properly elaborate effects implied bounds for super traits

Author: fee1-dead

compiler/rustc_hir_analysis/src/bounds.rs

@@ -85,12 +85,44 @@ impl<'tcx> Bounds<'tcx> {
                 }
                 tcx.consts.true_
             }
+            (DefKind::Trait, ty::BoundConstness::ConstIfConst) => {
+                // we are in a trait, where `bound_trait_ref` is a super trait `trait Foo: ~const Bar` or
+                // a where clause `where for<..> Something: ~const Bar`. In these cases, we generate
+                // `<Self as Foo>::Effects: TyCompat<<Self as Bar>::Effects>` and
+                // `for<..> <Self as Foo>::Effects: TyCompat<<Something as Bar>::Effects>`
+                let Some(own_fx) = tcx.associated_type_for_effects(defining_def_id) else {
+                    tcx.dcx().span_delayed_bug(span, "should not have allowed `~const` on a trait that doesn't have `#[const_trait]`");
+                    return;
+                };
+                let own_fx_ty = Ty::new_projection(
+                    tcx,
+                    own_fx,
+                    ty::GenericArgs::identity_for_item(tcx, own_fx),
+                );
+                let Some(their_fx) = tcx.associated_type_for_effects(bound_trait_ref.def_id())
+                else {
+                    tcx.dcx().span_delayed_bug(span, "`~const` on trait without Effects assoc");
+                    return;
+                };
+                let their_fx_ty =
+                    Ty::new_projection(tcx, their_fx, bound_trait_ref.skip_binder().args);
+                let compat = tcx.require_lang_item(LangItem::EffectsTyCompat, Some(span));
+                let clause = bound_trait_ref
+                    .map_bound(|_| {
+                        let trait_ref = ty::TraitRef::new(tcx, compat, [own_fx_ty, their_fx_ty]);
+                        ty::ClauseKind::Trait(ty::TraitPredicate {
+                            trait_ref,
+                            polarity: ty::PredicatePolarity::Positive,
+                        })
+                    })
+                    .upcast(tcx);
+
+                self.clauses.push((clause, span));
+                return;
+            }
 
-            (
-                DefKind::Trait | DefKind::Impl { of_trait: true },
-                ty::BoundConstness::ConstIfConst,
-            ) => {
-                // this is either a where clause on an impl/trait header or on a trait.
+            (DefKind::Impl { of_trait: true }, ty::BoundConstness::ConstIfConst) => {
+                // this is a where clause on an impl header.
                 // push `<T as Tr>::Effects` into the set for the `Min` bound.
                 let Some(assoc) = tcx.associated_type_for_effects(bound_trait_ref.def_id()) else {
                     tcx.dcx().span_delayed_bug(span, "`~const` on trait without Effects assoc");

compiler/rustc_hir_analysis/src/collect/item_bounds.rs

@@ -127,33 +127,10 @@ pub(super) fn explicit_item_bounds_with_filter(
         None => {}
     }
 
-    if tcx.is_effects_desugared_assoc_ty(def_id.to_def_id()) {
-        let mut predicates = Vec::new();
-
-        let parent = tcx.local_parent(def_id);
-
-        let preds = tcx.explicit_predicates_of(parent);
-
-        if let ty::AssocItemContainer::TraitContainer = tcx.associated_item(def_id).container {
-            // for traits, emit `type Effects: TyCompat<<(T1::Effects, ..) as Min>::Output>`
-            let tup = Ty::new(tcx, ty::Tuple(preds.effects_min_tys));
-            // FIXME(effects) span
-            let span = tcx.def_span(def_id);
-            let assoc = tcx.require_lang_item(hir::LangItem::EffectsIntersectionOutput, Some(span));
-            let proj = Ty::new_projection(tcx, assoc, [tup]);
-            let self_proj = Ty::new_projection(
-                tcx,
-                def_id.to_def_id(),
-                ty::GenericArgs::identity_for_item(tcx, def_id),
-            );
-            let trait_ = tcx.require_lang_item(hir::LangItem::EffectsTyCompat, Some(span));
-            let trait_ref = ty::TraitRef::new(tcx, trait_, [self_proj, proj]);
-            predicates.push((ty::Binder::dummy(trait_ref).upcast(tcx), span));
-        }
-        return ty::EarlyBinder::bind(tcx.arena.alloc_from_iter(predicates));
-    }
-
     let bounds = match tcx.hir_node_by_def_id(def_id) {
+        _ if tcx.is_effects_desugared_assoc_ty(def_id.to_def_id()) => {
+            associated_type_bounds(tcx, def_id, &[], tcx.def_span(def_id), filter)
+        }
         hir::Node::TraitItem(hir::TraitItem {
             kind: hir::TraitItemKind::Type(bounds, _),
             span,

compiler/rustc_middle/src/ty/context.rs

@@ -620,11 +620,13 @@ bidirectional_lang_item_map! {
     Destruct,
     DiscriminantKind,
     DynMetadata,
+    EffectsCompat,
     EffectsIntersection,
     EffectsIntersectionOutput,
     EffectsMaybe,
     EffectsNoRuntime,
     EffectsRuntime,
+    EffectsTyCompat,
     Fn,
     FnMut,
     FnOnce,

compiler/rustc_type_ir/src/elaborate.rs

@@ -4,8 +4,9 @@ use smallvec::smallvec;
 
 use crate::data_structures::HashSet;
 use crate::inherent::*;
+use crate::lang_items::TraitSolverLangItem;
 use crate::outlives::{push_outlives_components, Component};
-use crate::{self as ty, Interner, Upcast as _};
+use crate::{self as ty, AliasTy, Interner, Upcast as _};
 
