message: Fix compilation failure in tiering.rs.
diff: //! The tiering module supports switching between code compiled with different optimization levels
  //! as runtime.
  -use crate::backend::{Compiler, CompilerConfig};
  +use crate::backend::{Backend, Compiler, CompilerConfig};
  use crate::compile_with_config;
  use crate::fault::{
  catch_unsafe_unwind, ensure_sighandler, pop_code_version, push_code_version, with_ctx,
  