Handle index projections in call destinations in DSE

compiler/rustc_mir_transform/src/dead_store_elimination.rs

@@ -47,13 +47,21 @@ fn eliminate<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) -> bool {
     let mut patch = Vec::new();
 
     for (bb, bb_data) in traversal::preorder(body) {
-        if let TerminatorKind::Call { ref args, .. } = bb_data.terminator().kind {
+        if let TerminatorKind::Call { ref args, ref destination, .. } = bb_data.terminator().kind {
             let loc = Location { block: bb, statement_index: bb_data.statements.len() };
 
             // Position ourselves between the evaluation of `args` and the write to `destination`.
             live.seek_to_block_end(bb);
             let mut state = live.get().clone();
 
+            // Don't turn into a move if the local is used as an index
+            // projection for the destination place.
+            LivenessTransferFunction(&mut state).visit_place(
+                destination,
+                visit::PlaceContext::MutatingUse(visit::MutatingUseContext::Call),
+                loc,
+            );
+
             for (index, arg) in args.iter().map(|a| &a.node).enumerate().rev() {
                 if let Operand::Copy(place) = *arg
                     && !place.is_indirect()

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

@@ -0,0 +1,15 @@
+- // MIR for `move_index` before DeadStoreElimination-final
++ // MIR for `move_index` after DeadStoreElimination-final
+
+  fn move_index(_1: [usize; 10], _2: usize) -> () {
+      let mut _0: ();
+
+      bb0: {
+          _1[_2] = passthrough_usize(copy _2) -> [return: bb1, unwind unreachable];
+      }
+
+      bb1: {
+          return;
+      }
+  }
+

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

@@ -0,0 +1,15 @@
+- // MIR for `move_index` before DeadStoreElimination-final
++ // MIR for `move_index` after DeadStoreElimination-final
+
+  fn move_index(_1: [usize; 10], _2: usize) -> () {
+      let mut _0: ();
+
+      bb0: {
+          _1[_2] = passthrough_usize(copy _2) -> [return: bb1, unwind continue];
+      }
+
+      bb1: {
+          return;
+      }
+  }
+

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

@@ -40,7 +40,31 @@ fn move_packed(packed: Packed) {
     }
 }
 
+#[inline(never)]
+fn passthrough_usize(a: usize) -> usize {
+    a
+}
+
+// EMIT_MIR call_arg_copy.move_index.DeadStoreElimination-final.diff
+#[custom_mir(dialect = "analysis")]
+fn move_index(a: [usize; 10], b: usize) {
+    // CHECK-LABEL: fn move_index(
+    // CHECK: = passthrough_usize(copy _2)
+    mir! {
+        {
+            // The index is used again after the operand is evaluated to
+            // evaluate the destionation place, so the argument cannot be turned
+            // into a move.
+            Call(a[b] = passthrough_usize(b), ReturnTo(ret), UnwindContinue())
+        }
+        ret = {
+            Return()
+        }
+    }
+}
+
 fn main() {
     move_simple(1);
     move_packed(Packed { x: 0, y: 1 });
+    move_index([0; _], 1);
 }