feat: define SerFunction in lib.rs

NOTE: lib.rs can't use rustc_internal APIs. `extern crate rustc_*` in
lib.rs will cause compilation errrors:

error: cannot satisfy dependencies so `rustc_driver` only shows up once
  |
  = help: having upstream crates all available in one format will likely make this go away
  = note: `rustc_driver` was unavailable as a static crate, preventing fully static linking
  = help: `feature(rustc_private)` is needed to link to the compiler's `rustc_driver` library

error: cannot satisfy dependencies so `core` only shows up once
  |
  = help: having upstream crates all available in one format will likely make this go away
  = help: `feature(rustc_private)` is needed to link to the compiler's `rustc_driver` library

Author: zjp-CN

src/lib.rs

@@ -0,0 +1,26 @@
+use serde::{Deserialize, Serialize};
+
+/// A Rust funtion with its file source, attributes, and raw function content.
+#[derive(Debug, Serialize, Deserialize)]
+pub struct SerFunction {
+    pub hash: String,
+    /// DefId in stable_mir.
+    pub def_id: String,
+    /// Every funtion must be declared in a specific file, even for those
+    /// generated by macros.
+    pub file: String,
+    /// Attributes are attached the function, but it seems that attributes
+    /// and function must be separated to query.
+    pub attrs: Vec<String>,
+    /// Raw function string, including name, signature, and body.
+    pub func: String,
+    /// Recursive fnction calls inside the body.
+    pub callees: Vec<Callee>,
+}
+
+#[derive(Debug, Serialize, Deserialize)]
+pub struct Callee {
+    pub def_id: String,
+    pub file: String,
+    pub func: String,
+}