Implement MIR const trait stability checks

compiler/rustc_const_eval/messages.ftl

@@ -403,7 +403,7 @@ const_eval_uninhabited_enum_variant_read =
 const_eval_uninhabited_enum_variant_written =
     writing discriminant of an uninhabited enum variant
 
-const_eval_unmarked_const_fn_exposed = `{$def_path}` cannot be (indirectly) exposed to stable
+const_eval_unmarked_const_item_exposed = `{$def_path}` cannot be (indirectly) exposed to stable
     .help = either mark the callee as `#[rustc_const_stable_indirect]`, or the caller as `#[rustc_const_unstable]`
 const_eval_unmarked_intrinsic_exposed = intrinsic `{$def_path}` cannot be (indirectly) exposed to stable
     .help = mark the caller as `#[rustc_const_unstable]`, or mark the intrinsic `#[rustc_intrinsic_const_stable_indirect]` (but this requires team approval)
@@ -414,6 +414,7 @@ const_eval_unreachable_unwind =
 
 const_eval_unsized_local = unsized locals are not supported
 const_eval_unstable_const_fn = `{$def_path}` is not yet stable as a const fn
+const_eval_unstable_const_trait = `{$def_path}` is not yet stable as a const trait
 const_eval_unstable_in_stable_exposed =
     const function that might be (indirectly) exposed to stable cannot use `#[feature({$gate})]`
     .is_function_call = mark the callee as `#[rustc_const_stable_indirect]` if it does not itself require any unstable features

compiler/rustc_const_eval/src/check_consts/check.rs

@@ -8,6 +8,7 @@ use std::ops::Deref;
 
 use rustc_attr_parsing::{ConstStability, StabilityLevel};
 use rustc_errors::{Diag, ErrorGuaranteed};
+use rustc_hir::def::DefKind;
 use rustc_hir::def_id::DefId;
 use rustc_hir::{self as hir, LangItem};
 use rustc_index::bit_set::DenseBitSet;
@@ -29,7 +30,7 @@ use super::ops::{self, NonConstOp, Status};
 use super::qualifs::{self, HasMutInterior, NeedsDrop, NeedsNonConstDrop};
 use super::resolver::FlowSensitiveAnalysis;
 use super::{ConstCx, Qualif};
-use crate::check_consts::is_safe_to_expose_on_stable_const_fn;
+use crate::check_consts::is_fn_or_trait_safe_to_expose_on_stable;
 use crate::errors;
 
 type QualifResults<'mir, 'tcx, Q> =
@@ -470,6 +471,88 @@ impl<'mir, 'tcx> Checker<'mir, 'tcx> {
             self.tcx.crate_level_attribute_injection_span(self.tcx.local_def_id_to_hir_id(id))
         })
     }
+
+    /// Check the const stability of the given item (fn or trait).
+    fn check_callee_stability(&mut self, def_id: DefId) {
+        match self.tcx.lookup_const_stability(def_id) {
+            Some(ConstStability { level: StabilityLevel::Stable { .. }, .. }) => {
+                // All good.
+            }
+            None => {
+                // This doesn't need a separate const-stability check -- const-stability equals
+                // regular stability, and regular stability is checked separately.
+                // However, we *do* have to worry about *recursive* const stability.
+                if self.enforce_recursive_const_stability()
+                    && !is_fn_or_trait_safe_to_expose_on_stable(self.tcx, def_id)
+                {
+                    self.dcx().emit_err(errors::UnmarkedConstItemExposed {
+                        span: self.span,
+                        def_path: self.tcx.def_path_str(def_id),
+                    });
+                }
+            }
+            Some(ConstStability {
+                level: StabilityLevel::Unstable { implied_by: implied_feature, issue, .. },
+                feature,
+                ..
+            }) => {
+                // An unstable const fn/trait with a feature gate.
+                let callee_safe_to_expose_on_stable =
+                    is_fn_or_trait_safe_to_expose_on_stable(self.tcx, def_id);
+
+                // We only honor `span.allows_unstable` aka `#[allow_internal_unstable]` if
+                // the callee is safe to expose, to avoid bypassing recursive stability.
+                // This is not ideal since it means the user sees an error, not the macro
+                // author, but that's also the case if one forgets to set
+                // `#[allow_internal_unstable]` in the first place. Note that this cannot be
+                // integrated in the check below since we want to enforce
+                // `callee_safe_to_expose_on_stable` even if
+                // `!self.enforce_recursive_const_stability()`.
+                if (self.span.allows_unstable(feature)
+                    || implied_feature.is_some_and(|f| self.span.allows_unstable(f)))
+                    && callee_safe_to_expose_on_stable
+                {
+                    return;
+                }
+
+                // We can't use `check_op` to check whether the feature is enabled because
+                // the logic is a bit different than elsewhere: local functions don't need
+                // the feature gate, and there might be an "implied" gate that also suffices
+                // to allow this.
+                let feature_enabled = def_id.is_local()
+                    || self.tcx.features().enabled(feature)
+                    || implied_feature.is_some_and(|f| self.tcx.features().enabled(f))
+                    || {
+                        // When we're compiling the compiler itself we may pull in
+                        // crates from crates.io, but those crates may depend on other
+                        // crates also pulled in from crates.io. We want to ideally be
+                        // able to compile everything without requiring upstream
+                        // modifications, so in the case that this looks like a
+                        // `rustc_private` crate (e.g., a compiler crate) and we also have
+                        // the `-Z force-unstable-if-unmarked` flag present (we're
+                        // compiling a compiler crate), then let this missing feature
+                        // annotation slide.
+                        // This matches what we do in `eval_stability_allow_unstable` for
+                        // regular stability.
+                        feature == sym::rustc_private
+                            && issue == NonZero::new(27812)
+                            && self.tcx.sess.opts.unstable_opts.force_unstable_if_unmarked
+                    };
+                // Even if the feature is enabled, we still need check_op to double-check
+                // this if the callee is not safe to expose on stable.
+                if !feature_enabled || !callee_safe_to_expose_on_stable {
+                    self.check_op(ops::CallUnstable {
+                        def_id,
+                        feature,
+                        feature_enabled,
+                        safe_to_expose_on_stable: callee_safe_to_expose_on_stable,
+                        suggestion_span: self.crate_inject_span(),
+                        is_function_call: self.tcx.def_kind(def_id) != DefKind::Trait,
+                    });
+                }
+            }
+        }
+    }
 }
 
