Rollup merge of rust-lang#110512 - compiler-errors:fix-elaboration-with-associated-type-bounds, r=spastorino

Fix elaboration with associated type bounds

When computing a trait's supertrait predicates, do not add any associated type *trait* bounds to that list of supertrait predicates. This is because supertrait predicates are expected to have the same `Self` type as the trait.

For example, given:

```rust
trait Foo: Bar<Assoc: Send>
```

Before, we would compute that the supertrait predicates of `T: Foo` are `T: Bar` and `<T as Bar>::Assoc: Send`. However, the last bound is a trait predicate for a totally different type than `T`, and existing code that uses supertrait bounds such as vtable construction, closure fn signature deduction, etc. all rely on the invariant that we have a list of predicates for self type `T`.

Fixes rust-lang#76593

The reason for all the extra diagnostic noise is that we're recomputing predicates with a different filter now. These diagnostics should be deduplicated for any end-user though.

---

This does bring up an interesting question -- is the predicate `<T as Bar>::Assoc: Send` an implied bound of `T: Foo`? Because currently the only bounds implied by a (non-alias) trait are its supertraits. I guess I could fix this too, but it would require even more changes, and I'm inclined to punt this question along.

Author: Dylan-DPC

compiler/rustc_hir_analysis/src/astconv/mod.rs

@@ -56,6 +56,9 @@ use std::slice;
 #[derive(Debug)]
 pub struct PathSeg(pub DefId, pub usize);
 
+#[derive(Copy, Clone, Debug)]
+pub struct OnlySelfBounds(pub bool);
+
 pub trait AstConv<'tcx> {
     fn tcx(&self) -> TyCtxt<'tcx>;
 
@@ -670,6 +673,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
         args: &GenericArgs<'_>,
         infer_args: bool,
         self_ty: Ty<'tcx>,
+        only_self_bounds: OnlySelfBounds,
     ) -> GenericArgCountResult {
         let (substs, arg_count) = self.create_substs_for_ast_path(
             trait_ref_span,
@@ -706,6 +710,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
                 &mut dup_bindings,
                 binding_span.unwrap_or(binding.span),
                 constness,
+                only_self_bounds,
             );
             // Okay to ignore `Err` because of `ErrorGuaranteed` (see above).
         }
@@ -741,6 +746,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
         self_ty: Ty<'tcx>,
         bounds: &mut Bounds<'tcx>,
         speculative: bool,
+        only_self_bounds: OnlySelfBounds,
     ) -> GenericArgCountResult {
         let hir_id = trait_ref.hir_ref_id;
         let binding_span = None;
@@ -766,6 +772,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
             args,
             infer_args,
             self_ty,
+            only_self_bounds,
         )
     }
 
@@ -777,6 +784,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
         args: &GenericArgs<'_>,
         self_ty: Ty<'tcx>,
         bounds: &mut Bounds<'tcx>,
+        only_self_bounds: OnlySelfBounds,
     ) {
         let binding_span = Some(span);
         let constness = ty::BoundConstness::NotConst;
@@ -799,6 +807,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
             args,
             infer_args,
             self_ty,
+            only_self_bounds,
         );
     }
 
@@ -947,6 +956,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
         ast_bounds: I,
         bounds: &mut Bounds<'tcx>,
         bound_vars: &'tcx ty::List<ty::BoundVariableKind>,
+        only_self_bounds: OnlySelfBounds,
     ) {
         for ast_bound in ast_bounds {
             match ast_bound {
@@ -964,11 +974,18 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
                         param_ty,
                         bounds,
                         false,
+                        only_self_bounds,
                     );
                 }
                 &hir::GenericBound::LangItemTrait(lang_item, span, hir_id, args) => {
                     self.instantiate_lang_item_trait_ref(
-                        lang_item, span, hir_id, args, param_ty, bounds,
+                        lang_item,
+                        span,
+                        hir_id,
+                        args,
+                        param_ty,
+                        bounds,
+                        only_self_bounds,
                     );
                 }
                 hir::GenericBound::Outlives(lifetime) => {
@@ -1006,8 +1023,19 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
         &self,
         param_ty: Ty<'tcx>,
         ast_bounds: &[hir::GenericBound<'_>],
+        only_self_bounds: OnlySelfBounds,
     ) -> Bounds<'tcx> {
-        self.compute_bounds_inner(param_ty, ast_bounds)
+        let mut bounds = Bounds::default();
+        self.add_bounds(
+            param_ty,
+            ast_bounds.iter(),
+            &mut bounds,
+            ty::List::empty(),
+            only_self_bounds,
+        );
+        debug!(?bounds);
+
+        bounds
     }
 
