diff --git a/src/librustc_mir/const_eval/machine.rs b/src/librustc_mir/const_eval/machine.rs
index 8f4501cc3fb6..3f657089251e 100644
--- a/src/librustc_mir/const_eval/machine.rs
+++ b/src/librustc_mir/const_eval/machine.rs
@@ -178,7 +178,7 @@ impl<'mir, 'tcx> interpret::Machine<'mir, 'tcx> for CompileTimeInterpreter {
 
     type MemoryMap = FxHashMap<AllocId, (MemoryKind<!>, Allocation)>;
 
-    const GLOBAL_KIND: Option<!> = None; // no copying of globals allowed
+    const GLOBAL_KIND: Option<!> = None; // no copying of globals from `tcx` to machine memory
 
     // We do not check for alignment to avoid having to carry an `Align`
     // in `ConstValue::ByRef`.
diff --git a/src/librustc_mir/transform/const_prop.rs b/src/librustc_mir/transform/const_prop.rs
index cbb79637076b..232d33a35313 100644
--- a/src/librustc_mir/transform/const_prop.rs
+++ b/src/librustc_mir/transform/const_prop.rs
@@ -172,7 +172,7 @@ impl<'mir, 'tcx> interpret::Machine<'mir, 'tcx> for ConstPropMachine {
 
     type MemoryMap = FxHashMap<AllocId, (MemoryKind<!>, Allocation)>;
 
-    const GLOBAL_KIND: Option<!> = None;
+    const GLOBAL_KIND: Option<!> = None; // no copying of globals from `tcx` to machine memory
 
     const CHECK_ALIGN: bool = false;