Elaborate comment.

Author: cjgillot

compiler/rustc_mir_transform/src/dest_prop.rs

@@ -590,8 +590,12 @@ fn save_as_intervals<'tcx>(
             twostep = TwoStepIndex::from_u32(twostep.as_u32() + 1);
             debug_assert_eq!(twostep, two_step_loc(loc, Effect::After));
             append_at(&mut values, &state, twostep);
-            // Ensure we have a non-zero live range even for dead stores. This is done by marking
-            // all the written-to locals as live in the second half of the statement.
+            // Like terminators, ensure we have a non-zero live range even for dead stores.
+            // Some rvalues interleave reads and writes, for instance `Rvalue::Aggregate`, see
+            // https://github.com/rust-lang/rust/issues/146383. By precaution, treat statements
+            // as behaving so by default.
+            // We make an exception for simple assignments `_a.stuff = {copy|move} _b.stuff`,
+            // as marking `_b` live here would prevent unification.
             let is_simple_assignment =
                 matches!(stmt.kind, StatementKind::Assign(box (_, Rvalue::Use(_))));
             VisitPlacesWith(|place: Place<'tcx>, ctxt| {
@@ -600,10 +604,12 @@ fn save_as_intervals<'tcx>(
                         DefUse::Def | DefUse::PartialWrite => {
                             values.insert(relevant, twostep);
                         }
-                        DefUse::Use if !is_simple_assignment => {
-                            values.insert(relevant, twostep);
+                        DefUse::Use => {
+                            if !is_simple_assignment {
+                                values.insert(relevant, twostep);
+                            }
                         }
-                        DefUse::Use | DefUse::NonUse => {}
+                        DefUse::NonUse => {}
                     }
                 }
             })