expand comment on Pointee projection candidate assembly

Author: Lukas Markeffsky

compiler/rustc_trait_selection/src/traits/project.rs

@@ -1916,8 +1916,10 @@ fn assemble_candidates_from_impls<'cx, 'tcx>(
                         // Integers and floats are always Sized, and so have unit type metadata.
                         | ty::Infer(ty::InferTy::IntVar(_) | ty::InferTy::FloatVar(..)) => true,
 
-                        // type parameters, opaques, and unnormalized projections have pointer
-                        // metadata if they're known (e.g. by the param_env) to be sized
+                        // Type parameters, opaques, and unnormalized projections have unit
+                        // metadata if they're known (e.g. by the param_env) to be sized.
+                        // If they're not known to be sized, then we normalize from
+                        // `Wrapper<Tail>::Metadata` to `Tail::Metadata` if able.
                         ty::Param(_) | ty::Alias(..)
                             if self_ty != tail || selcx.infcx.predicate_must_hold_modulo_regions(
                                 &obligation.with(