better explain GLOBAL_KIND choice

Author: RalfJung

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`.

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;