trait solver: account for universes from replace_bound_vars

compiler/rustc_next_trait_solver/src/placeholder.rs

@@ -47,6 +47,7 @@ where
         IndexMap<ty::PlaceholderType<I>, ty::BoundTy<I>>,
         IndexMap<ty::PlaceholderConst<I>, ty::BoundConst<I>>,
     ) {
+        let old_universes = universe_indices.clone();
         let mut replacer = BoundVarReplacer {
             infcx,
             mapped_regions: Default::default(),
@@ -57,8 +58,29 @@ where
         };
 
         let value = value.fold_with(&mut replacer);
+        let BoundVarReplacer {
+            mapped_regions,
+            mapped_types,
+            mapped_consts,
+            universe_indices,
+            infcx: _,
+            current_index: _,
+        } = replacer;
+
+        if infcx.cx().assumptions_on_binders() {
+            for (old, new) in old_universes.into_iter().zip(universe_indices.iter()) {
+                if let (None, Some(new)) = (old, new) {
+                    // FIXME(-Zassumptions-on-binders): `replace_bound_vars` does not have enough
+                    // context to compute placeholder assumptions for the binders it enters.
+                    infcx.insert_placeholder_assumptions(
+                        *new,
+                        Some(rustc_type_ir::region_constraint::Assumptions::empty()),
+                    );
+                }
+            }
+        }
 
-        (value, replacer.mapped_regions, replacer.mapped_types, replacer.mapped_consts)
+        (value, mapped_regions, mapped_types, mapped_consts)
     }
 
     fn universe_for(&mut self, debruijn: ty::DebruijnIndex) -> ty::UniverseIndex {

tests/ui/assumptions_on_binders/placeholder-assumptions-issue-157840.rs

@@ -0,0 +1,17 @@
+//@ compile-flags: -Znext-solver=globally -Zassumptions-on-binders
+
+trait Trait<T> {}
+
+trait Proj<'a> {
+    type Assoc;
+}
+
+fn foo<'a, T>()
+where
+    T: Proj<'a, Assoc = fn(<T as Proj>::Assoc)>,
+    (): Trait<<T as Proj<'a>>::Assoc>,
+    //~^ ERROR the trait bound `(): Trait<fn(for<'a> fn(<T as Proj<'a>>::Assoc))>` is not satisfied
+{
+}
+
+fn main() {}

tests/ui/assumptions_on_binders/placeholder-assumptions-issue-157840.stderr

@@ -0,0 +1,14 @@
+error[E0277]: the trait bound `(): Trait<fn(for<'a> fn(<T as Proj<'a>>::Assoc))>` is not satisfied
+  --> $DIR/placeholder-assumptions-issue-157840.rs:12:9
+   |
+LL |     (): Trait<<T as Proj<'a>>::Assoc>,
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Trait<fn(for<'a> fn(<T as Proj<'a>>::Assoc))>` is not implemented for `()`
+   |
+help: consider extending the `where` clause, but there might be an alternative better way to express this requirement
+   |
+LL |     (): Trait<<T as Proj<'a>>::Assoc>, (): Trait<fn(for<'a> fn(<T as Proj<'a>>::Assoc))>
+   |                                        +++++++++++++++++++++++++++++++++++++++++++++++++
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0277`.