Do not discard ?Sized type params and suggest their removal

compiler/rustc_hir/src/hir.rs

@@ -442,6 +442,7 @@ pub enum GenericBound<'hir> {
     Trait(PolyTraitRef<'hir>, TraitBoundModifier),
     // FIXME(davidtwco): Introduce `PolyTraitRef::LangItem`
     LangItemTrait(LangItem, Span, HirId, &'hir GenericArgs<'hir>),
+    Unsized(Span),
     Outlives(Lifetime),
 }
 
@@ -458,6 +459,7 @@ impl GenericBound<'_> {
             GenericBound::Trait(t, ..) => t.span,
             GenericBound::LangItemTrait(_, span, ..) => *span,
             GenericBound::Outlives(l) => l.span,
+            GenericBound::Unsized(span) => *span,
         }
     }
 }

compiler/rustc_hir/src/intravisit.rs

@@ -889,6 +889,7 @@ pub fn walk_param_bound<'v, V: Visitor<'v>>(visitor: &mut V, bound: &'v GenericB
             visitor.visit_generic_args(span, args);
         }
         GenericBound::Outlives(ref lifetime) => visitor.visit_lifetime(lifetime),
+        GenericBound::Unsized(_) => {}
     }
 }