explain why Rvalue::Len still exists

Author: RalfJung

Diff for: 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`.