lower impl const to bind to host effect param

24 files changed, 237 insertions, 138 deletions

diff --git a/compiler/rustc_ast/src/ast.rs b/compiler/rustc_ast/src/ast.rs
index 2a268c2da85..f0434b0294f 100644
--- a/compiler/rustc_ast/src/ast.rs
+++ b/compiler/rustc_ast/src/ast.rs
@@ -313,6 +313,16 @@ pub enum TraitBoundModifier {
     MaybeConstMaybe,
 }
 
+impl TraitBoundModifier {
+    pub fn to_constness(self) -> Const {
+        match self {
+            // FIXME(effects) span
+            Self::MaybeConst => Const::Yes(DUMMY_SP),
+            _ => Const::No,
+        }
+    }
+}
+
 /// The AST represents all type param bounds as types.
 /// `typeck::collect::compute_bounds` matches these against
 /// the "special" built-in traits (see `middle::lang_items`) and
diff --git a/compiler/rustc_ast_lowering/src/asm.rs b/compiler/rustc_ast_lowering/src/asm.rs
index ab55c09465b..a1e62699680 100644
--- a/compiler/rustc_ast_lowering/src/asm.rs
+++ b/compiler/rustc_ast_lowering/src/asm.rs
@@ -207,6 +207,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                                 &sym.path,
                                 ParamMode::Optional,
                                 &ImplTraitContext::Disallowed(ImplTraitPosition::Path),
+                                None,
                             );
                             hir::InlineAsmOperand::SymStatic { path, def_id }
                         } else {
diff --git a/compiler/rustc_ast_lowering/src/expr.rs b/compiler/rustc_ast_lowering/src/expr.rs
index 0954cf03da9..f06d6fc6e5b 100644
--- a/compiler/rustc_ast_lowering/src/expr.rs
+++ b/compiler/rustc_ast_lowering/src/expr.rs
@@ -100,6 +100,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                         ParamMode::Optional,
                         ParenthesizedGenericArgs::Err,
                         &ImplTraitContext::Disallowed(ImplTraitPosition::Path),
+                        None,
                     ));
                     let receiver = self.lower_expr(receiver);
                     let args =
@@ -260,6 +261,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                         path,
                         ParamMode::Optional,
                         &ImplTraitContext::Disallowed(ImplTraitPosition::Path),
+                        None,
                     );
                     hir::ExprKind::Path(qpath)
                 }
@@ -307,6 +309,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                             &se.path,
                             ParamMode::Optional,
                             &ImplTraitContext::Disallowed(ImplTraitPosition::Path),
+                            None,
                         )),
                         self.arena
                             .alloc_from_iter(se.fields.iter().map(|x| self.lower_expr_field(x))),
@@ -1179,6 +1182,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                         path,
                         ParamMode::Optional,
                         &ImplTraitContext::Disallowed(ImplTraitPosition::Path),
+                        None,
                     );
                     // Destructure like a tuple struct.
                     let tuple_struct_pat = hir::PatKind::TupleStruct(
@@ -1198,6 +1202,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                         path,
                         ParamMode::Optional,
                         &ImplTraitContext::Disallowed(ImplTraitPosition::Path),
+                        None,
                     );
                     // Destructure like a unit struct.
                     let unit_struct_pat = hir::PatKind::Path(qpath);
@@ -1222,6 +1227,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                     &se.path,
                     ParamMode::Optional,
                     &ImplTraitContext::Disallowed(ImplTraitPosition::Path),
+                    None,
                 );
                 let fields_omitted = match &se.rest {
                     StructRest::Base(e) => {
diff --git a/compiler/rustc_ast_lowering/src/item.rs b/compiler/rustc_ast_lowering/src/item.rs
index 5a0474dbc01..201094667a1 100644
--- a/compiler/rustc_ast_lowering/src/item.rs
+++ b/compiler/rustc_ast_lowering/src/item.rs
@@ -85,6 +85,7 @@ impl<'a, 'hir> ItemLowerer<'a, 'hir> {
             allow_try_trait: Some([sym::try_trait_v2, sym::yeet_desugar_details][..].into()),
             allow_gen_future: Some([sym::gen_future, sym::closure_track_caller][..].into()),
             generics_def_id_map: Default::default(),
+            host_param_id: None,
         };
         lctx.with_hir_id_owner(owner, |lctx| f(lctx));
 
@@ -139,8 +140,24 @@ impl<'a, 'hir> ItemLowerer<'a, 'hir> {
             // This is used to track which lifetimes have already been defined,
             // and which need to be replicated when lowering an async fn.
 