 impl<'tcx> Visitor<'tcx> for Checker<'_, 'tcx> {
@@ -716,8 +799,7 @@ impl<'tcx> Visitor<'tcx> for Checker<'_, 'tcx> {
                             span: *fn_span,
                             call_source,
                         });
-                        // FIXME(const_trait_impl): do a more fine-grained check whether this
+                        self.check_callee_stability(trait_did);
-                        // particular trait can be const-stably called.
                     } else {
                         // Not even a const trait.
                         self.check_op(ops::FnCallNonConst {
@@ -793,7 +875,7 @@ impl<'tcx> Visitor<'tcx> for Checker<'_, 'tcx> {
                     // fallback body is safe to expose on stable.
                     let is_const_stable = intrinsic.const_stable
                         || (!intrinsic.must_be_overridden
-                            && is_safe_to_expose_on_stable_const_fn(tcx, callee));
+                            && is_fn_or_trait_safe_to_expose_on_stable(tcx, callee));
                     match tcx.lookup_const_stability(callee) {
                         None => {
                             // This doesn't need a separate const-stability check -- const-stability equals
@@ -842,83 +924,7 @@ impl<'tcx> Visitor<'tcx> for Checker<'_, 'tcx> {
                 }
 
                 // Finally, stability for regular function calls -- this is the big one.
-                match tcx.lookup_const_stability(callee) {
+                self.check_callee_stability(callee);
-                    Some(ConstStability { level: StabilityLevel::Stable { .. }, .. }) => {
-                        // All good.
-                    }
-                    None => {
-                        // This doesn't need a separate const-stability check -- const-stability equals
-                        // regular stability, and regular stability is checked separately.
-                        // However, we *do* have to worry about *recursive* const stability.
-                        if self.enforce_recursive_const_stability()
-                            && !is_safe_to_expose_on_stable_const_fn(tcx, callee)
-                        {
-                            self.dcx().emit_err(errors::UnmarkedConstFnExposed {
-                                span: self.span,
-                                def_path: self.tcx.def_path_str(callee),
-                            });
-                        }
-                    }
-                    Some(ConstStability {
-                        level: StabilityLevel::Unstable { implied_by: implied_feature, issue, .. },
-                        feature,
-                        ..
-                    }) => {
-                        // An unstable const fn with a feature gate.
-                        let callee_safe_to_expose_on_stable =
-                            is_safe_to_expose_on_stable_const_fn(tcx, callee);
-
-                        // We only honor `span.allows_unstable` aka `#[allow_internal_unstable]` if
-                        // the callee is safe to expose, to avoid bypassing recursive stability.
-                        // This is not ideal since it means the user sees an error, not the macro
-                        // author, but that's also the case if one forgets to set
-                        // `#[allow_internal_unstable]` in the first place. Note that this cannot be
-                        // integrated in the check below since we want to enforce
-                        // `callee_safe_to_expose_on_stable` even if
-                        // `!self.enforce_recursive_const_stability()`.
-                        if (self.span.allows_unstable(feature)
-                            || implied_feature.is_some_and(|f| self.span.allows_unstable(f)))
-                            && callee_safe_to_expose_on_stable
-                        {
-                            return;
-                        }
-
-                        // We can't use `check_op` to check whether the feature is enabled because
-                        // the logic is a bit different than elsewhere: local functions don't need
-                        // the feature gate, and there might be an "implied" gate that also suffices
-                        // to allow this.
-                        let feature_enabled = callee.is_local()
-                            || tcx.features().enabled(feature)
-                            || implied_feature.is_some_and(|f| tcx.features().enabled(f))
