diff --git a/src/librustc/ty/layout.rs b/src/librustc/ty/layout.rs
index e14a78a8c44..c67e6a0a13e 100644
--- a/src/librustc/ty/layout.rs
+++ b/src/librustc/ty/layout.rs
@@ -2529,8 +2529,9 @@ where
                     attrs.pointee_align = Some(pointee.align);
 
                     // `Box` (`UniqueBorrowed`) are not necessarily dereferencable
-                    // for the entire duration of the function, so set their size to 0.
-                    attrs.pointee_size =  match kind {
+                    // for the entire duration of the function as they can be deallocated
+                    // any time. Set their valid size to 0.
+                    attrs.pointee_size = match kind {
                         PointerKind::UniqueOwned => Size::ZERO,
                         _ => pointee.size
                     };
diff --git a/src/librustc_target/abi/call/mod.rs b/src/librustc_target/abi/call/mod.rs
index 1b55676fe4f..3a3ad763790 100644
--- a/src/librustc_target/abi/call/mod.rs
+++ b/src/librustc_target/abi/call/mod.rs
@@ -69,7 +69,8 @@ mod attr_impl {
 #[derive(Copy, Clone, PartialEq, Eq, Debug)]
 pub struct ArgAttributes {
     pub regular: ArgAttribute,
-    /// The dereferenceable size of the pointee.
+    /// The minimum size of the pointee, guaranteed to be valid for the duration of the whole call
+    /// (corresponding to LLVM's dereferenceable and dereferenceable_or_null attributes).
     pub pointee_size: Size,
     pub pointee_align: Option<Align>
 }