-            if let hir::ItemKind::Impl(impl_) = parent_hir.node().expect_item().kind {
-                lctx.is_in_trait_impl = impl_.of_trait.is_some();
+            match parent_hir.node().expect_item().kind {
+                hir::ItemKind::Impl(impl_) => {
+                    lctx.is_in_trait_impl = impl_.of_trait.is_some();
+                }
+                hir::ItemKind::Trait(_, _, generics, _, _) if lctx.tcx.features().effects => {
+                    lctx.host_param_id = generics
+                        .params
+                        .iter()
+                        .find(|param| {
+                            parent_hir
+                                .attrs
+                                .get(param.hir_id.local_id)
+                                .iter()
+                                .any(|attr| attr.has_name(sym::rustc_host))
+                        })
+                        .map(|param| param.def_id);
+                }
+                _ => {}
             }
 
             match ctxt {
@@ -384,6 +401,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                     self.lower_generics(ast_generics, *constness, id, &itctx, |this| {
                         let trait_ref = trait_ref.as_ref().map(|trait_ref| {
                             this.lower_trait_ref(
+                                *constness,
                                 trait_ref,
                                 &ImplTraitContext::Disallowed(ImplTraitPosition::Trait),
                             )
@@ -414,7 +432,6 @@ impl<'hir> LoweringContext<'_, 'hir> {
                     polarity,
                     defaultness,
                     defaultness_span,
-                    constness: self.lower_constness(*constness),
                     generics,
                     of_trait: trait_ref,
                     self_ty: lowered_ty,
@@ -1358,6 +1375,29 @@ impl<'hir> LoweringContext<'_, 'hir> {
             }
         }
 
+        // Desugar `~const` bound in generics into an additional `const host: bool` param
+        // if the effects feature is enabled. This needs to be done before we lower where
+        // clauses since where clauses need to bind to the DefId of the host param
+        let host_param_parts = if let Const::Yes(span) = constness && self.tcx.features().effects {
+            if let Some(param) = generics.params.iter().find(|x| {
+                x.attrs.iter().any(|x| x.has_name(sym::rustc_host))
+            }) {
+                // user has manually specified a `rustc_host` param, in this case, we set
+                // the param id so that lowering logic can use that. But we don't create
+                // another host param, so this gives `None`.
+                self.host_param_id = Some(self.local_def_id(param.id));
+                None
+            } else {
+                let param_node_id = self.next_node_id();
+                let hir_id = self.next_id();
+                let def_id = self.create_def(self.local_def_id(parent_node_id), param_node_id, DefPathData::TypeNs(sym::host), span);
+                self.host_param_id = Some(def_id);
+                Some((span, hir_id, def_id))
+            }
+        } else {
+            None
+        };
+
         let mut predicates: SmallVec<[hir::WherePredicate<'hir>; 4]> = SmallVec::new();
         predicates.extend(generics.params.iter().filter_map(|param| {
             self.lower_generic_bound_predicate(
@@ -1405,22 +1445,11 @@ impl<'hir> LoweringContext<'_, 'hir> {
         let impl_trait_bounds = std::mem::take(&mut self.impl_trait_bounds);
         predicates.extend(impl_trait_bounds.into_iter());
 
-        // Desugar `~const` bound in generics into an additional `const host: bool` param
-        // if the effects feature is enabled.
-        if let Const::Yes(span) = constness && self.tcx.features().effects
-            // Do not add host param if it already has it (manually specified)
-            && !params.iter().any(|x| {
-                self.attrs.get(&x.hir_id.local_id).map_or(false, |attrs| {
-                    attrs.iter().any(|x| x.has_name(sym::rustc_host))
-                })
-            })
-        {
-            let param_node_id = self.next_node_id();
+        if let Some((span, hir_id, def_id)) = host_param_parts {
             let const_node_id = self.next_node_id();
-            let def_id = self.create_def(self.local_def_id(parent_node_id), param_node_id, DefPathData::TypeNs(sym::host), span);
-            let anon_const: LocalDefId = self.create_def(def_id, const_node_id, DefPathData::AnonConst, span);
+            let anon_const: LocalDefId =
+                self.create_def(def_id, const_node_id, DefPathData::AnonConst, span);
 
-            let hir_id = self.next_id();
             let const_id = self.next_id();
             let const_expr_id = self.next_id();
             let bool_id = self.next_id();
@@ -1430,14 +1459,15 @@ impl<'hir> LoweringContext<'_, 'hir> {
 
             let attr_id = self.tcx.sess.parse_sess.attr_id_generator.mk_attr_id();
 
-            let attrs = self.arena.alloc_from_iter([
-                Attribute {
-                    kind: AttrKind::Normal(P(NormalAttr::from_ident(Ident::new(sym::rustc_host, span)))),
+            let attrs = self.arena.alloc_from_iter([Attribute {
+                kind: AttrKind::Normal(P(NormalAttr::from_ident(Ident::new(
+                    sym::rustc_host,
                     span,
-                    id: attr_id,
-                    style: AttrStyle::Outer,
-                },
-            ]);
+                )))),
+                span,
+                id: attr_id,
+                style: AttrStyle::Outer,
+            }]);
             self.attrs.insert(hir_id.local_id, attrs);
 
