Auto merge of #142712 - davidtwco:sized-hierarchy-missing-default-bounds, r=<try>

hir_analysis: add missing sizedness bounds

Default sizedness bounds were not being added to `explicit_super_predicates_of` and `explicit_implied_predicates_of` which meant that a trait bound added to a associated type projection would be missing the implied predicate of the default sizedness supertrait of that trait.

An unexpected consequence of this change was that the check for multiple principals was now finding an additional `MetaSized` principal when eagerly expanding trait aliases - this required modifying lowering to no longer add default bounds to trait aliases.

Author: bors

compiler/rustc_hir_analysis/src/collect/predicates_of.rs

@@ -660,6 +660,14 @@ pub(super) fn implied_predicates_with_filter<'tcx>(
         | PredicateFilter::SelfOnly
         | PredicateFilter::SelfTraitThatDefines(_)
         | PredicateFilter::SelfAndAssociatedTypeBounds => {
+            icx.lowerer().add_sizedness_bounds(
+                &mut bounds,
+                self_param_ty,
+                superbounds,
+                None,
+                Some(trait_def_id),
+                item.span,
+            );
             icx.lowerer().add_default_super_traits(
                 trait_def_id,
                 &mut bounds,

compiler/rustc_hir_analysis/src/hir_ty_lowering/dyn_compatibility.rs

@@ -64,8 +64,13 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
 
         self.add_default_traits(&mut user_written_bounds, dummy_self, &ast_bounds, None, span);
 
-        let (elaborated_trait_bounds, elaborated_projection_bounds) =
+        let (mut elaborated_trait_bounds, elaborated_projection_bounds) =
             traits::expand_trait_aliases(tcx, user_written_bounds.iter().copied());
+
+        // FIXME(sized-hierarchy): https://github.com/rust-lang/rust/pull/142712#issuecomment-3013231794
+        let meta_sized_did = tcx.require_lang_item(LangItem::MetaSized, span);
+        elaborated_trait_bounds.retain(|(pred, _)| pred.def_id() != meta_sized_did);
+
         let (regular_traits, mut auto_traits): (Vec<_>, Vec<_>) = elaborated_trait_bounds
             .into_iter()
             .partition(|(trait_ref, _)| !tcx.trait_is_auto(trait_ref.def_id()));

compiler/rustc_middle/src/ty/context.rs

@@ -492,6 +492,10 @@ impl<'tcx> Interner for TyCtxt<'tcx> {
         self.is_default_trait(def_id)
     }
 
+    fn is_sizedness_trait(self, def_id: DefId) -> bool {
+        self.is_sizedness_trait(def_id)
+    }
+
     fn as_lang_item(self, def_id: DefId) -> Option<TraitSolverLangItem> {
         lang_item_to_trait_lang_item(self.lang_items().from_def_id(def_id)?)
     }
@@ -1644,6 +1648,10 @@ impl<'tcx> TyCtxt<'tcx> {
             .any(|&default_trait| self.lang_items().get(default_trait) == Some(def_id))
     }
 
+    pub fn is_sizedness_trait(self, def_id: DefId) -> bool {
+        matches!(self.as_lang_item(def_id), Some(LangItem::Sized | LangItem::MetaSized))
+    }
+
     /// Returns a range of the start/end indices specified with the
     /// `rustc_layout_scalar_valid_range` attribute.
     // FIXME(eddyb) this is an awkward spot for this method, maybe move it?

compiler/rustc_next_trait_solver/src/solve/trait_goals.rs

@@ -1343,6 +1343,7 @@ where
     #[instrument(level = "debug", skip(self), ret)]
     pub(super) fn merge_trait_candidates(
         &mut self,
+        prefer_alias_over_param_candidates: bool,
         mut candidates: Vec<Candidate<I>>,
     ) -> Result<(CanonicalResponse<I>, Option<TraitGoalProvenVia>), NoSolution> {
         if let TypingMode::Coherence = self.typing_mode() {
@@ -1368,6 +1369,26 @@ where
             return Ok((candidate.result, Some(TraitGoalProvenVia::Misc)));
         }
 