-                            || {
-                                // When we're compiling the compiler itself we may pull in
-                                // crates from crates.io, but those crates may depend on other
-                                // crates also pulled in from crates.io. We want to ideally be
-                                // able to compile everything without requiring upstream
-                                // modifications, so in the case that this looks like a
-                                // `rustc_private` crate (e.g., a compiler crate) and we also have
-                                // the `-Z force-unstable-if-unmarked` flag present (we're
-                                // compiling a compiler crate), then let this missing feature
-                                // annotation slide.
-                                // This matches what we do in `eval_stability_allow_unstable` for
-                                // regular stability.
-                                feature == sym::rustc_private
-                                    && issue == NonZero::new(27812)
-                                    && tcx.sess.opts.unstable_opts.force_unstable_if_unmarked
-                            };
-                        // Even if the feature is enabled, we still need check_op to double-check
-                        // this if the callee is not safe to expose on stable.
-                        if !feature_enabled || !callee_safe_to_expose_on_stable {
-                            self.check_op(ops::FnCallUnstable {
-                                def_id: callee,
-                                feature,
-                                feature_enabled,
-                                safe_to_expose_on_stable: callee_safe_to_expose_on_stable,
-                                suggestion_span: self.crate_inject_span(),
-                            });
-                        }
-                    }
-                }
             }
 
             // Forbid all `Drop` terminators unless the place being dropped is a local with no

compiler/rustc_const_eval/src/check_consts/mod.rs

@@ -56,7 +56,7 @@ impl<'mir, 'tcx> ConstCx<'mir, 'tcx> {
         self.const_kind == Some(hir::ConstContext::ConstFn)
             && (self.tcx.features().staged_api()
                 || self.tcx.sess.opts.unstable_opts.force_unstable_if_unmarked)
-            && is_safe_to_expose_on_stable_const_fn(self.tcx, self.def_id().to_def_id())
+            && is_fn_or_trait_safe_to_expose_on_stable(self.tcx, self.def_id().to_def_id())
     }
 
     fn is_async(&self) -> bool {
@@ -84,28 +84,14 @@ pub fn rustc_allow_const_fn_unstable(
     attr::rustc_allow_const_fn_unstable(tcx.sess, attrs).any(|name| name == feature_gate)
 }
 
-/// Returns `true` if the given `const fn` is "safe to expose on stable".
+/// Returns `true` if the given `def_id` (trait or function) is "safe to expose on stable".
-///
-/// Panics if the given `DefId` does not refer to a `const fn`.
 ///
 /// This is relevant within a `staged_api` crate. Unlike with normal features, the use of unstable
 /// const features *recursively* taints the functions that use them. This is to avoid accidentally
 /// exposing e.g. the implementation of an unstable const intrinsic on stable. So we partition the
 /// world into two functions: those that are safe to expose on stable (and hence may not use
 /// unstable features, not even recursively), and those that are not.
-pub fn is_safe_to_expose_on_stable_const_fn(tcx: TyCtxt<'_>, def_id: DefId) -> bool {
+pub fn is_fn_or_trait_safe_to_expose_on_stable(tcx: TyCtxt<'_>, def_id: DefId) -> bool {
-    // A default body in a `#[const_trait]` is not const-stable because const trait fns currently
-    // cannot be const-stable. These functions can't be called from anything stable, so we shouldn't
-    // restrict them to only call const-stable functions.
-    if tcx.is_const_default_method(def_id) {
-        // FIXME(const_trait_impl): we have to eventually allow some of these if these things can ever be stable.
-        // They should probably behave like regular `const fn` for that...
-        return false;
-    }
-
-    // Const-stability is only relevant for `const fn`.
-    assert!(tcx.is_const_fn(def_id));
-
     match tcx.lookup_const_stability(def_id) {
         None => {
             // In a `staged_api` crate, we do enforce recursive const stability for all unmarked