             let const_body = self.lower_body(|this| {
@@ -1476,7 +1506,11 @@ impl<'hir> LoweringContext<'_, 'hir> {
                             }),
                         )),
                     )),
-                    default: Some(hir::AnonConst { def_id: anon_const, hir_id: const_id, body: const_body }),
+                    default: Some(hir::AnonConst {
+                        def_id: anon_const,
+                        hir_id: const_id,
+                        body: const_body,
+                    }),
                 },
                 colon_span: None,
                 pure_wrt_drop: false,
diff --git a/compiler/rustc_ast_lowering/src/lib.rs b/compiler/rustc_ast_lowering/src/lib.rs
index ac750690046..dd081fafafb 100644
--- a/compiler/rustc_ast_lowering/src/lib.rs
+++ b/compiler/rustc_ast_lowering/src/lib.rs
@@ -142,6 +142,8 @@ struct LoweringContext<'a, 'hir> {
     /// defined on the TAIT, so we have type Foo<'a1> = ... and we establish a mapping in this
     /// field from the original parameter 'a to the new parameter 'a1.
     generics_def_id_map: Vec<FxHashMap<LocalDefId, LocalDefId>>,
+
+    host_param_id: Option<LocalDefId>,
 }
 
 trait ResolverAstLoweringExt {
@@ -1267,6 +1269,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                         span: t.span
                     },
                     itctx,
+                    ast::Const::No,
                 );
                 let bounds = this.arena.alloc_from_iter([bound]);
                 let lifetime_bound = this.elided_dyn_bound(t.span);
@@ -1277,7 +1280,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         }
 
         let id = self.lower_node_id(t.id);
-        let qpath = self.lower_qpath(t.id, qself, path, param_mode, itctx);
+        let qpath = self.lower_qpath(t.id, qself, path, param_mode, itctx, None);
         self.ty_path(id, t.span, qpath)
     }
 
@@ -1361,10 +1364,12 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                         this.arena.alloc_from_iter(bounds.iter().filter_map(|bound| match bound {
                             GenericBound::Trait(
                                 ty,
-                                TraitBoundModifier::None
+                                modifier @ (TraitBoundModifier::None
                                 | TraitBoundModifier::MaybeConst
-                                | TraitBoundModifier::Negative,
-                            ) => Some(this.lower_poly_trait_ref(ty, itctx)),
+                                | TraitBoundModifier::Negative),
+                            ) => {
+                                Some(this.lower_poly_trait_ref(ty, itctx, modifier.to_constness()))
+                            }
                             // `~const ?Bound` will cause an error during AST validation
                             // anyways, so treat it like `?Bound` as compilation proceeds.
                             GenericBound::Trait(
@@ -2189,7 +2194,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
     ) -> hir::GenericBound<'hir> {
         match tpb {
             GenericBound::Trait(p, modifier) => hir::GenericBound::Trait(
-                self.lower_poly_trait_ref(p, itctx),
+                self.lower_poly_trait_ref(p, itctx, modifier.to_constness()),
                 self.lower_trait_bound_modifier(*modifier),
             ),
             GenericBound::Outlives(lifetime) => {
@@ -2332,8 +2337,20 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         }
     }
 
-    fn lower_trait_ref(&mut self, p: &TraitRef, itctx: &ImplTraitContext) -> hir::TraitRef<'hir> {
-        let path = match self.lower_qpath(p.ref_id, &None, &p.path, ParamMode::Explicit, itctx) {
+    fn lower_trait_ref(
+        &mut self,
+        constness: ast::Const,
+        p: &TraitRef,
+        itctx: &ImplTraitContext,
+    ) -> hir::TraitRef<'hir> {
+        let path = match self.lower_qpath(
+            p.ref_id,
+            &None,
+            &p.path,
+            ParamMode::Explicit,
+            itctx,
+            Some(constness),
+        ) {
             hir::QPath::Resolved(None, path) => path,
             qpath => panic!("lower_trait_ref: unexpected QPath `{qpath:?}`"),
         };
@@ -2345,10 +2362,11 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         &mut self,
         p: &PolyTraitRef,
         itctx: &ImplTraitContext,
+        constness: ast::Const,
     ) -> hir::PolyTraitRef<'hir> {
         let bound_generic_params =
             self.lower_lifetime_binder(p.trait_ref.ref_id, &p.bound_generic_params);
-        let trait_ref = self.lower_trait_ref(&p.trait_ref, itctx);
+        let trait_ref = self.lower_trait_ref(constness, &p.trait_ref, itctx);
         hir::PolyTraitRef { bound_generic_params, trait_ref, span: self.lower_span(p.span) }
     }
 
