Do not attempt to evaluate obligations on PointeeSized goals

Author: KiChjang

compiler/rustc_trait_selection/src/error_reporting/traits/fulfillment_errors.rs

@@ -2632,15 +2632,19 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
             trait_def_id != def_id
                 && trait_name == self.tcx.item_name(def_id)
                 && trait_has_same_params(def_id)
-                && self.predicate_must_hold_modulo_regions(&Obligation::new(
-                    self.tcx,
-                    obligation.cause.clone(),
-                    obligation.param_env,
-                    trait_pred.map_bound(|tr| ty::TraitPredicate {
-                        trait_ref: ty::TraitRef::new(self.tcx, def_id, tr.trait_ref.args),
-                        ..tr
-                    }),
-                ))
+                // `PointeeSized` is special -- it's a very weak/top sizedness marker that's
+                // effectively implemented for all types, and most importantly isn't lowered
+                // to the trait solver, so we skip doing any semantic checking for it.
+                && (self.tcx.is_lang_item(def_id, LangItem::PointeeSized)
+                    || self.predicate_must_hold_modulo_regions(&Obligation::new(
+                        self.tcx,
+                        obligation.cause.clone(),
+                        obligation.param_env,
+                        trait_pred.map_bound(|tr| ty::TraitPredicate {
+                            trait_ref: ty::TraitRef::new(self.tcx, def_id, tr.trait_ref.args),
+                            ..tr
+                        }),
+                    )))
         }) {
             err.note(format!(
                 "`{}` implements similarly named trait `{}`, but not `{}`",

tests/ui/traits/similarly-named-trait-pointee-sized.rs

@@ -0,0 +1,11 @@
+#![feature(sized_hierarchy)]
+trait PointeeSized {} //~ HELP this trait has no implementations, consider adding one
+
+fn require_trait<T: PointeeSized>() {} //~ NOTE required by a bound in `require_trait`
+    //~| NOTE required by this bound in `require_trait`
+
+fn main() {
+    require_trait::<i32>(); //~ ERROR the trait bound `i32: PointeeSized` is not satisfied
+    //~^ NOTE the trait `PointeeSized` is not implemented for `i32`
+    //~| NOTE `i32` implements similarly named trait `std::marker::PointeeSized`, but not `PointeeSized`
+}

tests/ui/traits/similarly-named-trait-pointee-sized.stderr

@@ -0,0 +1,21 @@
+error[E0277]: the trait bound `i32: PointeeSized` is not satisfied
+  --> $DIR/similarly-named-trait-pointee-sized.rs:8:21
+   |
+LL |     require_trait::<i32>();
+   |                     ^^^ the trait `PointeeSized` is not implemented for `i32`
+   |
+   = note: `i32` implements similarly named trait `std::marker::PointeeSized`, but not `PointeeSized`
+help: this trait has no implementations, consider adding one
+  --> $DIR/similarly-named-trait-pointee-sized.rs:2:1
+   |
+LL | trait PointeeSized {}
+   | ^^^^^^^^^^^^^^^^^^
+note: required by a bound in `require_trait`
+  --> $DIR/similarly-named-trait-pointee-sized.rs:4:21
+   |
+LL | fn require_trait<T: PointeeSized>() {}
+   |                     ^^^^^^^^^^^^ required by this bound in `require_trait`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0277`.