Do not convert copies of packed projections to moves.

compiler/rustc_mir_transform/src/dead_store_elimination.rs

@@ -12,6 +12,7 @@
 //!     will still not cause any further changes.
 //!
 
+use crate::util::is_disaligned;
 use rustc_index::bit_set::BitSet;
 use rustc_middle::mir::visit::Visitor;
 use rustc_middle::mir::*;
@@ -31,6 +32,8 @@ pub fn eliminate<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>, borrowed: &BitS
         .iterate_to_fixpoint()
         .into_results_cursor(body);
 
+    let param_env = tcx.param_env_reveal_all_normalized(body.source.def_id());
+
     // For blocks with a call terminator, if an argument copy can be turned into a move,
     // record it as (block, argument index).
     let mut call_operands_to_move = Vec::new();
@@ -49,6 +52,7 @@ pub fn eliminate<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>, borrowed: &BitS
                     && !place.is_indirect()
                     && !borrowed.contains(place.local)
                     && !state.contains(place.local)
+                    && !is_disaligned(tcx, body, param_env, place)
                 {
                     call_operands_to_move.push((bb, index));
                 }

tests/mir-opt/dead-store-elimination/call_arg_copy.move_packed.DeadStoreElimination.panic-abort.diff

@@ -0,0 +1,15 @@
+- // MIR for `move_packed` before DeadStoreElimination
++ // MIR for `move_packed` after DeadStoreElimination
+
+  fn move_packed(_1: Packed) -> () {
+      let mut _0: ();
+
+      bb0: {
+          _0 = use_both(const 0_i32, (_1.1: i32)) -> [return: bb1, unwind unreachable];
+      }
+
+      bb1: {
+          return;
+      }
+  }
+

tests/mir-opt/dead-store-elimination/call_arg_copy.move_packed.DeadStoreElimination.panic-unwind.diff

@@ -0,0 +1,15 @@
+- // MIR for `move_packed` before DeadStoreElimination
++ // MIR for `move_packed` after DeadStoreElimination
+
+  fn move_packed(_1: Packed) -> () {
+      let mut _0: ();
+
+      bb0: {
+          _0 = use_both(const 0_i32, (_1.1: i32)) -> [return: bb1, unwind continue];
+      }
+
+      bb1: {
+          return;
+      }
+  }
+

tests/mir-opt/dead-store-elimination/call_arg_copy.rs

@@ -2,6 +2,12 @@
 // unit-test: DeadStoreElimination
 // compile-flags: -Zmir-enable-passes=+CopyProp
 
+#![feature(core_intrinsics)]
+#![feature(custom_mir)]
+#![allow(internal_features)]
+
+use std::intrinsics::mir::*;
+
 #[inline(never)]
 fn use_both(_: i32, _: i32) {}
 
@@ -10,6 +16,26 @@ fn move_simple(x: i32) {
     use_both(x, x);
 }
 
+#[repr(packed)]
+struct Packed {
+    x: u8,
+    y: i32,
+}
+
+// EMIT_MIR call_arg_copy.move_packed.DeadStoreElimination.diff
+#[custom_mir(dialect = "analysis")]
+fn move_packed(packed: Packed) {
+    mir!(
+        {
+            Call(RET = use_both(0, packed.y), ret)
+        }
+        ret = {
+            Return()
+        }
+    )
+}
+
 fn main() {
     move_simple(1);
+    move_packed(Packed { x: 0, y: 1 });
 }