@@ -2702,6 +2720,57 @@ struct GenericArgsCtor<'hir> {
 }
 
 impl<'hir> GenericArgsCtor<'hir> {
+    fn push_constness(&mut self, lcx: &mut LoweringContext<'_, 'hir>, constness: ast::Const) {
+        if !lcx.tcx.features().effects {
+            return;
+        }
+
+        // if bound is non-const, don't add host effect param
+        let ast::Const::Yes(span) = constness else { return };
+
+        let span = lcx.lower_span(span);
+
+        let id = lcx.next_node_id();
+        let hir_id = lcx.next_id();
+        let body = lcx.lower_body(|lcx| {
+            (
+                &[],
+                match constness {
+                    ast::Const::Yes(_) => {
+                        let hir_id = lcx.next_id();
+                        let res =
+                            Res::Def(DefKind::ConstParam, lcx.host_param_id.unwrap().to_def_id());
+                        let expr_kind = hir::ExprKind::Path(hir::QPath::Resolved(
+                            None,
+                            lcx.arena.alloc(hir::Path {
+                                span,
+                                res,
+                                segments: arena_vec![lcx; hir::PathSegment::new(Ident {
+                                        name: sym::host,
+                                        span,
+                                    }, hir_id, res)],
+                            }),
+                        ));
+                        lcx.expr(span, expr_kind)
+                    }
+                    ast::Const::No => lcx.expr(
+                        span,
+                        hir::ExprKind::Lit(
+                            lcx.arena.alloc(hir::Lit { span, node: ast::LitKind::Bool(true) }),
+                        ),
+                    ),
+                },
+            )
+        });
+        let def_id =
+            lcx.create_def(lcx.current_hir_id_owner.def_id, id, DefPathData::AnonConst, span);
+        lcx.children.push((def_id, hir::MaybeOwner::NonOwner(hir_id)));
+        self.args.push(hir::GenericArg::Const(hir::ConstArg {
+            value: hir::AnonConst { def_id, hir_id, body },
+            span,
+        }))
+    }
+
     fn is_empty(&self) -> bool {
         self.args.is_empty()
             && self.bindings.is_empty()
diff --git a/compiler/rustc_ast_lowering/src/pat.rs b/compiler/rustc_ast_lowering/src/pat.rs
index 2509b705639..a30f264bc7d 100644
--- a/compiler/rustc_ast_lowering/src/pat.rs
+++ b/compiler/rustc_ast_lowering/src/pat.rs
@@ -38,6 +38,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                             path,
                             ParamMode::Optional,
                             &ImplTraitContext::Disallowed(ImplTraitPosition::Path),
+                            None,
                         );
                         let (pats, ddpos) = self.lower_pat_tuple(pats, "tuple struct");
                         break hir::PatKind::TupleStruct(qpath, pats, ddpos);
@@ -54,6 +55,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                             path,
                             ParamMode::Optional,
                             &ImplTraitContext::Disallowed(ImplTraitPosition::Path),
+                            None,
                         );
                         break hir::PatKind::Path(qpath);
                     }
@@ -64,6 +66,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                             path,
                             ParamMode::Optional,
                             &ImplTraitContext::Disallowed(ImplTraitPosition::Path),
+                            None,
                         );
 
                         let fs = self.arena.alloc_from_iter(fields.iter().map(|f| {
diff --git a/compiler/rustc_ast_lowering/src/path.rs b/compiler/rustc_ast_lowering/src/path.rs
index 441282c05b4..899f92a9958 100644
--- a/compiler/rustc_ast_lowering/src/path.rs
+++ b/compiler/rustc_ast_lowering/src/path.rs
@@ -23,6 +23,8 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         p: &Path,
         param_mode: ParamMode,
         itctx: &ImplTraitContext,
+        // constness of the impl/bound if this is a trait path
+        constness: Option<ast::Const>,
     ) -> hir::QPath<'hir> {
         let qself_position = qself.as_ref().map(|q| q.position);
         let qself = qself.as_ref().map(|q| self.lower_ty(&q.ty, itctx));
@@ -73,6 +75,8 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                         param_mode,
                         parenthesized_generic_args,
                         itctx,
+                        // if this is the last segment, add constness to the trait path
+                        if i == proj_start - 1 { constness } else { None },
                     )
                 },
             )),
@@ -119,6 +123,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                 param_mode,
                 ParenthesizedGenericArgs::Err,
                 itctx,
+                None,
             ));
             let qpath = hir::QPath::TypeRelative(ty, hir_segment);
 
@@ -159,6 +164,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                     param_mode,
                     ParenthesizedGenericArgs::Err,
                     &ImplTraitContext::Disallowed(ImplTraitPosition::Path),
+                    None,
                 )
             })),
             span: self.lower_span(p.span),
@@ -172,8 +178,9 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         param_mode: ParamMode,
         parenthesized_generic_args: ParenthesizedGenericArgs,
         itctx: &ImplTraitContext,
+        constness: Option<ast::Const>,
     ) -> hir::PathSegment<'hir> {
-        debug!("path_span: {:?}, lower_path_segment(segment: {:?})", path_span, segment,);
+        debug!("path_span: {:?}, lower_path_segment(segment: {:?})", path_span, segment);
         let (mut generic_args, infer_args) = if let Some(generic_args) = segment.args.as_deref() {
             match generic_args {
                 GenericArgs::AngleBracketed(data) => {
@@ -231,6 +238,10 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
             )
         };
 
