fix false negative for unused_mut

compiler/rustc_borrowck/src/lib.rs

@@ -935,6 +935,7 @@ enum InitializationRequiringAction {
     PartialAssignment,
 }
 
+#[derive(Debug)]
 struct RootPlace<'tcx> {
     place_local: Local,
     place_projection: &'tcx [PlaceElem<'tcx>],
@@ -1848,11 +1849,6 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
                         // is allowed, remove this match arm.
                         ty::Adt(..) | ty::Tuple(..) => {
                             check_parent_of_field(self, location, place_base, span, flow_state);
-
-                            // rust-lang/rust#21232, #54499, #54986: during period where we reject
-                            // partial initialization, do not complain about unnecessary `mut` on
-                            // an attempt to do a partial initialization.
-                            self.used_mut.insert(place.local);
                         }
 
                         _ => {}
@@ -1940,6 +1936,11 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
                     (prefix, base, span),
                     mpi,
                 );
+
+                // rust-lang/rust#21232, #54499, #54986: during period where we reject
+                // partial initialization, do not complain about unnecessary `mut` on
+                // an attempt to do a partial initialization.
+                this.used_mut.insert(base.local);
             }
         }
     }

tests/ui/lint/unused/lint-unused-mut-variables.rs

@@ -205,3 +205,11 @@ fn bar() {
     let mut b = vec![2]; //~ ERROR: variable does not need to be mutable
 
 }
+
+struct Arg(i32);
+
+// Regression test for https://github.com/rust-lang/rust/issues/110849
+fn write_through_reference(mut arg: &mut Arg) {
+    //~^ WARN: variable does not need to be mutable
+    arg.0 = 1
+}

tests/ui/lint/unused/lint-unused-mut-variables.stderr

@@ -218,5 +218,13 @@ note: the lint level is defined here
 LL | #[deny(unused_mut)]
    |        ^^^^^^^^^^
 
-error: aborting due to previous error; 25 warnings emitted
+warning: variable does not need to be mutable
+  --> $DIR/lint-unused-mut-variables.rs:212:28
+   |
+LL | fn write_through_reference(mut arg: &mut Arg) {
+   |                            ----^^^
+   |                            |
+   |                            help: remove this `mut`
+
+error: aborting due to previous error; 26 warnings emitted