Implement trait const stability

Author: fee1-dead

compiler/rustc_const_eval/src/check_consts/check.rs

@@ -709,8 +709,7 @@ impl<'tcx> Visitor<'tcx> for Checker<'_, 'tcx> {
                     if trait_is_const {
                         // Trait calls are always conditionally-const.
                         self.check_op(ops::ConditionallyConstCall { callee, args: fn_args });
-                        // FIXME(const_trait_impl): do a more fine-grained check whether this
-                        // particular trait can be const-stably called.
+                        self.tcx.enforce_trait_const_stability(trait_did, *fn_span);
                     } else {
                         // Not even a const trait.
                         self.check_op(ops::FnCallNonConst {

compiler/rustc_hir_analysis/src/collect.rs

@@ -1612,6 +1612,7 @@ fn check_impl_constness(
 
     let Some(trait_def_id) = hir_trait_ref.trait_def_id() else { return };
     if tcx.is_const_trait(trait_def_id) {
+        tcx.enforce_trait_const_stability(trait_def_id, hir_trait_ref.path.span);
         return;
     }
 

compiler/rustc_hir_analysis/src/hir_ty_lowering/mod.rs

@@ -741,9 +741,9 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
         };
 
         if let hir::BoundConstness::Always(span) | hir::BoundConstness::Maybe(span) = constness
-            && !self.tcx().is_const_trait(trait_def_id)
         {
-            let (def_span, suggestion, suggestion_pre) =
+            if !self.tcx().is_const_trait(trait_def_id) {
+                let (def_span, suggestion, suggestion_pre) =
                 match (trait_def_id.is_local(), self.tcx().sess.is_nightly_build()) {
                     (true, true) => (
                         None,
@@ -756,14 +756,17 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
                     ),
                     (false, _) | (_, false) => (Some(tcx.def_span(trait_def_id)), None, ""),
                 };
-            self.dcx().emit_err(crate::errors::ConstBoundForNonConstTrait {
-                span,
-                modifier: constness.as_str(),
-                def_span,
-                trait_name: self.tcx().def_path_str(trait_def_id),
-                suggestion_pre,
-                suggestion,
-            });
+                self.dcx().emit_err(crate::errors::ConstBoundForNonConstTrait {
+                    span,
+                    modifier: constness.as_str(),
+                    def_span,
+                    trait_name: self.tcx().def_path_str(trait_def_id),
+                    suggestion_pre,
+                    suggestion,
+                });
+            }
+
+            tcx.enforce_trait_const_stability(trait_def_id, span)
         }
 
         match predicate_filter {

compiler/rustc_middle/messages.ftl

@@ -105,6 +105,8 @@ middle_type_length_limit = reached the type-length limit while instantiating `{$
 middle_unknown_layout =
     the type `{$ty}` has an unknown layout
 
+middle_unstable_const_trait = `{$def_path}` is not yet stable as a const trait
+
 middle_values_too_big =
     values of the type `{$ty}` are too big for the target architecture
 middle_written_to_path = the full type name has been written to '{$path}'

compiler/rustc_middle/src/error.rs

@@ -171,3 +171,11 @@ pub struct TypeLengthLimit {
     pub path: PathBuf,
     pub type_length: usize,
 }
+
+#[derive(Diagnostic)]
+#[diag(middle_unstable_const_trait)]
+pub struct UnstableConstTrait {
+    #[primary_span]
+    pub span: Span,
+    pub def_path: String,
+}

compiler/rustc_middle/src/ty/context.rs

@@ -15,6 +15,7 @@ use std::{fmt, iter, mem};
 
 use rustc_abi::{ExternAbi, FieldIdx, Layout, LayoutData, TargetDataLayout, VariantIdx};
 use rustc_ast as ast;
+use rustc_attr_parsing::{ConstStability, StabilityLevel};
 use rustc_data_structures::defer;
 use rustc_data_structures::fingerprint::Fingerprint;
 use rustc_data_structures::fx::FxHashMap;
@@ -3132,6 +3133,35 @@ impl<'tcx> TyCtxt<'tcx> {
             && self.impl_trait_header(def_id).unwrap().constness == hir::Constness::Const
     }
 