+        if let Some(constness) = constness {
+            generic_args.push_constness(self, constness);
+        }
+
         let has_lifetimes =
             generic_args.args.iter().any(|arg| matches!(arg, GenericArg::Lifetime(_)));
 
diff --git a/compiler/rustc_const_eval/src/const_eval/fn_queries.rs b/compiler/rustc_const_eval/src/const_eval/fn_queries.rs
index cc39387c41f..4ee4ebbb9e4 100644
--- a/compiler/rustc_const_eval/src/const_eval/fn_queries.rs
+++ b/compiler/rustc_const_eval/src/const_eval/fn_queries.rs
@@ -40,7 +40,7 @@ fn constness(tcx: TyCtxt<'_>, def_id: LocalDefId) -> hir::Constness {
         | hir::Node::AnonConst(_)
         | hir::Node::ConstBlock(_)
         | hir::Node::ImplItem(hir::ImplItem { kind: hir::ImplItemKind::Const(..), .. }) => hir::Constness::Const,
-        hir::Node::Item(hir::Item { kind: hir::ItemKind::Impl(impl_), .. }) => impl_.constness,
+        hir::Node::Item(hir::Item { kind: hir::ItemKind::Impl(_), .. }) => tcx.generics_of(def_id).host_effect_index.map_or(hir::Constness::NotConst, |_| hir::Constness::Const),
         hir::Node::ForeignItem(hir::ForeignItem { kind: hir::ForeignItemKind::Fn(..), .. }) => {
             // Intrinsics use `rustc_const_{un,}stable` attributes to indicate constness. All other
             // foreign items cannot be evaluated at compile-time.
diff --git a/compiler/rustc_const_eval/src/transform/check_consts/mod.rs b/compiler/rustc_const_eval/src/transform/check_consts/mod.rs
index e77fb4ea2a2..e51082e1ec0 100644
--- a/compiler/rustc_const_eval/src/transform/check_consts/mod.rs
+++ b/compiler/rustc_const_eval/src/transform/check_consts/mod.rs
@@ -127,15 +127,8 @@ fn is_parent_const_stable_trait(tcx: TyCtxt<'_>, def_id: DefId) -> bool {
     let hir_id = tcx.local_def_id_to_hir_id(local_def_id);
 
     let Some(parent) = tcx.hir().opt_parent_id(hir_id) else { return false };
-    let parent_def = tcx.hir().get(parent);
-
-    if !matches!(
-        parent_def,
-        hir::Node::Item(hir::Item {
-            kind: hir::ItemKind::Impl(hir::Impl { constness: hir::Constness::Const, .. }),
-            ..
-        })
-    ) {
+
+    if !tcx.is_const_trait_impl_raw(parent.owner.def_id.to_def_id()) {
         return false;
     }
 
diff --git a/compiler/rustc_const_eval/src/transform/check_consts/ops.rs b/compiler/rustc_const_eval/src/transform/check_consts/ops.rs
index e785196c744..1f3cda35c2b 100644
--- a/compiler/rustc_const_eval/src/transform/check_consts/ops.rs
+++ b/compiler/rustc_const_eval/src/transform/check_consts/ops.rs
@@ -145,8 +145,11 @@ impl<'tcx> NonConstOp<'tcx> for FnCallNonConst<'tcx> {
                     let implsrc = selcx.select(&obligation);
 
                     if let Ok(Some(ImplSource::UserDefined(data))) = implsrc {
-                        let span = tcx.def_span(data.impl_def_id);
-                        err.subdiagnostic(errors::NonConstImplNote { span });
+                        // FIXME(effects) revisit this
+                        if !tcx.is_const_trait_impl_raw(data.impl_def_id) {
+                            let span = tcx.def_span(data.impl_def_id);
+                            err.subdiagnostic(errors::NonConstImplNote { span });
+                        }
                     }
                 }
                 _ => {}
diff --git a/compiler/rustc_hir/src/hir.rs b/compiler/rustc_hir/src/hir.rs
index 6b76e16825f..3922dadfdb1 100644
--- a/compiler/rustc_hir/src/hir.rs
+++ b/compiler/rustc_hir/src/hir.rs
@@ -3357,7 +3357,6 @@ pub struct Impl<'hir> {
     // We do not put a `Span` in `Defaultness` because it breaks foreign crate metadata
     // decoding as `Span`s cannot be decoded when a `Session` is not available.
     pub defaultness_span: Option<Span>,
-    pub constness: Constness,
     pub generics: &'hir Generics<'hir>,
 
     /// The trait being implemented, if any.
diff --git a/compiler/rustc_hir/src/intravisit.rs b/compiler/rustc_hir/src/intravisit.rs
index a8a94e6a476..056b68949a2 100644
--- a/compiler/rustc_hir/src/intravisit.rs
+++ b/compiler/rustc_hir/src/intravisit.rs
@@ -522,7 +522,6 @@ pub fn walk_item<'v, V: Visitor<'v>>(visitor: &mut V, item: &'v Item<'v>) {
             unsafety: _,
             defaultness: _,
             polarity: _,
-            constness: _,
             defaultness_span: _,
             ref generics,
             ref of_trait,
diff --git a/compiler/rustc_hir_analysis/src/astconv/mod.rs b/compiler/rustc_hir_analysis/src/astconv/mod.rs
index b40e3123522..7c03c1794f0 100644
--- a/compiler/rustc_hir_analysis/src/astconv/mod.rs
+++ b/compiler/rustc_hir_analysis/src/astconv/mod.rs
@@ -532,6 +532,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
                         if let Err(guar) = ty.error_reported() {
                             return ty::Const::new_error(tcx, guar, ty).into();
                         }
+                        // FIXME(effects) see if we should special case effect params here
                         if !infer_args && has_default {
                             tcx.const_param_default(param.def_id)
                                 .instantiate(tcx, args.unwrap())
@@ -659,7 +660,6 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
         &self,
         trait_ref: &hir::TraitRef<'_>,
         self_ty: Ty<'tcx>,
-        constness: ty::BoundConstness,
     ) -> ty::TraitRef<'tcx> {
         self.prohibit_generics(trait_ref.path.segments.split_last().unwrap().1.iter(), |_| {});
 