+        let potential_alias_bound_response =
+            candidates.iter().any(|c| matches!(c.source, CandidateSource::AliasBound)).then(|| {
+                let alias_bounds: Vec<_> = candidates
+                    .iter()
+                    .filter(|c| matches!(c.source, CandidateSource::AliasBound))
+                    .map(|c| c.result)
+                    .collect();
+                if let Some(response) = self.try_merge_responses(&alias_bounds) {
+                    (response, Some(TraitGoalProvenVia::AliasBound))
+                } else {
+                    (self.bail_with_ambiguity(&alias_bounds), None)
+                }
+            });
+
+        if prefer_alias_over_param_candidates
+            && let Some(alias_bound_response) = potential_alias_bound_response
+        {
+            return Ok(alias_bound_response);
+        }
+
         // If there are non-global where-bounds, prefer where-bounds
         // (including global ones) over everything else.
         let has_non_global_where_bounds = candidates
@@ -1386,17 +1407,8 @@ where
             };
         }
 
-        if candidates.iter().any(|c| matches!(c.source, CandidateSource::AliasBound)) {
-            let alias_bounds: Vec<_> = candidates
-                .iter()
-                .filter(|c| matches!(c.source, CandidateSource::AliasBound))
-                .map(|c| c.result)
-                .collect();
-            return if let Some(response) = self.try_merge_responses(&alias_bounds) {
-                Ok((response, Some(TraitGoalProvenVia::AliasBound)))
-            } else {
-                Ok((self.bail_with_ambiguity(&alias_bounds), None))
-            };
+        if let Some(response) = potential_alias_bound_response {
+            return Ok(response);
         }
 
         self.filter_specialized_impls(AllowInferenceConstraints::No, &mut candidates);
@@ -1431,7 +1443,12 @@ where
         goal: Goal<I, TraitPredicate<I>>,
     ) -> Result<(CanonicalResponse<I>, Option<TraitGoalProvenVia>), NoSolution> {
         let candidates = self.assemble_and_evaluate_candidates(goal, AssembleCandidatesFrom::All);
-        self.merge_trait_candidates(candidates)
+
+        let did = goal.predicate.def_id();
+        let is_sizedness_or_auto_or_default_goal = self.cx().is_sizedness_trait(did)
+            || self.cx().trait_is_auto(did)
+            || self.cx().is_default_trait(did);
+        self.merge_trait_candidates(is_sizedness_or_auto_or_default_goal, candidates)
     }
 
     fn try_stall_coroutine_witness(

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

@@ -144,8 +144,6 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         obligation: &PolyTraitObligation<'tcx>,
         idx: usize,
     ) -> Result<PredicateObligations<'tcx>, SelectionError<'tcx>> {
-        let tcx = self.tcx();
-
         let placeholder_trait_predicate =
             self.infcx.enter_forall_and_leak_universe(obligation.predicate).trait_ref;
         let placeholder_self_ty = self.infcx.shallow_resolve(placeholder_trait_predicate.self_ty());
@@ -194,28 +192,6 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 .map_err(|_| SelectionError::Unimplemented)?,
         );
 
-        // FIXME(compiler-errors): I don't think this is needed.
-        if let ty::Alias(ty::Projection, alias_ty) = placeholder_self_ty.kind() {
-            let predicates = tcx.predicates_of(alias_ty.def_id).instantiate_own(tcx, alias_ty.args);
-            for (predicate, _) in predicates {
-                let normalized = normalize_with_depth_to(
-                    self,
-                    obligation.param_env,
-                    obligation.cause.clone(),
-                    obligation.recursion_depth + 1,
-                    predicate,
-                    &mut obligations,
-                );
-                obligations.push(Obligation::with_depth(
-                    self.tcx(),
-                    obligation.cause.clone(),
-                    obligation.recursion_depth + 1,
-                    obligation.param_env,
-                    normalized,
-                ));
-            }
-        }
-
         Ok(obligations)
     }
 

