Error on moving unsized values rather than ICE'ing

compiler/rustc_codegen_ssa/src/mir/mod.rs

@@ -141,7 +141,12 @@ enum LocalRef<'tcx, V> {
     /// `UnsizedPlace(p)`: `p` itself is a thin pointer (indirect place).
     /// `*p` is the wide pointer that references the actual unsized place.
     ///
-    /// Rust has no alloca and thus no ability to move the unsized place.
+    /// MIR only supports unsized args, not dynamically-sized locals, so
+    /// new unsized temps don't exist and we must reuse the referred-to place.
+    ///
+    /// FIXME: Since the removal of unsized locals in <https://github.com/rust-lang/rust/pull/142911>,
+    /// can we maybe use `Place` here? Or refactor it in another way? There are quite a few
+    /// `UnsizedPlace => bug` branches now.
     UnsizedPlace(PlaceRef<'tcx, V>),
     /// The backend [`OperandValue`] has already been generated.
     Operand(OperandRef<'tcx, V>),