     /// Convert the bounds in `ast_bounds` that refer to traits which define an associated type
@@ -1029,17 +1057,14 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
             }
         }
 
-        self.compute_bounds_inner(param_ty, &result)
-    }
-
-    fn compute_bounds_inner(
-        &self,
-        param_ty: Ty<'tcx>,
-        ast_bounds: &[hir::GenericBound<'_>],
-    ) -> Bounds<'tcx> {
         let mut bounds = Bounds::default();
-
-        self.add_bounds(param_ty, ast_bounds.iter(), &mut bounds, ty::List::empty());
+        self.add_bounds(
+            param_ty,
+            result.iter(),
+            &mut bounds,
+            ty::List::empty(),
+            OnlySelfBounds(true),
+        );
         debug!(?bounds);
 
         bounds
@@ -1062,6 +1087,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
         dup_bindings: &mut FxHashMap<DefId, Span>,
         path_span: Span,
         constness: ty::BoundConstness,
+        only_self_bounds: OnlySelfBounds,
     ) -> Result<(), ErrorGuaranteed> {
         // Given something like `U: SomeTrait<T = X>`, we want to produce a
         // predicate like `<U as SomeTrait>::T = X`. This is somewhat
@@ -1361,8 +1387,20 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
                 //
                 // Calling `skip_binder` is okay, because `add_bounds` expects the `param_ty`
                 // parameter to have a skipped binder.
-                let param_ty = tcx.mk_alias(ty::Projection, projection_ty.skip_binder());
-                self.add_bounds(param_ty, ast_bounds.iter(), bounds, projection_ty.bound_vars());
+                //
+                // NOTE: If `only_self_bounds` is true, do NOT expand this associated
+                // type bound into a trait predicate, since we only want to add predicates
+                // for the `Self` type.
+                if !only_self_bounds.0 {
+                    let param_ty = tcx.mk_alias(ty::Projection, projection_ty.skip_binder());
+                    self.add_bounds(
+                        param_ty,
+                        ast_bounds.iter(),
+                        bounds,
+                        projection_ty.bound_vars(),
+                        only_self_bounds,
+                    );
+                }
             }
         }
         Ok(())
@@ -1403,6 +1441,10 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
                 dummy_self,
                 &mut bounds,
                 false,
+                // FIXME: This should be `true`, but we don't really handle
+                // associated type bounds or type aliases in objects in a way
+                // that makes this meaningful, I think.
+                OnlySelfBounds(false),
             ) {
                 potential_assoc_types.extend(cur_potential_assoc_types);
             }

compiler/rustc_hir_analysis/src/collect/item_bounds.rs

@@ -1,5 +1,5 @@
 use super::ItemCtxt;
-use crate::astconv::AstConv;
+use crate::astconv::{AstConv, OnlySelfBounds};
 use rustc_hir as hir;
 use rustc_infer::traits::util;
 use rustc_middle::ty::subst::InternalSubsts;
@@ -26,7 +26,7 @@ fn associated_type_bounds<'tcx>(
     );
 
     let icx = ItemCtxt::new(tcx, assoc_item_def_id);
-    let mut bounds = icx.astconv().compute_bounds(item_ty, ast_bounds);
+    let mut bounds = icx.astconv().compute_bounds(item_ty, ast_bounds, OnlySelfBounds(false));
     // Associated types are implicitly sized unless a `?Sized` bound is found
     icx.astconv().add_implicitly_sized(&mut bounds, item_ty, ast_bounds, None, span);
 