compiler/rustc_const_eval/src/check_consts/ops.rs

@@ -377,11 +377,11 @@ fn build_error_for_const_call<'tcx>(
     err
 }
 
-/// A call to an `#[unstable]` const fn or `#[rustc_const_unstable]` function.
+/// A call to an `#[unstable]` const fn, `#[rustc_const_unstable]` function or trait.
 ///
-/// Contains the name of the feature that would allow the use of this function.
+/// Contains the name of the feature that would allow the use of this function/trait.
 #[derive(Debug)]
-pub(crate) struct FnCallUnstable {
+pub(crate) struct CallUnstable {
     pub def_id: DefId,
     pub feature: Symbol,
     /// If this is true, then the feature is enabled, but we need to still check if it is safe to
@@ -389,24 +389,33 @@ pub(crate) struct FnCallUnstable {
     pub feature_enabled: bool,
     pub safe_to_expose_on_stable: bool,
     pub suggestion_span: Option<Span>,
+    /// true if `def_id` is the function we are calling, false if `def_id` is an unstable trait.
+    pub is_function_call: bool,
 }
 
-impl<'tcx> NonConstOp<'tcx> for FnCallUnstable {
+impl<'tcx> NonConstOp<'tcx> for CallUnstable {
     fn status_in_item(&self, _ccx: &ConstCx<'_, 'tcx>) -> Status {
         Status::Unstable {
             gate: self.feature,
             gate_already_checked: self.feature_enabled,
             safe_to_expose_on_stable: self.safe_to_expose_on_stable,
-            is_function_call: true,
+            is_function_call: self.is_function_call,
         }
     }
 
     fn build_error(&self, ccx: &ConstCx<'_, 'tcx>, span: Span) -> Diag<'tcx> {
         assert!(!self.feature_enabled);
-        let mut err = ccx.dcx().create_err(errors::UnstableConstFn {
+        let mut err = if self.is_function_call {
-            span,
+            ccx.dcx().create_err(errors::UnstableConstFn {
-            def_path: ccx.tcx.def_path_str(self.def_id),
+                span,
-        });
+                def_path: ccx.tcx.def_path_str(self.def_id),
+            })
+        } else {
+            ccx.dcx().create_err(errors::UnstableConstTrait {
+                span,
+                def_path: ccx.tcx.def_path_str(self.def_id),
+            })
+        };
         // FIXME: make this translatable
         let msg = format!("add `#![feature({})]` to the crate attributes to enable", self.feature);
         #[allow(rustc::untranslatable_diagnostic)]

compiler/rustc_const_eval/src/errors.rs

@@ -121,6 +121,14 @@ pub(crate) struct UnstableConstFn {
     pub def_path: String,
 }
 
+#[derive(Diagnostic)]
+#[diag(const_eval_unstable_const_trait)]
+pub(crate) struct UnstableConstTrait {
+    #[primary_span]
+    pub span: Span,
+    pub def_path: String,
+}
+
 #[derive(Diagnostic)]
 #[diag(const_eval_unstable_intrinsic)]
 pub(crate) struct UnstableIntrinsic {
@@ -139,9 +147,9 @@ pub(crate) struct UnstableIntrinsic {
 }
 
 #[derive(Diagnostic)]
-#[diag(const_eval_unmarked_const_fn_exposed)]
+#[diag(const_eval_unmarked_const_item_exposed)]
 #[help]
-pub(crate) struct UnmarkedConstFnExposed {
+pub(crate) struct UnmarkedConstItemExposed {
     #[primary_span]
     pub span: Span,
     pub def_path: String,

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

@@ -11,6 +11,7 @@ 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 `staged_api::MyTrait` is not yet stable as a const trait
 }
 
 fn main() {}

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

@@ -9,6 +9,17 @@ LL |     Unstable::func();
    = 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: `staged_api::MyTrait` is not yet stable as a const trait
+  --> $DIR/staged-api-user-crate.rs:12:5
+   |
+LL |     Unstable::func();
+   |     ^^^^^^^^^^^^^^^^
+   |
+help: add `#![feature(unstable)]` to the crate attributes to enable
+   |
+LL + #![feature(unstable)]
+   |
+
+error: aborting due to 2 previous errors
 
 For more information about this error, try `rustc --explain E0658`.

