Rust 过程宏：用 syn Fold 优雅替换 Panic

#[panic_to_result] // use our macrofn create_person_with_result(name: String, age: u32) -> Result<Person, String> {   if age > 30 {       panic!("I hope I die before I get old"); // <- panic will be replaced by an Err   }   Ok(Person {       name,       age,   })}
fn main() {   // the assertion shows we got back an Err instead of a panic!   assert!(create_person_with_result("name".to_string(), 31).is_err());}

match expression {   // check the if expressions for panics   Expr::If(mut ex_if) => {       let new_statements: Vec<Stmt> = // modify existing statements       ex_if.then_branch.stmts = new_statements; // and put them inside the if       Stmt::Expr(Expr::If(ex_if), token)   },   // return all other expressions without modification   _ => Stmt::Expr(expression, token)}

[dependencies]quote = "1.0.33"syn = { version = "2.0.39", features = ["fold", "full"]}
[lib]proc-macro = true

use proc_macro::TokenStream;use quote::quote;
#[proc_macro_attribute]pub fn panic_to_result(_attr: TokenStream, input: TokenStream) -> TokenStream {   let item: ItemFn = syn::parse(input).unwrap(); // parse the input   let result = ChangePanicIntoResult.fold_item_fn(item); // fold it   quote!(#result).into() // and return the result}

fn extract_panic_content(mac: &Macro) -> Option<TokenStream2> {    let does_panic = mac.path.segments.iter()        .any(|v| v.ident.to_string().eq("panic"));
    if does_panic {        Some(mac.tokens.clone())    } else {        None    }}

use quote::quote;use syn::{fold, ItemFn, Macro, Stmt};use syn::fold::Fold;
struct ChangePanicIntoResult; // the struct that we were calling in the entry point
impl Fold for ChangePanicIntoResult {   fn fold_stmt(&mut self, stmt: Stmt) -> Stmt {       let new_statement: Stmt = match stmt {           Stmt::Macro(ref mac) => {               let output = extract_panic_content(&mac.mac); // helper to get the panic message               output                   .map(|t| quote! {                       return Err(#t.to_string());                   })                   .map(syn::parse2)                   .map(Result::unwrap)                   .unwrap_or(stmt)           }           // panics should be inside a 'Macro', so in every other case we return           _ => stmt       };       // keep folding       fold::fold_stmt(self, new_statement)   }}

use fold_macro::panic_to_result;
#[derive(Debug)]pub struct Person {   name: String,   age: u32,}
#[panic_to_result]fn create_person_with_result(name: String, age: u32) -> Result<Person, String> {   // 'if' works   if age > 30 && age < 50 {       panic!("I hope I die before I get old");   }   // but now loop does as well   loop {       if age > 50 {           panic!("This person is old... very old");       }       break   }   Ok(Person {       name,       age,   })}
fn main() {   let first = create_person_with_result("name".to_string(), 20);   println!("{first:?}");   let second = create_person_with_result("name".to_string(), 40);   println!("{second:?}");   let third = create_person_with_result("name".to_string(), 51);   println!("{third:?}");;}

Ok(Person { name: "name", age: 20 }) Err("I hope I die before I get old") Err("This person is old... very old")