Rollup merge of rust-lang#135423 - compiler-errors:enforce-const-trait-syntactical, r=oli-obk,RalfJung

Enforce syntactical stability of const traits in HIR

This PR enforces what I'm calling *syntactical* const stability of traits. In other words, it enforces the ability to name `~const`/`const` traits in trait bounds in various syntax positions in HIR (including in the trait of an impl header). This functionality is analogous to the *regular* item stability checker, which is concerned with making sure that you cannot refer to unstable items by name, and is implemented as an extension of that pass.

This is separate from enforcing the *recursive* const stability of const trait methods, which is implemented in MIR and runs on MIR bodies. That will require adding a new `NonConstOp` to the const checker and probably adjusting some logic to deduplicate redundant errors.

However, this check is separate and necessary for making sure that users don't add `~const`/`const` bounds to items when the trait is not const-stable in the first place. I chose to separate enforcing recursive const stability out of this PR to make it easier to review. I'll probably open a follow-up following this one, blocked on this PR.

r? `@RalfJung` cc `@rust-lang/project-const-traits`

Author: workingjubilee

compiler/rustc_middle/src/middle/stability.rs

@@ -30,6 +30,14 @@ pub enum StabilityLevel {
     Stable,
 }
 
+#[derive(Copy, Clone)]
+pub enum UnstableKind {
+    /// Enforcing regular stability of an item
+    Regular,
+    /// Enforcing const stability of an item
+    Const(Span),
+}
+
 /// An entry in the `depr_map`.
 #[derive(Copy, Clone, HashStable, Debug, Encodable, Decodable)]
 pub struct DeprecationEntry {
@@ -108,10 +116,16 @@ pub fn report_unstable(
     is_soft: bool,
     span: Span,
     soft_handler: impl FnOnce(&'static Lint, Span, String),
+    kind: UnstableKind,
 ) {
+    let qual = match kind {
+        UnstableKind::Regular => "",
+        UnstableKind::Const(_) => " const",
+    };
+
     let msg = match reason {
-        Some(r) => format!("use of unstable library feature `{feature}`: {r}"),
-        None => format!("use of unstable library feature `{feature}`"),
+        Some(r) => format!("use of unstable{qual} library feature `{feature}`: {r}"),
+        None => format!("use of unstable{qual} library feature `{feature}`"),
     };
 
     if is_soft {
@@ -121,6 +135,9 @@ pub fn report_unstable(
         if let Some((inner_types, msg, sugg, applicability)) = suggestion {
             err.span_suggestion(inner_types, msg, sugg, applicability);
         }
+        if let UnstableKind::Const(kw) = kind {
+            err.span_label(kw, "trait is not stable as const yet");
+        }
         err.emit();
     }
 }
@@ -587,13 +604,81 @@ impl<'tcx> TyCtxt<'tcx> {
                 is_soft,
                 span,
                 soft_handler,
+                UnstableKind::Regular,
             ),
             EvalResult::Unmarked => unmarked(span, def_id),
         }
 