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

@@ -22,7 +22,7 @@ impl const MyTrait for Foo {
     fn func() {}
 }
 
-#[rustc_allow_const_fn_unstable(const_trait_impl)]
+#[rustc_allow_const_fn_unstable(const_trait_impl, unstable)]
 const fn conditionally_const<T: ~const MyTrait>() {
     T::func();
 }
@@ -37,10 +37,13 @@ fn non_const_context() {
 const fn const_context() {
     Unstable::func();
     //~^ ERROR cannot use `#[feature(const_trait_impl)]`
+    //~| ERROR cannot use `#[feature(unstable)]`
     Foo::func();
     //~^ ERROR cannot use `#[feature(const_trait_impl)]`
+    //~| ERROR cannot use `#[feature(unstable)]`
     Unstable2::func();
     //~^ ERROR cannot use `#[feature(const_trait_impl)]`
+    //~| ERROR cannot use `#[feature(unstable)]`
     conditionally_const::<Foo>();
     //~^ ERROR cannot use `#[feature(const_trait_impl)]`
 }
@@ -59,8 +62,10 @@ pub const fn const_context_not_const_stable() {
 const fn stable_const_context() {
     Unstable::func();
     //~^ ERROR cannot use `#[feature(const_trait_impl)]`
+    //~| ERROR cannot use `#[feature(unstable)]`
     Foo::func();
     //~^ ERROR cannot use `#[feature(const_trait_impl)]`
+    //~| ERROR cannot use `#[feature(unstable)]`
     const_context_not_const_stable();
     //~^ ERROR cannot use `#[feature(local_feature)]`
     conditionally_const::<Foo>();

tests/ui/traits/const-traits/staged-api.stderr

@@ -15,8 +15,25 @@ LL + #[rustc_allow_const_fn_unstable(const_trait_impl)]
 LL | const fn const_context() {
    |
 
+error: const function that might be (indirectly) exposed to stable cannot use `#[feature(unstable)]`
+  --> $DIR/staged-api.rs:38:5
+   |
+LL |     Unstable::func();
+   |     ^^^^^^^^^^^^^^^^
+   |
+help: if the function is not (yet) meant to be exposed to stable, add `#[rustc_const_unstable]` (this is what you probably want to do)
+   |
+LL + #[rustc_const_unstable(feature = "...", issue = "...")]
+LL | const fn const_context() {
+   |
+help: otherwise, as a last resort `#[rustc_allow_const_fn_unstable]` can be used to bypass stability checks (this requires team approval)
+   |
+LL + #[rustc_allow_const_fn_unstable(unstable)]
+LL | const fn const_context() {
+   |
+
 error: const function that might be (indirectly) exposed to stable cannot use `#[feature(const_trait_impl)]`
-  --> $DIR/staged-api.rs:40:5
+  --> $DIR/staged-api.rs:41:5
    |
 LL |     Foo::func();
    |     ^^^^^^^^^^^
@@ -32,8 +49,25 @@ LL + #[rustc_allow_const_fn_unstable(const_trait_impl)]
 LL | const fn const_context() {
    |
 
+error: const function that might be (indirectly) exposed to stable cannot use `#[feature(unstable)]`
+  --> $DIR/staged-api.rs:41:5
+   |
+LL |     Foo::func();
+   |     ^^^^^^^^^^^
+   |
+help: if the function is not (yet) meant to be exposed to stable, add `#[rustc_const_unstable]` (this is what you probably want to do)
+   |
+LL + #[rustc_const_unstable(feature = "...", issue = "...")]
+LL | const fn const_context() {
+   |
+help: otherwise, as a last resort `#[rustc_allow_const_fn_unstable]` can be used to bypass stability checks (this requires team approval)
+   |
+LL + #[rustc_allow_const_fn_unstable(unstable)]
+LL | const fn const_context() {
+   |
+
 error: const function that might be (indirectly) exposed to stable cannot use `#[feature(const_trait_impl)]`
-  --> $DIR/staged-api.rs:42:5
+  --> $DIR/staged-api.rs:44:5
    |
 LL |     Unstable2::func();
    |     ^^^^^^^^^^^^^^^^^
@@ -49,9 +83,26 @@ LL + #[rustc_allow_const_fn_unstable(const_trait_impl)]
 LL | const fn const_context() {
    |
 
