layout: deal with placeholders, ICE on bound types

a placeholder type is the same as a param as they
represent "this could be any type". A bound type
represents a type inside of a `for<T>` or `exists<T>`.
When entering a forall or exists `T` should be
instantiated as a existential (inference var) or universal
(placeholder). You should never observe a bound variable
without its binder.

compiler/rustc_ty_utils/src/layout.rs

@@ -470,14 +470,11 @@ fn layout_of_uncached<'tcx>(
  return Err(LayoutError::Unknown(ty));
  }
 
- ty::Placeholder(..)
- | ty::GeneratorWitness(..)
- | ty::GeneratorWitnessMIR(..)
- | ty::Infer(_) => {
+ ty::Bound(..) | ty::GeneratorWitness(..) | ty::GeneratorWitnessMIR(..) | ty::Infer(_) => {
  bug!("Layout::compute: unexpected type `{}`", ty)
  }
 
- ty::Bound(..) | ty::Param(_) | ty::Error(_) => {
+ ty::Placeholder(..) | ty::Param(_) | ty::Error(_) => {
  return Err(LayoutError::Unknown(ty));
  }
  })