From 29c372bf8b612dd9e23eb2503a1f1167e2f2f47c Mon Sep 17 00:00:00 2001
From: Oliver Scherer <github35764891676564198441@oli-obk.de>
Date: Tue, 7 Jan 2020 15:51:43 +0100
Subject: [PATCH] Add more documentation

---
 src/librustc_mir/const_eval/eval_queries.rs | 7 ++++++-
 src/librustc_mir/interpret/place.rs         | 2 ++
 2 files changed, 8 insertions(+), 1 deletion(-)

diff --git a/src/librustc_mir/const_eval/eval_queries.rs b/src/librustc_mir/const_eval/eval_queries.rs
index 9fbb6e1a39d9..d2a0798249ad 100644
--- a/src/librustc_mir/const_eval/eval_queries.rs
+++ b/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())
         }
diff --git a/src/librustc_mir/interpret/place.rs b/src/librustc_mir/interpret/place.rs
index 1f7db45ccff5..69563c5a08de 100644
--- a/src/librustc_mir/interpret/place.rs
+++ b/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,