Begin to implement type system layer of unsafe binders

compiler/rustc_trait_selection/src/traits/project.rs

@@ -1047,6 +1047,8 @@ fn assemble_candidates_from_impls<'cx, 'tcx>(
                         // Integers and floats always have `u8` as their discriminant.
                         | ty::Infer(ty::InferTy::IntVar(_) | ty::InferTy::FloatVar(..)) => true,
 
+                        ty::UnsafeBinder(_) => todo!("FIXME(unsafe_binder)"),
+
                         // type parameters, opaques, and unnormalized projections don't have
                         // a known discriminant and may need to be normalized further or rely
                         // on param env for discriminant projections
@@ -1072,6 +1074,7 @@ fn assemble_candidates_from_impls<'cx, 'tcx>(
                         | ty::Ref(..)
                         | ty::FnDef(..)
                         | ty::FnPtr(..)
+                        | ty::UnsafeBinder(_)
                         | ty::Dynamic(..)
                         | ty::Closure(..)
                         | ty::CoroutineClosure(..)
@@ -1163,6 +1166,8 @@ fn assemble_candidates_from_impls<'cx, 'tcx>(
                             true
                         }
 
+                        ty::UnsafeBinder(_) => todo!("FIXME(unsafe_binder)"),
+
                         // FIXME(compiler-errors): are Bound and Placeholder types ever known sized?
                         ty::Param(_)
                         | ty::Alias(..)