@@ -669,7 +669,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
             self_ty,
             trait_ref.path.segments.last().unwrap(),
             true,
-            constness,
+            ty::BoundConstness::NotConst,
         )
     }
 
@@ -849,6 +849,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
         self_ty: Ty<'tcx>,
         trait_segment: &hir::PathSegment<'_>,
         is_impl: bool,
+        // FIXME(effects) move all host param things in astconv to hir lowering
         constness: ty::BoundConstness,
     ) -> ty::TraitRef<'tcx> {
         let (generic_args, _) = self.create_args_for_ast_trait_ref(
@@ -2712,11 +2713,8 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
         };
         let i = hir.get_parent(fn_hir_id).expect_item().expect_impl();
 
-        let trait_ref = self.instantiate_mono_trait_ref(
-            i.of_trait.as_ref()?,
-            self.ast_ty_to_ty(i.self_ty),
-            ty::BoundConstness::NotConst,
-        );
+        let trait_ref =
+            self.instantiate_mono_trait_ref(i.of_trait.as_ref()?, self.ast_ty_to_ty(i.self_ty));
 
         let assoc = tcx.associated_items(trait_ref.def_id).find_by_name_and_kind(
             tcx,
diff --git a/compiler/rustc_hir_analysis/src/collect.rs b/compiler/rustc_hir_analysis/src/collect.rs
index f568b751951..c65cd25f12c 100644
--- a/compiler/rustc_hir_analysis/src/collect.rs
+++ b/compiler/rustc_hir_analysis/src/collect.rs
@@ -1358,38 +1358,29 @@ fn impl_trait_ref(
         .of_trait
         .as_ref()
         .map(|ast_trait_ref| {
-            let selfty = tcx.type_of(def_id).instantiate_identity();
-            icx.astconv().instantiate_mono_trait_ref(
+            check_impl_constness(
+                tcx,
+                tcx.is_const_trait_impl_raw(def_id.to_def_id()),
                 ast_trait_ref,
-                selfty,
-                check_impl_constness(tcx, impl_.constness, ast_trait_ref),
-            )
+            );
+            let selfty = tcx.type_of(def_id).instantiate_identity();
+            icx.astconv().instantiate_mono_trait_ref(ast_trait_ref, selfty)
         })
         .map(ty::EarlyBinder::bind)
 }
 
-fn check_impl_constness(
-    tcx: TyCtxt<'_>,
-    constness: hir::Constness,
-    ast_trait_ref: &hir::TraitRef<'_>,
-) -> ty::BoundConstness {
-    match constness {
-        hir::Constness::Const => {
-            if let Some(trait_def_id) = ast_trait_ref.trait_def_id() && !tcx.has_attr(trait_def_id, sym::const_trait) {
-                let trait_name = tcx.item_name(trait_def_id).to_string();
-                tcx.sess.emit_err(errors::ConstImplForNonConstTrait {
-                    trait_ref_span: ast_trait_ref.path.span,
-                    trait_name,
-                    local_trait_span: trait_def_id.as_local().map(|_| tcx.def_span(trait_def_id).shrink_to_lo()),
-                    marking: (),
-                    adding: (),
-                });
-                ty::BoundConstness::NotConst
-            } else {
-                ty::BoundConstness::ConstIfConst
-            }
-        },
-        hir::Constness::NotConst => ty::BoundConstness::NotConst,
+fn check_impl_constness(tcx: TyCtxt<'_>, is_const: bool, ast_trait_ref: &hir::TraitRef<'_>) {
+    if is_const {
+        if let Some(trait_def_id) = ast_trait_ref.trait_def_id() && !tcx.has_attr(trait_def_id, sym::const_trait) {
+            let trait_name = tcx.item_name(trait_def_id).to_string();
+            tcx.sess.emit_err(errors::ConstImplForNonConstTrait {
+                trait_ref_span: ast_trait_ref.path.span,
+                trait_name,
+                local_trait_span: trait_def_id.as_local().map(|_| tcx.def_span(trait_def_id).shrink_to_lo()),
+                marking: (),
+                adding: (),
+            });
+        }
     }
 }
 
diff --git a/compiler/rustc_hir_analysis/src/collect/generics_of.rs b/compiler/rustc_hir_analysis/src/collect/generics_of.rs
index 6e1762c54f2..4842008279a 100644
--- a/compiler/rustc_hir_analysis/src/collect/generics_of.rs
+++ b/compiler/rustc_hir_analysis/src/collect/generics_of.rs
@@ -320,7 +320,7 @@ pub(super) fn generics_of(tcx: TyCtxt<'_>, def_id: LocalDefId) -> ty::Generics {
                     bug!("parent also has host effect param? index: {idx}, def: {def_id:?}");
                 }
 
