Update 3.1-frontend.md

src/3.1-frontend.md

@@ -14,5 +14,25 @@ Cargo will automatically search the binaries named cargo-toolname from the paths
 Note that we cannot directly invoke rap in the first round but through cargo check because we need cargo to manage the project-level compilation and append detailed compilation options for launching rustc. However, we want to hook rustc execution and execute rap instead for analysis. Therefore, we set [RUSTC_WRAPPER](https://doc.rust-lang.org/cargo/reference/environment-variables.html) with the value of cargo-rap. In this way, cargo check will actually run `cargo-rap rustc appended_options`. We then dispath the execution to rap with appended options.
 
 ## Register Analysis Callbacks
+Supposing the purpose is to execute a function named my_analysis, developers should design a new struct and implement the [Callbacks Trait](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_driver/trait.Callbacks.html) for the struct.
+```
+pub struct MyCallback {...}
+impl Callbacks for MyCallback {
+    fn after_analysis<'tcx>(&mut self, compiler: &Compiler, queries: &'tcx Queries<'tcx>) -> Compilation {
+        compiler.session().abort_if_errors();
+        queries.global_ctxt().unwrap().enter(
+            |tcx| my_analysis(tcx, *self) // the analysis function to execute after compilation.
+        );
+        compiler.session().abort_if_errors();
+        Compilation::Continue
+    }
+}
+```
+To execute the compiler and callback function, developers can employ the APIs [rustc_driver::RunCompiler](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_driver/struct.RunCompiler.html) provided by Rust.
+```
+let mut callback = MyCallback::default();
+let run_compiler = rustc_driver::RunCompiler::new(&args, callback);
+let exit_code = rustc_driver::catch_with_exit_code(move || run_compiler.run());
+```