+    pub fn enforce_trait_const_stability(self, trait_def_id: DefId, span: Span) {
+        match self.lookup_const_stability(trait_def_id) {
+            Some(ConstStability {
+                level: StabilityLevel::Unstable { implied_by: implied_feature, .. },
+                feature,
+                ..
+            }) => {
+                if span.allows_unstable(feature)
+                    || implied_feature.is_some_and(|f| span.allows_unstable(f))
+                {
+                    return;
+                }
+                let feature_enabled = trait_def_id.is_local()
+                    || self.features().enabled(feature)
+                    || implied_feature.is_some_and(|f| self.features().enabled(f));
+
+                if !feature_enabled {
+                    let mut diag = self.dcx().create_err(crate::error::UnstableConstTrait {
+                        span,
+                        def_path: self.def_path_str(trait_def_id),
+                    });
+                    self.disabled_nightly_features(&mut diag, None, [(String::new(), feature)]);
+                    diag.emit();
+                }
+            }
+            _ => {}
+        }
+    }
+
     pub fn intrinsic(self, def_id: impl IntoQueryParam<DefId> + Copy) -> Option<ty::IntrinsicDef> {
         match self.def_kind(def_id) {
             DefKind::Fn | DefKind::AssocFn => {}

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()

library/core/src/marker.rs

@@ -960,6 +960,7 @@ 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)]
 #[cfg_attr(bootstrap, rustc_deny_explicit_impl(implement_via_object = false))]

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"]
 #[cfg_attr(not(bootstrap), 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")]
@@ -287,6 +288,7 @@ 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")]
 #[cfg_attr(not(bootstrap), 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

@@ -1,6 +1,6 @@
 //@ 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!(); } }

tests/ui/issues/issue-25901.rs

@@ -3,6 +3,7 @@ struct B;
 
 static S: &'static B = &A;
 //~^ ERROR cannot call conditionally-const method
+//~| ERROR `Deref` is not yet stable as a const trait
 
 use std::ops::Deref;
 

tests/ui/issues/issue-25901.stderr

@@ -8,6 +8,14 @@ LL | static S: &'static B = &A;
    = help: add `#![feature(const_trait_impl)]` to the crate attributes to enable
    = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
 
-error: aborting due to 1 previous error
+error: `Deref` is not yet stable as a const trait
+  --> $DIR/issue-25901.rs:4:24
+   |
+LL | static S: &'static B = &A;
+   |                        ^^
+   |
+   = help: add `#![feature(const_deref)]` to the crate attributes to enable
+
+error: aborting due to 2 previous errors
 
 For more information about this error, try `rustc --explain E0658`.

tests/ui/self/arbitrary-self-from-method-substs-ice.rs

@@ -12,6 +12,7 @@ impl Foo {
         //~| ERROR destructor of `R` cannot be evaluated at compile-time
         self.0
         //~^ ERROR cannot call conditionally-const method `<R as Deref>::deref` in constant function
+        //~| ERROR `Deref` is not yet stable as a const trait
     }
 }
 

tests/ui/self/arbitrary-self-from-method-substs-ice.stderr

@@ -8,6 +8,14 @@ LL |         self.0
    = help: add `#![feature(const_trait_impl)]` to the crate attributes to enable
    = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
 
+error: `Deref` is not yet stable as a const trait
+  --> $DIR/arbitrary-self-from-method-substs-ice.rs:13:9
+   |
+LL |         self.0
+   |         ^^^^^^
+   |
+   = help: add `#![feature(const_deref)]` to the crate attributes to enable
+
 error[E0493]: destructor of `R` cannot be evaluated at compile-time
   --> $DIR/arbitrary-self-from-method-substs-ice.rs:10:43
    |
@@ -26,7 +34,7 @@ LL |     const fn get<R: Deref<Target = Self>>(self: R) -> u32 {
    = note: type of `self` must not be a method generic parameter type
    = help: use a concrete type such as `self`, `&self`, `&mut self`, `self: Box<Self>`, `self: Rc<Self>`, `self: Arc<Self>`, or `self: Pin<P>` (where P is one of the previous types except `Self`)
 
-error: aborting due to 3 previous errors
+error: aborting due to 4 previous errors
 
 Some errors have detailed explanations: E0493, E0658, E0801.
 For more information about an error, try `rustc --explain E0493`.

tests/ui/stability-attribute/missing-const-stability.rs

@@ -22,6 +22,7 @@ impl Foo {
 #[stable(feature = "stable", since = "1.0.0")]
 #[const_trait]
 pub trait Bar {
+    //~^ ERROR: trait has missing const stability attribute
     #[stable(feature = "stable", since = "1.0.0")]
     fn fun();
 }

tests/ui/stability-attribute/missing-const-stability.stderr

@@ -4,8 +4,18 @@ error: function has missing const stability attribute
 LL | pub const fn foo() {}
    | ^^^^^^^^^^^^^^^^^^^^^
 