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

@@ -485,7 +485,15 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             }
         } else {
             let has_non_region_infer = stack.obligation.predicate.has_non_region_infer();
-            if let Some(candidate) = self.winnow_candidates(has_non_region_infer, candidates) {
+            let did = stack.obligation.predicate.def_id();
+            let is_sizedness_or_auto_or_default_predicate = self.tcx().is_sizedness_trait(did)
+                || self.tcx().trait_is_auto(did)
+                || self.tcx().is_default_trait(did);
+            if let Some(candidate) = self.winnow_candidates(
+                has_non_region_infer,
+                is_sizedness_or_auto_or_default_predicate,
+                candidates,
+            ) {
                 self.filter_reservation_impls(candidate)
             } else {
                 Ok(None)
@@ -1826,6 +1834,7 @@ impl<'tcx> SelectionContext<'_, 'tcx> {
     fn winnow_candidates(
         &mut self,
         has_non_region_infer: bool,
+        prefer_alias_over_param_candidates: bool,
         mut candidates: Vec<EvaluatedCandidate<'tcx>>,
     ) -> Option<SelectionCandidate<'tcx>> {
         if candidates.len() == 1 {
@@ -1879,6 +1888,19 @@ impl<'tcx> SelectionContext<'_, 'tcx> {
             break;
         }
 
+        let alias_bound = candidates
+            .iter()
+            .filter_map(|c| if let ProjectionCandidate(i) = c.candidate { Some(i) } else { None })
+            .try_reduce(|c1, c2| if has_non_region_infer { None } else { Some(c1.min(c2)) });
+
+        if prefer_alias_over_param_candidates {
+            match alias_bound {
+                Some(Some(index)) => return Some(ProjectionCandidate(index)),
+                Some(None) => {}
+                None => return None,
+            }
+        }
+
         // The next highest priority is for non-global where-bounds. However, while we don't
         // prefer global where-clauses here, we do bail with ambiguity when encountering both
         // a global and a non-global where-clause.
@@ -1912,10 +1934,6 @@ impl<'tcx> SelectionContext<'_, 'tcx> {
         // fairly arbitrary but once again necessary for backwards compatibility.
         // If there are multiple applicable candidates which don't affect type inference,
         // choose the one with the lowest index.
-        let alias_bound = candidates
-            .iter()
-            .filter_map(|c| if let ProjectionCandidate(i) = c.candidate { Some(i) } else { None })
-            .try_reduce(|c1, c2| if has_non_region_infer { None } else { Some(c1.min(c2)) });
         match alias_bound {
             Some(Some(index)) => return Some(ProjectionCandidate(index)),
             Some(None) => {}

compiler/rustc_type_ir/src/interner.rs

@@ -300,6 +300,8 @@ pub trait Interner:
 
     fn is_default_trait(self, def_id: Self::DefId) -> bool;
 
+    fn is_sizedness_trait(self, def_id: Self::DefId) -> bool;
+
     fn as_lang_item(self, def_id: Self::DefId) -> Option<TraitSolverLangItem>;
 
     fn associated_type_def_ids(self, def_id: Self::DefId) -> impl IntoIterator<Item = Self::DefId>;

tests/ui/dyn-compatibility/assoc_type_bounds_sized_used.rs

@@ -9,7 +9,8 @@ trait Bop {
 
 fn bop<T: Bop + ?Sized>() {
     let _ = <T as Bop>::Bar::default();
-    //~^ ERROR: trait bounds were not satisfied
+    //~^ ERROR: the size for values of type `T` cannot be known at compilation time
+    //~| ERROR: the size for values of type `T` cannot be known at compilation time
     //~| ERROR: the size for values of type `T` cannot be known at compilation time
 }
 

tests/ui/dyn-compatibility/assoc_type_bounds_sized_used.stderr

@@ -24,21 +24,58 @@ help: consider relaxing the implicit `Sized` restriction
 LL |     type Bar: Default + ?Sized
    |                       ++++++++
 