 /// "Elaboration" is the process of identifying all the predicates that
 /// are implied by a source predicate. Currently, this basically means
@@ -89,6 +90,55 @@ impl<I: Interner, O: Elaboratable<I>> Elaborator<I, O> {
                     return;
                 }
 
+                // N.B. this is a bit of a hack to get implied bounds for effects working..
+                if cx.is_lang_item(data.def_id(), TraitSolverLangItem::EffectsCompat)
+                    && matches!(self.mode, Filter::All)
+                {
+                    // first, ensure that the predicate we've got looks like a `<T as Tr>::Fx: EffectsCompat<somebool>`.
+                    // our plan is to gather information such that we elaborate into `<T as SuperTr>::Fx: EffectsCompat<somebool>`
+                    // when we know that `trait Tr: ~const SuperTr`.
+                    if let ty::Alias(ty::AliasTyKind::Projection, alias_ty) = data.self_ty().kind()
+                    {
+                        // look for effects-level bounds that look like `<Self as Tr>::Fx: TyCompat<<Self as SuperTr>::Fx>`,
+                        // which is proof to us that `Tr: ~const SuperTr`.
+                        let bounds = cx.explicit_implied_predicates_of(cx.parent(alias_ty.def_id));
+                        let expected_args_with_self =
+                            I::GenericArgs::identity_for_item(cx, alias_ty.def_id);
+                        let elaborated = bounds.iter_identity().filter_map(|(clause, _)| {
+                            let ty::ClauseKind::Trait(data2) = clause.kind().skip_binder() else {
+                                return None;
+                            };
+                            if !cx
+                                .is_lang_item(data2.def_id(), TraitSolverLangItem::EffectsTyCompat)
+                            {
+                                return None;
+                            };
+                            let ty2 = data2.trait_ref.args.type_at(1);
+                            let ty::Alias(ty::AliasTyKind::Projection, alias_ty2) = ty2.kind()
+                            else {
+                                return None;
+                            };
+                            if alias_ty2.args != expected_args_with_self {
+                                return None;
+                            };
+
+                            // important: create `<T as SuperTr>::Fx`, this will need to use the original `alias_ty`'s args
+                            // for the `T` as Self type.
+                            let alias_new =
+                                AliasTy::new_from_args(cx, alias_ty2.def_id, alias_ty.args);
+                            let alias =
+                                I::Ty::new_alias(cx, ty::AliasTyKind::Projection, alias_new);
+                            let mut data = data;
+                            let mut new_args = data.trait_ref.args.to_vec();
+                            new_args[0] = alias.into();
+                            data.trait_ref.args = cx.mk_args(&new_args);
+
+                            Some(elaboratable.child(bound_clause.rebind(data).upcast(cx)))
+                        });
+                        self.extend_deduped(elaborated);
+                    }
+                }
+
                 let map_to_child_clause =
                     |(index, (clause, span)): (usize, (I::Clause, I::Span))| {
                         elaboratable.child_with_derived_cause(

compiler/rustc_type_ir/src/lang_items.rs

@@ -20,11 +20,13 @@ pub enum TraitSolverLangItem {
     Destruct,
     DiscriminantKind,
     DynMetadata,
+    EffectsCompat,
     EffectsIntersection,
     EffectsIntersectionOutput,
     EffectsMaybe,
     EffectsNoRuntime,
     EffectsRuntime,
+    EffectsTyCompat,
     Fn,
     FnMut,
     FnOnce,

tests/ui/rfcs/rfc-2632-const-trait-impl/super-traits-fail-3.rs

@@ -3,7 +3,7 @@
 #![feature(const_trait_impl, effects)]
 
 //@ revisions: yy yn ny nn
-//@[yy] known-bug: #110395
+//@[yy] check-pass
 
 #[cfg_attr(any(yy, yn), const_trait)]
 trait Foo {

tests/ui/rfcs/rfc-2632-const-trait-impl/super-traits-fail-3.yy.stderr

@@ -1,5 +1,4 @@
-// FIXME(effects) check-pass
-//@ known-bug: #110395
+//@ check-pass
 //@ compile-flags: -Znext-solver
 #![allow(incomplete_features)]
 #![feature(const_trait_impl, effects)]