-error: const function that might be (indirectly) exposed to stable cannot use `#[feature(const_trait_impl)]`
+error: const function that might be (indirectly) exposed to stable cannot use `#[feature(unstable)]`
   --> $DIR/staged-api.rs:44:5
    |
+LL |     Unstable2::func();
+   |     ^^^^^^^^^^^^^^^^^
+   |
+help: if the function is not (yet) meant to be exposed to stable, add `#[rustc_const_unstable]` (this is what you probably want to do)
+   |
+LL + #[rustc_const_unstable(feature = "...", issue = "...")]
+LL | const fn const_context() {
+   |
+help: otherwise, as a last resort `#[rustc_allow_const_fn_unstable]` can be used to bypass stability checks (this requires team approval)
+   |
+LL + #[rustc_allow_const_fn_unstable(unstable)]
+LL | const fn const_context() {
+   |
+
+error: const function that might be (indirectly) exposed to stable cannot use `#[feature(const_trait_impl)]`
+  --> $DIR/staged-api.rs:47:5
+   |
 LL |     conditionally_const::<Foo>();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
@@ -67,7 +118,7 @@ LL | const fn const_context() {
    |
 
 error: const function that might be (indirectly) exposed to stable cannot use `#[feature(const_trait_impl)]`
-  --> $DIR/staged-api.rs:60:5
+  --> $DIR/staged-api.rs:63:5
    |
 LL |     Unstable::func();
    |     ^^^^^^^^^^^^^^^^
@@ -83,8 +134,25 @@ LL + #[rustc_allow_const_fn_unstable(const_trait_impl)]
 LL | const fn stable_const_context() {
    |
 
+error: const function that might be (indirectly) exposed to stable cannot use `#[feature(unstable)]`
+  --> $DIR/staged-api.rs:63:5
+   |
+LL |     Unstable::func();
+   |     ^^^^^^^^^^^^^^^^
+   |
+help: if the function is not (yet) meant to be exposed to stable, add `#[rustc_const_unstable]` (this is what you probably want to do)
+   |
+LL + #[rustc_const_unstable(feature = "...", issue = "...")]
+LL | const fn stable_const_context() {
+   |
+help: otherwise, as a last resort `#[rustc_allow_const_fn_unstable]` can be used to bypass stability checks (this requires team approval)
+   |
+LL + #[rustc_allow_const_fn_unstable(unstable)]
+LL | const fn stable_const_context() {
+   |
+
 error: const function that might be (indirectly) exposed to stable cannot use `#[feature(const_trait_impl)]`
-  --> $DIR/staged-api.rs:62:5
+  --> $DIR/staged-api.rs:66:5
    |
 LL |     Foo::func();
    |     ^^^^^^^^^^^
@@ -100,8 +168,25 @@ LL + #[rustc_allow_const_fn_unstable(const_trait_impl)]
 LL | const fn stable_const_context() {
    |
 
+error: const function that might be (indirectly) exposed to stable cannot use `#[feature(unstable)]`
+  --> $DIR/staged-api.rs:66:5
+   |
+LL |     Foo::func();
+   |     ^^^^^^^^^^^
+   |
+help: if the function is not (yet) meant to be exposed to stable, add `#[rustc_const_unstable]` (this is what you probably want to do)
+   |
+LL + #[rustc_const_unstable(feature = "...", issue = "...")]
+LL | const fn stable_const_context() {
+   |
+help: otherwise, as a last resort `#[rustc_allow_const_fn_unstable]` can be used to bypass stability checks (this requires team approval)
+   |
+LL + #[rustc_allow_const_fn_unstable(unstable)]
+LL | const fn stable_const_context() {
+   |
+
 error: const function that might be (indirectly) exposed to stable cannot use `#[feature(local_feature)]`
-  --> $DIR/staged-api.rs:64:5
+  --> $DIR/staged-api.rs:69:5
    |
 LL |     const_context_not_const_stable();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -119,7 +204,7 @@ LL | const fn stable_const_context() {
    |
 
 error: const function that might be (indirectly) exposed to stable cannot use `#[feature(const_trait_impl)]`
-  --> $DIR/staged-api.rs:66:5
+  --> $DIR/staged-api.rs:71:5
    |
 LL |     conditionally_const::<Foo>();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -135,5 +220,5 @@ LL + #[rustc_allow_const_fn_unstable(const_trait_impl)]
 LL | const fn stable_const_context() {
    |
 
-error: aborting due to 8 previous errors
+error: aborting due to 13 previous errors