@@ -67,7 +67,7 @@ fn opaque_type_bounds<'tcx>(
 ) -> &'tcx [(ty::Predicate<'tcx>, Span)] {
     ty::print::with_no_queries!({
         let icx = ItemCtxt::new(tcx, opaque_def_id);
-        let mut bounds = icx.astconv().compute_bounds(item_ty, ast_bounds);
+        let mut bounds = icx.astconv().compute_bounds(item_ty, ast_bounds, OnlySelfBounds(false));
         // Opaque types are implicitly sized unless a `?Sized` bound is found
         icx.astconv().add_implicitly_sized(&mut bounds, item_ty, ast_bounds, None, span);
         debug!(?bounds);

compiler/rustc_hir_analysis/src/collect/predicates_of.rs

@@ -1,4 +1,4 @@
-use crate::astconv::AstConv;
+use crate::astconv::{AstConv, OnlySelfBounds};
 use crate::bounds::Bounds;
 use crate::collect::ItemCtxt;
 use crate::constrained_generic_params as cgp;
@@ -14,9 +14,6 @@ use rustc_middle::ty::{GenericPredicates, ToPredicate};
 use rustc_span::symbol::{sym, Ident};
 use rustc_span::{Span, DUMMY_SP};
 
-#[derive(Debug)]
-struct OnlySelfBounds(bool);
-
 /// 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
 /// `Self: Trait` predicates for traits.
@@ -99,8 +96,9 @@ fn gather_explicit_predicates_of(tcx: TyCtxt<'_>, def_id: LocalDefId) -> ty::Gen
             | ItemKind::Struct(_, generics)
             | ItemKind::Union(_, generics) => generics,
 
-            ItemKind::Trait(_, _, generics, ..) | ItemKind::TraitAlias(generics, _) => {
-                is_trait = Some(ty::TraitRef::identity(tcx, def_id.to_def_id()));
+            ItemKind::Trait(_, _, generics, self_bounds, ..)
+            | ItemKind::TraitAlias(generics, self_bounds) => {
+                is_trait = Some(self_bounds);
                 generics
             }
             ItemKind::OpaqueTy(OpaqueTy { generics, .. }) => generics,
@@ -122,10 +120,14 @@ fn gather_explicit_predicates_of(tcx: TyCtxt<'_>, def_id: LocalDefId) -> ty::Gen
 
     // Below we'll consider the bounds on the type parameters (including `Self`)
     // and the explicit where-clauses, but to get the full set of predicates
-    // on a trait we need to add in the supertrait bounds and bounds found on
-    // associated types.
-    if let Some(_trait_ref) = is_trait {
-        predicates.extend(tcx.implied_predicates_of(def_id).predicates.iter().cloned());
+    // on a trait we must also consider the bounds that follow the trait's name,
+    // like `trait Foo: A + B + C`.
+    if let Some(self_bounds) = is_trait {
+        predicates.extend(
+            icx.astconv()
+                .compute_bounds(tcx.types.self_param, self_bounds, OnlySelfBounds(false))
+                .predicates(),
+        );
     }
 
     // In default impls, we can assume that the self type implements
@@ -225,7 +227,13 @@ fn gather_explicit_predicates_of(tcx: TyCtxt<'_>, def_id: LocalDefId) -> ty::Gen
                 }
 
                 let mut bounds = Bounds::default();
-                icx.astconv().add_bounds(ty, bound_pred.bounds.iter(), &mut bounds, bound_vars);
+                icx.astconv().add_bounds(
+                    ty,
+                    bound_pred.bounds.iter(),
+                    &mut bounds,
+                    bound_vars,
+                    OnlySelfBounds(false),
+                );
                 predicates.extend(bounds.predicates());
             }
 
