explain why Rvalue::Len still exists

compiler/rustc_middle/src/mir/syntax.rs

@@ -1307,6 +1307,9 @@ pub enum Rvalue<'tcx> {
  /// If the type of the place is an array, this is the array length. For slices (`[T]`, not
  /// `&[T]`) this accesses the place's metadata to determine the length. This rvalue is
  /// ill-formed for places of other types.
+ ///
+ /// This cannot be a `UnOp(PtrMetadata, _)` because that expects a value, and we only
+ /// have a place, and `UnOp(PtrMetadata, RawPtr(place))` is not a thing.
  Len(Place<'tcx>),
 
  /// Performs essentially all of the casts that can be performed via `as`.