+error: trait has missing const stability attribute
+  --> $DIR/missing-const-stability.rs:24:1
+   |
+LL | / pub trait Bar {
+LL | |
+LL | |     #[stable(feature = "stable", since = "1.0.0")]
+LL | |     fn fun();
+LL | | }
+   | |_^
+
 error: function has missing const stability attribute
-  --> $DIR/missing-const-stability.rs:36:1
+  --> $DIR/missing-const-stability.rs:37:1
    |
 LL | pub const unsafe fn size_of_val<T>(x: *const T) -> usize { 42 }
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -16,5 +26,5 @@ error: associated function has missing const stability attribute
 LL |     pub const fn foo() {}
    |     ^^^^^^^^^^^^^^^^^^^^^
 
-error: aborting due to 3 previous errors
+error: aborting due to 4 previous errors
 

tests/ui/traits/const-traits/auxiliary/staged-api.rs

@@ -4,6 +4,7 @@
 #![stable(feature = "rust1", since = "1.0.0")]
 
 #[stable(feature = "rust1", since = "1.0.0")]
+#[rustc_const_unstable(feature = "unstable", issue = "none")]
 #[const_trait]
 pub trait MyTrait {
     #[stable(feature = "rust1", since = "1.0.0")]
@@ -14,16 +15,6 @@ pub trait MyTrait {
 pub struct Unstable;
 
 #[stable(feature = "rust1", since = "1.0.0")]
-#[rustc_const_unstable(feature = "unstable", issue = "none")]
 impl const MyTrait for Unstable {
     fn func() {}
 }
-
-#[stable(feature = "rust1", since = "1.0.0")]
-pub struct Unstable2;
-
-#[stable(feature = "rust1", since = "1.0.0")]
-#[rustc_const_unstable(feature = "unstable2", issue = "none")]
-impl const MyTrait for Unstable2 {
-    fn func() {}
-}

tests/ui/traits/const-traits/call-const-trait-method-pass.rs

@@ -1,6 +1,6 @@
 //@ known-bug: #110395
 
-#![feature(const_trait_impl)]
+#![feature(const_trait_impl, const_ops)]
 
 struct Int(i32);
 

tests/ui/traits/const-traits/const-and-non-const-impl.rs

@@ -1,6 +1,6 @@
 //@ known-bug: #110395
 
-#![feature(const_trait_impl)]
+#![feature(const_trait_impl, const_ops)]
 
 pub struct Int(i32);
 

tests/ui/traits/const-traits/generic-bound.rs

@@ -1,6 +1,6 @@
 //@ check-pass
 
-#![feature(const_trait_impl)]
+#![feature(const_trait_impl, const_ops)]
 
 use std::marker::PhantomData;
 

tests/ui/traits/const-traits/staged-api-user-crate.rs

@@ -10,7 +10,30 @@ fn non_const_context() {
 
 const fn stable_const_context() {
     Unstable::func();
-    //~^ ERROR cannot call conditionally-const associated function `<staged_api::Unstable as staged_api::MyTrait>::func` in constant functions
+    //~^ ERROR: cannot call conditionally-const associated function `<staged_api::Unstable as staged_api::MyTrait>::func` in constant functions
+    //~| ERROR: `staged_api::MyTrait` is not yet stable as a const trait
 }
 
+struct S1;
+impl MyTrait for S1 {
+    fn func() {}
+}
+
+// const impls are gated
+struct S2;
+impl const MyTrait for S2 {
+    //~^ ERROR: `staged_api::MyTrait` is not yet stable as a const trait
+    //~| ERROR: const trait impls are experimental
+    fn func() {}
+}
+
+fn bound1<T: MyTrait>() {}
+const fn bound2<T: ~const MyTrait>() {}
+//~^ ERROR: `staged_api::MyTrait` is not yet stable as a const trait
+//~| ERROR: `staged_api::MyTrait` is not yet stable as a const trait
+//~| ERROR: const trait impls are experimental
+fn bound3<T: const MyTrait>() {}
+//~^ ERROR: `staged_api::MyTrait` is not yet stable as a const trait
+//~| ERROR: const trait impls are experimental
+
 fn main() {}