-                host_effect_index = Some(parent_count + index as usize);
+                host_effect_index = Some(index as usize);
             }
 
             Some(ty::GenericParamDef {
diff --git a/compiler/rustc_hir_analysis/src/structured_errors/wrong_number_of_generic_args.rs b/compiler/rustc_hir_analysis/src/structured_errors/wrong_number_of_generic_args.rs
index 6be8d72aed2..61b182b1be7 100644
--- a/compiler/rustc_hir_analysis/src/structured_errors/wrong_number_of_generic_args.rs
+++ b/compiler/rustc_hir_analysis/src/structured_errors/wrong_number_of_generic_args.rs
@@ -578,6 +578,9 @@ impl<'a, 'tcx> WrongNumberOfGenericArgs<'a, 'tcx> {
             MissingTypesOrConsts { .. } => {
                 self.suggest_adding_type_and_const_args(err);
             }
+            ExcessTypesOrConsts { .. } => {
+                // this can happen with `~const T` where T isn't a const_trait.
+            }
             _ => unreachable!(),
         }
     }
diff --git a/compiler/rustc_hir_pretty/src/lib.rs b/compiler/rustc_hir_pretty/src/lib.rs
index 2d8b956771b..e2aecccb6e9 100644
--- a/compiler/rustc_hir_pretty/src/lib.rs
+++ b/compiler/rustc_hir_pretty/src/lib.rs
@@ -626,7 +626,6 @@ impl<'a> State<'a> {
                 unsafety,
                 polarity,
                 defaultness,
-                constness,
                 defaultness_span: _,
                 generics,
                 ref of_trait,
@@ -643,10 +642,6 @@ impl<'a> State<'a> {
                     self.space();
                 }
 
-                if constness == hir::Constness::Const {
-                    self.word_nbsp("const");
-                }
-
                 if let hir::ImplPolarity::Negative(_) = polarity {
                     self.word("!");
                 }
diff --git a/compiler/rustc_hir_typeck/src/callee.rs b/compiler/rustc_hir_typeck/src/callee.rs
index c68f2d94f35..ba00954db91 100644
--- a/compiler/rustc_hir_typeck/src/callee.rs
+++ b/compiler/rustc_hir_typeck/src/callee.rs
@@ -767,9 +767,15 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     ) {
         let tcx = self.tcx;
 
-        if !tcx.features().effects || tcx.sess.opts.unstable_opts.unleash_the_miri_inside_of_you {
-            return;
-        }
+        // fast-reject if callee doesn't have the host effect param (non-const)
+        let generics = tcx.generics_of(callee_did);
+        let Some(host_effect_index) = generics.host_effect_index else { return };
+
+        // if the callee does have the param, we need to equate the param to some const
+        // value no matter whether the effects feature is enabled in the local crate,
+        // because inference will fail if we don't.
+        let mut host_always_on =
+            !tcx.features().effects || tcx.sess.opts.unstable_opts.unleash_the_miri_inside_of_you;
 
         // Compute the constness required by the context.
         let context = tcx.hir().enclosing_body_owner(call_expr_hir);
@@ -780,10 +786,11 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 
         if tcx.has_attr(context.to_def_id(), sym::rustc_do_not_const_check) {
             trace!("do not const check this context");
-            return;
+            host_always_on = true;
         }
 
         let effect = match const_context {
+            _ if host_always_on => tcx.consts.true_,
             Some(hir::ConstContext::Static(_) | hir::ConstContext::Const) => tcx.consts.false_,
             Some(hir::ConstContext::ConstFn) => {
                 let args = ty::GenericArgs::identity_for_item(tcx, context);
@@ -792,21 +799,17 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             None => tcx.consts.true_,
         };
 
-        let generics = tcx.generics_of(callee_did);
-
         trace!(?effect, ?generics, ?callee_args);
 