@@ -608,7 +616,7 @@ pub(super) fn implied_predicates_with_filter(
     let (superbounds, where_bounds_that_match) = match filter {
         PredicateFilter::All => (
             // Convert the bounds that follow the colon (or equal in trait aliases)
-            icx.astconv().compute_bounds(self_param_ty, bounds),
+            icx.astconv().compute_bounds(self_param_ty, bounds, OnlySelfBounds(false)),
             // Also include all where clause bounds
             icx.type_parameter_bounds_in_generics(
                 generics,
@@ -620,7 +628,7 @@ pub(super) fn implied_predicates_with_filter(
         ),
         PredicateFilter::SelfOnly => (
             // Convert the bounds that follow the colon (or equal in trait aliases)
-            icx.astconv().compute_bounds(self_param_ty, bounds),
+            icx.astconv().compute_bounds(self_param_ty, bounds, OnlySelfBounds(true)),
             // Include where clause bounds for `Self`
             icx.type_parameter_bounds_in_generics(
                 generics,
@@ -774,32 +782,35 @@ impl<'tcx> ItemCtxt<'tcx> {
         only_self_bounds: OnlySelfBounds,
         assoc_name: Option<Ident>,
     ) -> Vec<(ty::Predicate<'tcx>, Span)> {
-        ast_generics
-            .predicates
-            .iter()
-            .filter_map(|wp| match wp {
-                hir::WherePredicate::BoundPredicate(bp) => Some(bp),
-                _ => None,
-            })
-            .flat_map(|bp| {
-                let bt = if bp.is_param_bound(param_def_id.to_def_id()) {
-                    Some(ty)
-                } else if !only_self_bounds.0 {
-                    Some(self.to_ty(bp.bounded_ty))
-                } else {
-                    None
-                };
-                let bvars = self.tcx.late_bound_vars(bp.hir_id);
-
-                bp.bounds.iter().filter_map(move |b| bt.map(|bt| (bt, b, bvars))).filter(
-                    |(_, b, _)| match assoc_name {
-                        Some(assoc_name) => self.bound_defines_assoc_item(b, assoc_name),
-                        None => true,
-                    },
-                )
-            })
-            .flat_map(|(bt, b, bvars)| predicates_from_bound(self, bt, b, bvars))
-            .collect()
+        let mut bounds = Bounds::default();
+
+        for predicate in ast_generics.predicates {
+            let hir::WherePredicate::BoundPredicate(predicate) = predicate else {
+                continue;
+            };
+
+            let bound_ty = if predicate.is_param_bound(param_def_id.to_def_id()) {
+                ty
+            } else if !only_self_bounds.0 {
+                self.to_ty(predicate.bounded_ty)
+            } else {
+                continue;
+            };
+
+            let bound_vars = self.tcx.late_bound_vars(predicate.hir_id);
+            self.astconv().add_bounds(
+                bound_ty,
+                predicate.bounds.iter().filter(|bound| {
+                    assoc_name
+                        .map_or(true, |assoc_name| self.bound_defines_assoc_item(bound, assoc_name))
+                }),
+                &mut bounds,
+                bound_vars,
+                only_self_bounds,
+            );
+        }
+
+        bounds.predicates().collect()
     }
 
     #[instrument(level = "trace", skip(self))]
@@ -817,19 +828,3 @@ impl<'tcx> ItemCtxt<'tcx> {
         }
     }
 }
-
-/// Converts a specific `GenericBound` from the AST into a set of
-/// predicates that apply to the self type. A vector is returned
-/// because this can be anywhere from zero predicates (`T: ?Sized` adds no
-/// predicates) to one (`T: Foo`) to many (`T: Bar<X = i32>` adds `T: Bar`
-/// and `<T as Bar>::X == i32`).
-fn predicates_from_bound<'tcx>(
-    astconv: &dyn AstConv<'tcx>,
-    param_ty: Ty<'tcx>,
-    bound: &'tcx hir::GenericBound<'tcx>,
-    bound_vars: &'tcx ty::List<ty::BoundVariableKind>,
-) -> Vec<(ty::Predicate<'tcx>, Span)> {
-    let mut bounds = Bounds::default();
-    astconv.add_bounds(param_ty, [bound].into_iter(), &mut bounds, bound_vars);
-    bounds.predicates().collect()
-}

compiler/rustc_hir_analysis/src/lib.rs

@@ -116,7 +116,7 @@ use rustc_trait_selection::traits::{self, ObligationCause, ObligationCauseCode,
 
 use std::ops::Not;
 
-use astconv::AstConv;
+use astconv::{AstConv, OnlySelfBounds};
 use bounds::Bounds;
 
 fluent_messages! { "../messages.ftl" }
@@ -531,6 +531,7 @@ pub fn hir_trait_to_predicates<'tcx>(
         self_ty,
         &mut bounds,
         true,
+        OnlySelfBounds(false),
     );
 
     bounds