Rollup merge of #101388 - compiler-errors:issue-101376, r=fee1-dead

Don't delay invalid LHS bug unless it will be covered by an error in `check_overloaded_binop`

Fixes #101376

compiler/rustc_typeck/src/check/op.rs

@@ -57,9 +57,28 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                     )
                     .is_ok()
                 {
-                    // Suppress this error, since we already emitted
-                    // a deref suggestion in check_overloaded_binop
-                    err.downgrade_to_delayed_bug();
+                    // If LHS += RHS is an error, but *LHS += RHS is successful, then we will have
+                    // emitted a better suggestion during error handling in check_overloaded_binop.
+                    if self
+                        .lookup_op_method(
+                            lhs_ty,
+                            Some(rhs_ty),
+                            Some(rhs),
+                            Op::Binary(op, IsAssign::Yes),
+                            expected,
+                        )
+                        .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,
+                        );
+                    }
                 }
             }
         });