Fix wording in comments

Author: ohadravid

compiler/rustc_mir_transform/src/copy_prop.rs

@@ -42,7 +42,7 @@ impl<'tcx> crate::MirPass<'tcx> for CopyProp {
         for (local, &head) in ssa.copy_classes().iter_enumerated() {
             if local != head {
                 // We need to determine if we can keep the head's storage statements (which enables better optimizations).
-                // For every local's usage location, if the head is in `maybe_storage_dead`, we have to remove the storage statements for it.
+                // For every local's usage location, if the head is maybe-uninitialized, we'll need to remove it's storage statements.
                 head_storage_to_check.insert(head);
             }
         }
@@ -130,7 +130,7 @@ fn fully_moved_locals(ssa: &SsaLocals, body: &Body<'_>) -> DenseBitSet<Local> {
     fully_moved
 }
 
-// The replacer will remove tautological moves from the head to the local, so we want the `MaybeUninitializedPlaces`
+// We don't want to consider tautological moves in `MaybeUninitializedPlaces`, so we remove them before running it.
 struct TautologicalMoveAssignmentRemover<'a, 'tcx> {
     tcx: TyCtxt<'tcx>,
     copy_classes: &'a IndexSlice<Local, Local>,

tests/mir-opt/copy-prop/copy_prop_storage_dead_twice.rs

@@ -4,7 +4,8 @@
 
 #![feature(custom_mir, core_intrinsics)]
 
-// Check that we do not introduce out-of-bounds access.
+// Check that we remove the storage statements if the head
+// becomes uninitialized before it is used again.
 
 use std::intrinsics::mir::*;