         is_allowed
     }
 
+    /// This function is analogous to `check_optional_stability` but with the logic in
+    /// `eval_stability_allow_unstable` inlined, and which operating on const stability
+    /// instead of regular stability.
+    ///
+    /// This enforces *syntactical* const stability of const traits. In other words,
+    /// it enforces the ability to name `~const`/`const` traits in trait bounds in various
+    /// syntax positions in HIR (including in the trait of an impl header).
+    pub fn check_const_stability(self, def_id: DefId, span: Span, const_kw_span: Span) {
+        let is_staged_api = self.lookup_stability(def_id.krate.as_def_id()).is_some();
+        if !is_staged_api {
+            return;
+        }
+
+        // Only the cross-crate scenario matters when checking unstable APIs
+        let cross_crate = !def_id.is_local();
+        if !cross_crate {
+            return;
+        }
+
+        let stability = self.lookup_const_stability(def_id);
+        debug!(
+            "stability: \
+                inspecting def_id={:?} span={:?} of stability={:?}",
+            def_id, span, stability
+        );
+
+        match stability {
+            Some(ConstStability {
+                level: attr::StabilityLevel::Unstable { reason, issue, is_soft, implied_by, .. },
+                feature,
+                ..
+            }) => {
+                assert!(!is_soft);
+
+                if span.allows_unstable(feature) {
+                    debug!("body stability: skipping span={:?} since it is internal", span);
+                    return;
+                }
+                if self.features().enabled(feature) {
+                    return;
+                }
+
+                // If this item was previously part of a now-stabilized feature which is still
+                // enabled (i.e. the user hasn't removed the attribute for the stabilized feature
+                // yet) then allow use of this item.
+                if let Some(implied_by) = implied_by
+                    && self.features().enabled(implied_by)
+                {
+                    return;
+                }
+
+                report_unstable(
+                    self.sess,
+                    feature,
+                    reason.to_opt_reason(),
+                    issue,
+                    None,
+                    false,
+                    span,
+                    |_, _, _| {},
+                    UnstableKind::Const(const_kw_span),
+                );
+            }
+            Some(_) | None => {}
+        }
+    }
+
     pub fn lookup_deprecation(self, id: DefId) -> Option<Deprecation> {
         self.lookup_deprecation_entry(id).map(|depr| depr.attr)
     }

compiler/rustc_passes/src/stability.rs

@@ -593,9 +593,11 @@ impl<'tcx> MissingStabilityAnnotations<'tcx> {
     }
 
     fn check_missing_const_stability(&self, def_id: LocalDefId, span: Span) {
-        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.def_kind(def_id.to_def_id()) == DefKind::Trait
+                && self.tcx.is_const_trait(def_id.to_def_id()));
 
-        // Reachable const fn must have a stability attribute.
+        // Reachable const fn/trait must have a stability attribute.
         if is_const
             && self.effective_visibilities.is_reachable(def_id)
             && self.tcx.lookup_const_stability(def_id).is_none()
@@ -772,7 +774,13 @@ 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, .. }) => {
+            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());
@@ -814,6 +822,16 @@ impl<'tcx> Visitor<'tcx> for Checker<'tcx> {
                     }
                 }
 
+                match constness {
+                    rustc_hir::Constness::Const => {
+                        if let Some(def_id) = t.trait_def_id() {
+                            // FIXME(const_trait_impl): Improve the span here.
+                            self.tcx.check_const_stability(def_id, t.path.span, t.path.span);
+                        }
+                    }
+                    rustc_hir::Constness::NotConst => {}
+                }
+
                 for impl_item_ref in *items {
                     let impl_item = self.tcx.associated_item(impl_item_ref.id.owner_id);
 
@@ -829,6 +847,18 @@ impl<'tcx> Visitor<'tcx> for Checker<'tcx> {
         intravisit::walk_item(self, item);
     }
 
+    fn visit_poly_trait_ref(&mut self, t: &'tcx hir::PolyTraitRef<'tcx>) {
+        match t.modifiers.constness {
+            hir::BoundConstness::Always(span) | hir::BoundConstness::Maybe(span) => {
+                if let Some(def_id) = t.trait_ref.trait_def_id() {
+                    self.tcx.check_const_stability(def_id, t.trait_ref.path.span, span);
+                }
+            }
+            hir::BoundConstness::Never => {}
+        }
+        intravisit::walk_poly_trait_ref(self, t);
+    }
+
     fn visit_path(&mut self, path: &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);

compiler/rustc_resolve/src/macros.rs

@@ -1031,6 +1031,7 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
                         is_soft,
                         span,
                         soft_handler,
+                        stability::UnstableKind::Regular,
                     );
                 }
             }

library/core/src/intrinsics/fallback.rs

@@ -8,6 +8,7 @@
 #![allow(missing_docs)]
 
