Rollup merge of #93358 - compiler-errors:is-not-const, r=fee1-dead

Add note suggesting that predicate may be satisfied, but is not `const`

Not sure if we should be printing this in addition to, or perhaps _instead_ of the help message:
```
help: the trait `~const Add` is not implemented for `NonConstAdd`
```

Also added `ParamEnv::is_const` and `PolyTraitPredicate::is_const_if_const` and, in a separate commit, used those in other places instead of `== hir::Constness::Const`, etc.

r? ````@fee1-dead````

compiler/rustc_const_eval/src/const_eval/eval_queries.rs

@@ -7,7 +7,6 @@ use crate::interpret::{
 };
 
 use rustc_errors::ErrorReported;
-use rustc_hir as hir;
 use rustc_hir::def::DefKind;
 use rustc_middle::mir;
 use rustc_middle::mir::interpret::ErrorHandled;
@@ -216,7 +215,7 @@ pub fn eval_to_const_value_raw_provider<'tcx>(
     tcx: TyCtxt<'tcx>,
     key: ty::ParamEnvAnd<'tcx, GlobalId<'tcx>>,
 ) -> ::rustc_middle::mir::interpret::EvalToConstValueResult<'tcx> {
-    assert!(key.param_env.constness() == hir::Constness::Const);
+    assert!(key.param_env.is_const());
     // see comment in eval_to_allocation_raw_provider for what we're doing here
     if key.param_env.reveal() == Reveal::All {
         let mut key = key;
@@ -251,7 +250,7 @@ pub fn eval_to_allocation_raw_provider<'tcx>(
     tcx: TyCtxt<'tcx>,
     key: ty::ParamEnvAnd<'tcx, GlobalId<'tcx>>,
 ) -> ::rustc_middle::mir::interpret::EvalToAllocationRawResult<'tcx> {
-    assert!(key.param_env.constness() == hir::Constness::Const);
+    assert!(key.param_env.is_const());
     // Because the constant is computed twice (once per value of `Reveal`), we are at risk of
     // reporting the same error twice here. To resolve this, we check whether we can evaluate the
     // constant in the more restrictive `Reveal::UserFacing`, which most likely already was

compiler/rustc_lint/src/traits.rs

@@ -86,7 +86,6 @@ declare_lint_pass!(
 
 impl<'tcx> LateLintPass<'tcx> for DropTraitConstraints {
     fn check_item(&mut self, cx: &LateContext<'tcx>, item: &'tcx hir::Item<'tcx>) {
-        use rustc_middle::ty;
         use rustc_middle::ty::PredicateKind::*;
 
         let predicates = cx.tcx.explicit_predicates_of(item.def_id);
@@ -94,7 +93,7 @@ impl<'tcx> LateLintPass<'tcx> for DropTraitConstraints {
             let Trait(trait_predicate) = predicate.kind().skip_binder() else {
                 continue
             };
-            if trait_predicate.constness == ty::BoundConstness::ConstIfConst {
+            if trait_predicate.is_const_if_const() {
                 // `~const Drop` definitely have meanings so avoid linting here.
                 continue;
             }

compiler/rustc_middle/src/ty/mod.rs

@@ -784,6 +784,11 @@ impl<'tcx> TraitPredicate<'tcx> {
     pub fn self_ty(self) -> Ty<'tcx> {
         self.trait_ref.self_ty()
     }
+
+    #[inline]
+    pub fn is_const_if_const(self) -> bool {
+        self.constness == BoundConstness::ConstIfConst
+    }
 }
 
 impl<'tcx> PolyTraitPredicate<'tcx> {
@@ -803,6 +808,11 @@ impl<'tcx> PolyTraitPredicate<'tcx> {
             p
         });
     }
+
+    #[inline]
+    pub fn is_const_if_const(self) -> bool {
+        self.skip_binder().is_const_if_const()
+    }
 }
 
 #[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash, Debug, TyEncodable, TyDecodable)]
@@ -1388,6 +1398,11 @@ impl<'tcx> ParamEnv<'tcx> {
         self.packed.tag().constness
     }
 
+    #[inline]
+    pub fn is_const(self) -> bool {
+        self.packed.tag().constness == hir::Constness::Const
+    }
+
     /// Construct a trait environment with no where-clauses in scope
     /// where the values of all `impl Trait` and other hidden types
     /// are revealed. This is suitable for monomorphized, post-typeck
@@ -1503,6 +1518,7 @@ impl<'tcx> PolyTraitRef<'tcx> {
             polarity: ty::ImplPolarity::Positive,
         })
     }
+
     #[inline]
     pub fn without_const(self) -> PolyTraitPredicate<'tcx> {
         self.with_constness(BoundConstness::NotConst)

compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs

@@ -439,6 +439,28 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
                         } else {
                             err.span_label(span, explanation);
                         }
+
+                        if trait_predicate.is_const_if_const() && obligation.param_env.is_const() {
+                            let non_const_predicate = trait_ref.without_const();
+                            let non_const_obligation = Obligation {
+                                cause: obligation.cause.clone(),
+                                param_env: obligation.param_env.without_const(),
+                                predicate: non_const_predicate.to_predicate(tcx),
+                                recursion_depth: obligation.recursion_depth,
+                            };
+                            if self.predicate_may_hold(&non_const_obligation) {
+                                err.span_note(
+                                    span,
+                                    &format!(
+                                        "the trait `{}` is implemented for `{}`, \
+                                        but that implementation is not `const`",
+                                        non_const_predicate.print_modifiers_and_trait_path(),
+                                        trait_ref.skip_binder().self_ty(),
+                                    ),
+                                );
+                            }
+                        }
+
                         if let Some((msg, span)) = type_def {
                             err.span_label(span, &msg);
                         }

compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs

@@ -305,7 +305,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             } else if lang_items.unsize_trait() == Some(def_id) {
                 self.assemble_candidates_for_unsizing(obligation, &mut candidates);
             } else if lang_items.drop_trait() == Some(def_id)
-                && obligation.predicate.skip_binder().constness == ty::BoundConstness::ConstIfConst
+                && obligation.predicate.is_const_if_const()
             {
                 self.assemble_const_drop_candidates(obligation, &mut candidates);
             } else {

compiler/rustc_trait_selection/src/traits/select/confirmation.rs

@@ -72,9 +72,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         // CheckPredicate(&A: Super)
         // CheckPredicate(A: ~const Super) // <- still const env, failure
         // ```
-        if obligation.param_env.constness() == Constness::Const
-            && obligation.predicate.skip_binder().constness == ty::BoundConstness::NotConst
-        {
+        if obligation.param_env.is_const() && !obligation.predicate.is_const_if_const() {
             new_obligation = TraitObligation {
                 cause: obligation.cause.clone(),
                 param_env: obligation.param_env.without_const(),

compiler/rustc_trait_selection/src/traits/select/mod.rs

@@ -1173,9 +1173,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                     ImplCandidate(def_id)
                         if tcx.impl_constness(def_id) == hir::Constness::Const => {}
                     // const param
-                    ParamCandidate(trait_pred)
-                        if trait_pred.skip_binder().constness
-                            == ty::BoundConstness::ConstIfConst => {}
+                    ParamCandidate(trait_pred) if trait_pred.is_const_if_const() => {}
                     // auto trait impl
                     AutoImplCandidate(..) => {}
                     // generator, this will raise error in other places