-        if let Some(idx) = generics.host_effect_index {
-            let param = callee_args.const_at(idx);
-            let cause = self.misc(span);
-            match self.at(&cause, self.param_env).eq(infer::DefineOpaqueTypes::No, effect, param) {
-                Ok(infer::InferOk { obligations, value: () }) => {
-                    self.register_predicates(obligations);
-                }
-                Err(e) => {
-                    // FIXME(effects): better diagnostic
-                    self.err_ctxt().report_mismatched_consts(&cause, effect, param, e).emit();
-                }
+        let param = callee_args.const_at(host_effect_index);
+        let cause = self.misc(span);
+        match self.at(&cause, self.param_env).eq(infer::DefineOpaqueTypes::No, effect, param) {
+            Ok(infer::InferOk { obligations, value: () }) => {
+                self.register_predicates(obligations);
+            }
+            Err(e) => {
+                // FIXME(effects): better diagnostic
+                self.err_ctxt().report_mismatched_consts(&cause, effect, param, e).emit();
             }
         }
     }
diff --git a/compiler/rustc_hir_typeck/src/fn_ctxt/_impl.rs b/compiler/rustc_hir_typeck/src/fn_ctxt/_impl.rs
index 1433c67d55d..6b118ea5a29 100644
--- a/compiler/rustc_hir_typeck/src/fn_ctxt/_impl.rs
+++ b/compiler/rustc_hir_typeck/src/fn_ctxt/_impl.rs
@@ -1349,7 +1349,10 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                         }
                     }
                     GenericParamDefKind::Const { has_default } => {
-                        if !infer_args && has_default {
+                        if !infer_args
+                            && has_default
+                            && !tcx.has_attr(param.def_id, sym::rustc_host)
+                        {
                             tcx.const_param_default(param.def_id)
                                 .instantiate(tcx, args.unwrap())
                                 .into()
diff --git a/compiler/rustc_middle/src/ty/context.rs b/compiler/rustc_middle/src/ty/context.rs
index 5eef3e45f32..30f60abe31c 100644
--- a/compiler/rustc_middle/src/ty/context.rs
+++ b/compiler/rustc_middle/src/ty/context.rs
@@ -1963,9 +1963,9 @@ impl<'tcx> TyCtxt<'tcx> {
         matches!(
             node,
             hir::Node::Item(hir::Item {
-                kind: hir::ItemKind::Impl(hir::Impl { constness: hir::Constness::Const, .. }),
+                kind: hir::ItemKind::Impl(hir::Impl { generics, .. }),
                 ..
-            })
+            }) if generics.params.iter().any(|p| self.has_attr(p.def_id, sym::rustc_host))
         )
     }
 
diff --git a/compiler/rustc_middle/src/ty/print/pretty.rs b/compiler/rustc_middle/src/ty/print/pretty.rs
index d3fd49150ba..ed0af6f5eb1 100644
--- a/compiler/rustc_middle/src/ty/print/pretty.rs
+++ b/compiler/rustc_middle/src/ty/print/pretty.rs
@@ -2841,6 +2841,11 @@ define_print_and_forward_display! {
 
     ty::TraitPredicate<'tcx> {
         p!(print(self.trait_ref.self_ty()), ": ");
+        if let Some(idx) = cx.tcx().generics_of(self.trait_ref.def_id).host_effect_index {
+            if self.trait_ref.args.const_at(idx) != cx.tcx().consts.true_ {
+                p!("~const ");
+            }
+        }
         // FIXME(effects) print `~const` here
         if let ty::ImplPolarity::Negative = self.polarity {
             p!("!");
diff --git a/compiler/rustc_passes/src/stability.rs b/compiler/rustc_passes/src/stability.rs
index 25a3d38c144..3950ac9dd31 100644
--- a/compiler/rustc_passes/src/stability.rs
+++ b/compiler/rustc_passes/src/stability.rs
@@ -732,13 +732,7 @@ 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,
-                constness,
-                ..
-            }) => {
+            hir::ItemKind::Impl(hir::Impl { of_trait: Some(ref t), self_ty, items, .. }) => {
                 let features = self.tcx.features();
                 if features.staged_api {
                     let attrs = self.tcx.hir().attrs(item.hir_id());
@@ -769,7 +763,7 @@ 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
+                        && self.tcx.is_const_trait_impl_raw(item.owner_id.to_def_id())
                         && const_stab.is_some_and(|(stab, _)| stab.is_const_stable())
                     {
                         self.tcx.sess.emit_err(errors::TraitImplConstStable { span: item.span });
diff --git a/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs b/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs
index eae13eb6302..f56b4425b12 100644
--- a/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs
+++ b/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs
@@ -3102,27 +3102,6 @@ impl<'tcx> InferCtxtPrivExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
     ) -> UnsatisfiedConst {
         let unsatisfied_const = UnsatisfiedConst(false);
         // FIXME(effects)
-        /* if trait_predicate.is_const_if_const() {
-            let non_const_predicate = trait_ref.without_const();
-            let non_const_obligation = Obligation {
-                cause: obligation.cause.clone(),
-                param_env: obligation.param_env,
-                predicate: non_const_predicate.to_predicate(self.tcx),
-                recursion_depth: obligation.recursion_depth,
-            };
-            if self.predicate_may_hold(&non_const_obligation) {
-                unsatisfied_const = UnsatisfiedConst(true);
-                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(),
-                    ),
-                );
-            }
-        } */
         unsatisfied_const
     }
 
diff --git a/tests/ui/rfcs/rfc-2632-const-trait-impl/const-drop-fail-2.rs b/tests/ui/rfcs/rfc-2632-const-trait-impl/const-drop-fail-2.rs
index 3de9d37d493..10f6c5c064a 100644
--- a/tests/ui/rfcs/rfc-2632-const-trait-impl/const-drop-fail-2.rs
+++ b/tests/ui/rfcs/rfc-2632-const-trait-impl/const-drop-fail-2.rs
@@ -1,5 +1,5 @@
 // known-bug: #110395
-#![feature(const_trait_impl)]
+#![feature(const_trait_impl, effects)]
 #![feature(const_mut_refs)]
 #![cfg_attr(precise, feature(const_precise_live_drops))]