 #[const_trait]
+#[rustc_const_unstable(feature = "core_intrinsics_fallbacks", issue = "none")]
 pub trait CarryingMulAdd: Copy + 'static {
     type Unsigned: Copy + 'static;
     fn carrying_mul_add(

library/core/src/marker.rs

@@ -952,6 +952,7 @@ marker_impls! {
 /// This should be used for `~const` bounds,
 /// as non-const bounds will always hold for every type.
 #[unstable(feature = "const_destruct", issue = "133214")]
+#[rustc_const_unstable(feature = "const_destruct", issue = "133214")]
 #[lang = "destruct"]
 #[rustc_on_unimplemented(message = "can't drop `{Self}`", append_const_msg)]
 #[rustc_deny_explicit_impl]

library/core/src/ops/arith.rs

@@ -65,6 +65,7 @@
 /// ```
 #[lang = "add"]
 #[stable(feature = "rust1", since = "1.0.0")]
+#[rustc_const_unstable(feature = "const_ops", issue = "90080")]
 #[rustc_on_unimplemented(
     on(all(_Self = "{integer}", Rhs = "{float}"), message = "cannot add a float to an integer",),
     on(all(_Self = "{float}", Rhs = "{integer}"), message = "cannot add an integer to a float",),

library/core/src/ops/deref.rs

@@ -134,6 +134,7 @@
 #[stable(feature = "rust1", since = "1.0.0")]
 #[rustc_diagnostic_item = "Deref"]
 #[const_trait]
+#[rustc_const_unstable(feature = "const_deref", issue = "88955")]
 pub trait Deref {
     /// The resulting type after dereferencing.
     #[stable(feature = "rust1", since = "1.0.0")]
@@ -263,6 +264,7 @@ impl<T: ?Sized> const Deref for &mut T {
 #[doc(alias = "*")]
 #[stable(feature = "rust1", since = "1.0.0")]
 #[const_trait]
+#[rustc_const_unstable(feature = "const_deref", issue = "88955")]
 pub trait DerefMut: ~const Deref {
     /// Mutably dereferences the value.
     #[stable(feature = "rust1", since = "1.0.0")]

library/core/src/ops/drop.rs

@@ -204,6 +204,7 @@
 #[lang = "drop"]
 #[stable(feature = "rust1", since = "1.0.0")]
 #[const_trait]
+#[rustc_const_unstable(feature = "const_destruct", issue = "133214")]
 pub trait Drop {
     /// Executes the destructor for this type.
     ///

tests/ui/consts/promoted-const-drop.rs

@@ -1,4 +1,4 @@
-#![feature(const_trait_impl)]
+#![feature(const_trait_impl, const_destruct)]
 
 struct A();
 

tests/ui/consts/promoted_const_call.rs

@@ -1,22 +1,20 @@
-//@ known-bug: #103507
-
-#![feature(const_trait_impl)]
+#![feature(const_trait_impl, const_destruct)]
 
 struct Panic;
 impl const Drop for Panic { fn drop(&mut self) { panic!(); } }
 
 pub const fn id<T>(x: T) -> T { x }
 pub const C: () = {
     let _: &'static _ = &id(&Panic);
-    //FIXME ~^ ERROR: temporary value dropped while borrowed
-    //FIXME ~| ERROR: temporary value dropped while borrowed
+    //~^ ERROR: temporary value dropped while borrowed
+    //~| ERROR: temporary value dropped while borrowed
 };
 
 fn main() {
     let _: &'static _ = &id(&Panic);
-    //FIXME ~^ ERROR: temporary value dropped while borrowed
-    //FIXME ~| ERROR: temporary value dropped while borrowed
+    //~^ ERROR: temporary value dropped while borrowed
+    //~| ERROR: temporary value dropped while borrowed
     let _: &'static _ = &&(Panic, 0).1;
-    //FIXME~^ ERROR: temporary value dropped while borrowed
-    //FIXME~| ERROR: temporary value dropped while borrowed
+    //~^ ERROR: temporary value dropped while borrowed
+    //~| ERROR: temporary value dropped while borrowed
 }