Duplicate ~const bounds with a non-const one in effects desugaring

compiler/rustc_hir_analysis/src/astconv/mod.rs

@@ -1088,7 +1088,8 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
             self.trait_defines_associated_item_named(r.def_id(), ty::AssocKind::Const, assoc_name)
         });
 
-        let (bound, next_cand) = match (matching_candidates.next(), const_candidates.next()) {
+        let (mut bound, mut next_cand) = match (matching_candidates.next(), const_candidates.next())
+        {
             (Some(bound), _) => (bound, matching_candidates.next()),
             (None, Some(bound)) => (bound, const_candidates.next()),
             (None, None) => {
@@ -1103,6 +1104,37 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
         };
         debug!(?bound);
 
+        // look for a candidate that is not the same as our first bound, disregarding
+        // whether the bound is const.
+        while let Some(mut bound2) = next_cand {
+            debug!(?bound2);
+            let tcx = self.tcx();
+            if bound2.bound_vars() != bound.bound_vars() {
+                break;
+            }
+
+            let generics = tcx.generics_of(bound.def_id());
+            let Some(host_index) = generics.host_effect_index else { break };
+
+            // always return the bound that contains the host param.
+            if let ty::ConstKind::Param(_) = bound2.skip_binder().args.const_at(host_index).kind() {
+                (bound, bound2) = (bound2, bound);
+            }
+
+            let unconsted_args = bound
+                .skip_binder()
+                .args
+                .iter()
+                .enumerate()
+                .map(|(n, arg)| if host_index == n { tcx.consts.true_.into() } else { arg });
+
+            if unconsted_args.eq(bound2.skip_binder().args.iter()) {
+                next_cand = matching_candidates.next().or_else(|| const_candidates.next());
+            } else {
+                break;
+            }
+        }
+
         if let Some(bound2) = next_cand {
             debug!(?bound2);
 

compiler/rustc_hir_analysis/src/bounds.rs

@@ -43,6 +43,30 @@ impl<'tcx> Bounds<'tcx> {
         trait_ref: ty::PolyTraitRef<'tcx>,
         span: Span,
         polarity: ty::ImplPolarity,
+    ) {
+        self.push_trait_bound_inner(tcx, trait_ref, span, polarity);
+
+        if tcx.features().effects && trait_ref.skip_binder().args.host_effect_param().is_some() {
+            let generics = tcx.generics_of(trait_ref.def_id());
+            let Some(host_index) = generics.host_effect_index else { return };
+            let trait_ref = trait_ref.map_bound(|mut trait_ref| {
+                trait_ref.args =
+                    tcx.mk_args_from_iter(trait_ref.args.iter().enumerate().map(|(n, arg)| {
+                        if host_index == n { tcx.consts.true_.into() } else { arg }
+                    }));
+                trait_ref
+            });
+
+            self.push_trait_bound_inner(tcx, trait_ref, span, polarity);
+        }
+    }
+
+    fn push_trait_bound_inner(
+        &mut self,
+        tcx: TyCtxt<'tcx>,
+        trait_ref: ty::PolyTraitRef<'tcx>,
+        span: Span,
+        polarity: ty::ImplPolarity,
     ) {
         self.clauses.push((
             trait_ref

compiler/rustc_hir_analysis/src/collect/predicates_of.rs

@@ -11,7 +11,7 @@ use rustc_hir::intravisit::{self, Visitor};
 use rustc_middle::ty::{self, Ty, TyCtxt};
 use rustc_middle::ty::{GenericPredicates, ImplTraitInTraitData, ToPredicate};
 use rustc_span::symbol::Ident;
-use rustc_span::{Span, DUMMY_SP};
+use rustc_span::{sym, Span, DUMMY_SP};
 
