl const Default for Data {
-    #[rustc_const_unstable(feature = "data_foo", issue = "none")]
     fn default() -> Data {
         Data { _data: 42 }
     }