check is_indirect_first_projection when replacing in RefProp

compiler/rustc_mir_transform/src/ref_prop.rs

@@ -335,7 +335,7 @@ fn compute_replacement<'tcx>(
                     //
                     // This also allows to detect cases where `target.local` is not replaceable,
                     // and mark it as such.
-                    if let &[PlaceElem::Deref] = &target.projection[..] {
+                    if target.is_indirect_first_projection() {
                         assert!(perform_opt);
                         self.allowed_replacements.insert((target.local, loc));
                         place.local = target.local;

tests/mir-opt/reference_prop.maybe_dead_field_reborrow.ReferencePropagation.diff

@@ -0,0 +1,30 @@
+- // MIR for `maybe_dead_field_reborrow` before ReferencePropagation
++ // MIR for `maybe_dead_field_reborrow` after ReferencePropagation
+
+  fn maybe_dead_field_reborrow(_1: bool) -> i32 {
+      let mut _0: i32;
+      let mut _2: (i32, i32);
+      let mut _3: &mut (i32, i32);
+      let mut _4: &mut i32;
+
+      bb0: {
+          StorageLive(_2);
+          _2 = (const 0_i32, const 0_i32);
+          _3 = &mut _2;
+          ((*_3).0: i32) = const 1_i32;
+-         _4 = &mut ((*_3).0: i32);
+          switchInt(copy _1) -> [1: bb1, otherwise: bb2];
+      }
+
+      bb1: {
+          StorageDead(_2);
+          goto -> bb2;
+      }
+
+      bb2: {
+-         _0 = copy (*_4);
++         _0 = copy ((*_3).0: i32);
+          return;
+      }
+  }
+

tests/mir-opt/reference_prop.rs

@@ -776,6 +776,36 @@ fn maybe_dead(m: bool) {
     }
 }
 
+#[custom_mir(dialect = "runtime", phase = "post-cleanup")]
+fn maybe_dead_field_reborrow(m: bool) -> i32 {
+    // CHECK-LABEL: fn maybe_dead_field_reborrow(
+    // CHECK: [[r:_.*]] = &mut {{_.*}};
+    // CHECK: ((*[[r]]).0: i32) = const 1_i32;
+
+    use std::intrinsics::mir::*;
+    mir! {
+        let x: (i32, i32);
+        let r: &mut (i32, i32);
+        let r2: &mut i32;
+        {
+            StorageLive(x);
+            x = (0, 0);
+            r = &mut x;
+            (*r).0 = 1;
+            r2 = &mut (*r).0;
+            match m { true => bb1, _ => bb2 }
+        }
+        bb1 = {
+            StorageDead(x);
+            Goto(bb2)
+        }
+        bb2 = {
+            RET = *r2;
+            Return()
+        }
+    }
+}
+
 fn mut_raw_then_mut_shr() -> (i32, i32) {
     // CHECK-LABEL: fn mut_raw_then_mut_shr(
     // CHECK-NOT: (*{{_.*}})
@@ -875,6 +905,7 @@ fn main() {
     multiple_storage();
     dominate_storage();
     maybe_dead(true);
+    maybe_dead_field_reborrow(true);
     mut_raw_then_mut_shr();
     unique_with_copies();
     debuginfo();
@@ -889,6 +920,7 @@ fn main() {
 // EMIT_MIR reference_prop.multiple_storage.ReferencePropagation.diff
 // EMIT_MIR reference_prop.dominate_storage.ReferencePropagation.diff
 // EMIT_MIR reference_prop.maybe_dead.ReferencePropagation.diff
+// EMIT_MIR reference_prop.maybe_dead_field_reborrow.ReferencePropagation.diff
 // EMIT_MIR reference_prop.mut_raw_then_mut_shr.ReferencePropagation.diff
 // EMIT_MIR reference_prop.unique_with_copies.ReferencePropagation.diff
 // EMIT_MIR reference_prop.debuginfo.ReferencePropagation.diff