-error[E0599]: the function or associated item `default` exists for associated type `<T as Bop>::Bar`, but its trait bounds were not satisfied
-  --> $DIR/assoc_type_bounds_sized_used.rs:11:30
+error[E0277]: the size for values of type `T` cannot be known at compilation time
+  --> $DIR/assoc_type_bounds_sized_used.rs:11:13
    |
+LL | fn bop<T: Bop + ?Sized>() {
+   |        - this type parameter needs to be `Sized`
 LL |     let _ = <T as Bop>::Bar::default();
-   |                              ^^^^^^^ function or associated item cannot be called on `<T as Bop>::Bar` due to unsatisfied trait bounds
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
+   |
+note: required by a bound in `Bop::Bar`
+  --> $DIR/assoc_type_bounds_sized_used.rs:7:15
+   |
+LL |     type Bar: Default
+   |          --- required by a bound in this associated type
+LL |     where
+LL |         Self: Sized;
+   |               ^^^^^ required by this bound in `Bop::Bar`
+help: consider removing the `?Sized` bound to make the type parameter `Sized`
+   |
+LL - fn bop<T: Bop + ?Sized>() {
+LL + fn bop<T: Bop>() {
    |
-   = note: the following trait bounds were not satisfied:
-           `T: Sized`
-           which is required by `<T as Bop>::Bar: Default`
-help: consider restricting the type parameter to satisfy the trait bound
+help: consider relaxing the implicit `Sized` restriction
+   |
+LL |     type Bar: Default + ?Sized
+   |                       ++++++++
+
+error[E0277]: the size for values of type `T` cannot be known at compilation time
+  --> $DIR/assoc_type_bounds_sized_used.rs:11:13
    |
-LL | fn bop<T: Bop + ?Sized>() where T: Sized {
-   |                           ++++++++++++++
+LL | fn bop<T: Bop + ?Sized>() {
+   |        - this type parameter needs to be `Sized`
+LL |     let _ = <T as Bop>::Bar::default();
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
+   |
+note: required by a bound in `Bop::Bar`
+  --> $DIR/assoc_type_bounds_sized_used.rs:7:15
+   |
+LL |     type Bar: Default
+   |          --- required by a bound in this associated type
+LL |     where
+LL |         Self: Sized;
+   |               ^^^^^ required by this bound in `Bop::Bar`
+help: consider removing the `?Sized` bound to make the type parameter `Sized`
+   |
+LL - fn bop<T: Bop + ?Sized>() {
+LL + fn bop<T: Bop>() {
+   |
+help: consider relaxing the implicit `Sized` restriction
+   |
+LL |     type Bar: Default + ?Sized
+   |                       ++++++++
 
-error: aborting due to 2 previous errors
+error: aborting due to 3 previous errors
 
-Some errors have detailed explanations: E0277, E0599.
-For more information about an error, try `rustc --explain E0277`.
+For more information about this error, try `rustc --explain E0277`.

tests/ui/generic-associated-types/bugs/issue-100013.stderr

@@ -1,25 +1,3 @@
-error: lifetime bound not satisfied
-  --> $DIR/issue-100013.rs:15:5
-   |
-LL | /     async { // a coroutine checked for autotrait impl `Send`
-LL | |         let x = None::<I::Future<'_, '_>>; // a type referencing GAT
-LL | |         async {}.await; // a yield point
-LL | |     }
-   | |_____^
-   |
-   = note: this is a known limitation that will be removed in the future (see issue #100013 <https://github.com/rust-lang/rust/issues/100013> for more information)
-
-error: lifetime bound not satisfied
-  --> $DIR/issue-100013.rs:22:5
-   |
-LL | /     async { // a coroutine checked for autotrait impl `Send`
-LL | |         let x = None::<I::Future<'a, 'b>>; // a type referencing GAT
-LL | |         async {}.await; // a yield point
-LL | |     }
-   | |_____^
-   |
-   = note: this is a known limitation that will be removed in the future (see issue #100013 <https://github.com/rust-lang/rust/issues/100013> for more information)
-
 error: lifetime may not live long enough
   --> $DIR/issue-100013.rs:23:17
    |
@@ -33,16 +11,5 @@ LL |         let x = None::<I::Future<'a, 'b>>; // a type referencing GAT
    |
    = help: consider adding the following bound: `'a: 'b`
 
-error: lifetime bound not satisfied
-  --> $DIR/issue-100013.rs:29:5
-   |
-LL | /     async { // a coroutine checked for autotrait impl `Send`
-LL | |         let x = None::<I::Future<'a, 'b>>; // a type referencing GAT
-LL | |         async {}.await; // a yield point
-LL | |     }
-   | |_____^
-   |
-   = note: this is a known limitation that will be removed in the future (see issue #100013 <https://github.com/rust-lang/rust/issues/100013> for more information)
-
-error: aborting due to 4 previous errors
+error: aborting due to 1 previous error