Add more documentation

Author: oli-obk

src/librustc_mir/const_eval/eval_queries.rs

@@ -124,7 +124,12 @@ pub(super) fn op_to_const<'tcx>(
             ConstValue::ByRef { alloc, offset: ptr.offset }
         }
         Scalar::Raw { data, .. } => {
-            assert_eq!(data, mplace.layout.align.abi.bytes().into());
+            assert_eq!(
+                data,
+                mplace.layout.align.abi.bytes().into(),
+                "this MPlaceTy must come from `try_as_mplace` being used on a zst, so we know what
+                 value this integer address must have",
+            );
             assert!(mplace.layout.is_zst());
             ConstValue::Scalar(Scalar::zst())
         }

src/librustc_mir/interpret/place.rs

@@ -21,7 +21,9 @@ use super::{
 };
 
 #[derive(Copy, Clone, Debug, Hash, PartialEq, Eq, HashStable)]
+/// Information required for the sound usage of a `MemPlace`.
 pub enum MemPlaceMeta<Tag = (), Id = AllocId> {
+    /// The unsized payload (e.g. length for slices or vtable pointer for trait objects).
     Unsized(Scalar<Tag, Id>),
     /// `Sized` types or unsized `extern type`
     None,