Also suggest dereferencing LHS when both &mut T and T are valid binop…

… LHS

compiler/rustc_typeck/src/check/op.rs

@@ -70,6 +70,14 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                         .is_err()
                     {
                         err.downgrade_to_delayed_bug();
+                    } else {
+                        // Otherwise, it's valid to suggest dereferencing the LHS here.
+                        err.span_suggestion_verbose(
+                            lhs.span.shrink_to_lo(),
+                            "consider dereferencing the left-hand side of this operation",
+                            "*",
+                            Applicability::MaybeIncorrect,
+                        );
                     }
                 }
             }

src/test/ui/typeck/assign-non-lval-needs-deref.rs

@@ -0,0 +1,19 @@
+// issue #101376
+
+use std::ops::AddAssign;
+struct Foo;
+
+impl AddAssign<()> for Foo {
+    fn add_assign(&mut self, _: ()) {}
+}
+
+impl AddAssign<()> for &mut Foo {
+    fn add_assign(&mut self, _: ()) {}
+}
+
+fn main() {
+    (&mut Foo) += ();
+    //~^ ERROR invalid left-hand side of assignment
+    //~| NOTE cannot assign to this expression
+    //~| HELP consider dereferencing the left-hand side of this operation
+}

src/test/ui/typeck/assign-non-lval-needs-deref.stderr

@@ -0,0 +1,16 @@
+error[E0067]: invalid left-hand side of assignment
+  --> $DIR/assign-non-lval-needs-deref.rs:15:16
+   |
+LL |     (&mut Foo) += ();
+   |     ---------- ^^
+   |     |
+   |     cannot assign to this expression
+   |
+help: consider dereferencing the left-hand side of this operation
+   |
+LL |     *(&mut Foo) += ();
+   |     +
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0067`.