 /// Returns a list of all type predicates (explicit and implicit) for the definition with
 /// ID `def_id`. This includes all predicates returned by `predicates_defined_on`, plus
@@ -38,11 +38,38 @@ pub(super) fn predicates_of(tcx: TyCtxt<'_>, def_id: DefId) -> ty::GenericPredic
         // an obligation and instead be skipped. Otherwise we'd use
         // `tcx.def_span(def_id);`
         let span = rustc_span::DUMMY_SP;
-        result.predicates =
-            tcx.arena.alloc_from_iter(result.predicates.iter().copied().chain(std::iter::once((
-                ty::TraitRef::identity(tcx, def_id).to_predicate(tcx),
+        let non_const_bound = if tcx.features().effects && tcx.has_attr(def_id, sym::const_trait) {
+            // when `Self` is a const trait, also add `Self: Trait<.., true>` as implied bound,
+            // because only implementing `Self: Trait<.., false>` is currently not possible.
+            Some((
+                ty::TraitRef::new(
+                    tcx,
+                    def_id,
+                    ty::GenericArgs::for_item(tcx, def_id, |param, _| {
+                        if param.is_host_effect() {
+                            tcx.consts.true_.into()
+                        } else {
+                            tcx.mk_param_from_def(param)
+                        }
+                    }),
+                )
+                .to_predicate(tcx),
                 span,
-            ))));
+            ))
+        } else {
+            None
+        };
+        result.predicates = tcx.arena.alloc_from_iter(
+            result
+                .predicates
+                .iter()
+                .copied()
+                .chain(std::iter::once((
+                    ty::TraitRef::identity(tcx, def_id).to_predicate(tcx),
+                    span,
+                )))
+                .chain(non_const_bound),
+        );
     }
     debug!("predicates_of(def_id={:?}) = {:?}", def_id, result);
     result

compiler/rustc_hir_typeck/src/fn_ctxt/mod.rs

@@ -221,14 +221,14 @@ impl<'a, 'tcx> AstConv<'tcx> for FnCtxt<'a, 'tcx> {
         let item_def_id = tcx.hir().ty_param_owner(def_id);
         let generics = tcx.generics_of(item_def_id);
         let index = generics.param_def_id_to_index[&def_id.to_def_id()];
+        // HACK(eddyb) should get the original `Span`.
+        let span = tcx.def_span(def_id);
         ty::GenericPredicates {
             parent: None,
             predicates: tcx.arena.alloc_from_iter(
                 self.param_env.caller_bounds().iter().filter_map(|predicate| {
                     match predicate.kind().skip_binder() {
                         ty::ClauseKind::Trait(data) if data.self_ty().is_param(index) => {
-                            // HACK(eddyb) should get the original `Span`.
-                            let span = tcx.def_span(def_id);
                             Some((predicate, span))
                         }
                         _ => None,

compiler/rustc_middle/src/ty/generics.rs

@@ -79,6 +79,10 @@ impl GenericParamDef {
         }
     }
 
+    pub fn is_host_effect(&self) -> bool {
+        matches!(self.kind, GenericParamDefKind::Const { is_host_effect: true, .. })
+    }
+
     pub fn default_value<'tcx>(
         &self,
         tcx: TyCtxt<'tcx>,

tests/ui/rfcs/rfc-2632-const-trait-impl/assoc-type-const-bound-usage.stderr

@@ -1,14 +1,12 @@
-error[E0277]: the trait bound `T: Foo` is not satisfied
-  --> $DIR/assoc-type-const-bound-usage.rs:12:6
+error[E0308]: mismatched types
+  --> $DIR/assoc-type-const-bound-usage.rs:12:5
    |
 LL |     <T as Foo>::Assoc::foo();
-   |      ^ the trait `Foo` is not implemented for `T`
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^ expected `host`, found `true`
    |
-help: consider further restricting this bound
-   |
-LL | const fn foo<T: ~const Foo + Foo>() {
-   |                            +++++
+   = note: expected constant `host`
+              found constant `true`
 
 error: aborting due to previous error
 
-For more information about this error, try `rustc --explain E0277`.
+For more information about this error, try `rustc --explain E0308`.