Use elaboration to turn ~const bounds into non-const bounds

Author: compiler-errors

compiler/rustc_hir_analysis/src/astconv/mod.rs

@@ -1084,15 +1084,42 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
     where
         I: Iterator<Item = ty::PolyTraitRef<'tcx>>,
     {
-        let mut matching_candidates = all_candidates().filter(|r| {
-            self.trait_defines_associated_item_named(r.def_id(), ty::AssocKind::Type, assoc_name)
-        });
-        let mut const_candidates = all_candidates().filter(|r| {
-            self.trait_defines_associated_item_named(r.def_id(), ty::AssocKind::Const, assoc_name)
-        });
+        let mut matching_candidates = all_candidates()
+            .filter(|r| {
+                self.trait_defines_associated_item_named(
+                    r.def_id(),
+                    ty::AssocKind::Type,
+                    assoc_name,
+                )
+            })
+            // For the purposes of an associated type, we only care
+            // about supertraits where the host effect is set to true.
+            .filter(|trait_ref| {
+                let tcx = self.tcx();
+                let generics = tcx.generics_of(trait_ref.def_id());
+                generics.host_effect_index.map_or(true, |idx| {
+                    trait_ref.skip_binder().args.const_at(idx) == tcx.consts.true_
+                })
+            });
+        let mut const_candidates = all_candidates()
+            .filter(|r| {
+                self.trait_defines_associated_item_named(
+                    r.def_id(),
+                    ty::AssocKind::Const,
+                    assoc_name,
+                )
+            })
+            // For the purposes of an associated const, we only care
+            // about supertraits where the host effect is set to true.
+            .filter(|trait_ref| {
+                let tcx = self.tcx();
+                let generics = tcx.generics_of(trait_ref.def_id());
+                generics.host_effect_index.map_or(true, |idx| {
+                    trait_ref.skip_binder().args.const_at(idx) == tcx.consts.true_
+                })
+            });
 
-        let (mut bound, mut next_cand) = match (matching_candidates.next(), const_candidates.next())
-        {
+        let (bound, next_cand) = match (matching_candidates.next(), const_candidates.next()) {
             (Some(bound), _) => (bound, matching_candidates.next()),
             (None, Some(bound)) => (bound, const_candidates.next()),
             (None, None) => {
@@ -1108,37 +1135,6 @@ 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);
 
@@ -1226,9 +1222,19 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
         assoc_name: Ident,
         span: Span,
     ) -> Result<ty::PolyTraitRef<'tcx>, ErrorGuaranteed> {
-        let mut matching_candidates = all_candidates.filter(|r| {
-            self.trait_defines_associated_item_named(r.def_id(), ty::AssocKind::Fn, assoc_name)
-        });
+        let mut matching_candidates = all_candidates
+            .filter(|r| {
+                self.trait_defines_associated_item_named(r.def_id(), ty::AssocKind::Fn, assoc_name)
+            })
+            // For the purposes of an RTN bound, we only care
+            // about supertraits where the host effect is set to true.
+            .filter(|trait_ref| {
+                let tcx = self.tcx();
+                let generics = tcx.generics_of(trait_ref.def_id());
+                generics.host_effect_index.map_or(true, |idx| {
+                    trait_ref.skip_binder().args.const_at(idx) == tcx.consts.true_
+                })
+            });
 
         let candidate = match matching_candidates.next() {
             Some(candidate) => candidate,

compiler/rustc_hir_analysis/src/bounds.rs

@@ -43,33 +43,6 @@ 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 we have a host param, we push an unconst trait bound in addition
-        // to the const one.
-        // FIXME(effects) we should find a better way than name matching
-        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::{sym, Span, DUMMY_SP};
+use rustc_span::{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
@@ -38,38 +38,11 @@ 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;
-        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),
+        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,
-            ))
-        } 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_infer/src/traits/util.rs

@@ -288,6 +288,22 @@ impl<'tcx, O: Elaboratable<'tcx>> Elaborator<'tcx, O> {
                     });
                 debug!(?data, ?obligations, "super_predicates");
                 self.extend_deduped(obligations);
+
+                // Also elaborate `~const Trait` into `Trait`.
+                let generics = tcx.generics_of(data.def_id());
+                if let Some(host_param_idx) = generics.host_effect_index
+                    && data.trait_ref.args.const_at(host_param_idx) != tcx.consts.true_
+                {
+                    let mut trait_predicate_with_host = data;
+                    trait_predicate_with_host.trait_ref.args = tcx.mk_args_from_iter(
+                        data.trait_ref.args.iter().enumerate().map(|(idx, arg)| {
+                            if idx == host_param_idx { tcx.consts.true_.into() } else { arg }
+                        }),
+                    );
+                    self.extend_deduped([
+                        elaboratable.child(trait_predicate_with_host.to_predicate(tcx))
+                    ]);
+                }
             }
             ty::PredicateKind::Clause(ty::ClauseKind::WellFormed(..)) => {
                 // Currently, we do not elaborate WF predicates,

compiler/rustc_middle/src/ty/generics.rs

@@ -79,10 +79,6 @@ 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>,