syn 3.0.3: syn::Expr

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Clean-room public code sample — generation instructions

Write a brand-new, minimal, self-contained code sample in this clean-room directory.
Do not copy, paraphrase, or reference any existing project source. Work only from this spec.

A csx.json manifest scaffold already exists. Do not recreate it from memory. Preserve its case.goal, packages and symbols; fill its empty case.contract with exact assertions and correct its environment, commands and verifierAdapter for the files you generate.

Goal: verify syn::Expr in pkg:cargo/syn@3.0.3
Kind: HOW

Use EXACTLY these public packages and versions:
  - pkg:cargo/syn@3.0.3
Demonstrate these symbols/APIs:
  - syn::Expr

Rules:
  - One focused purpose; the smallest project that proves the goal.
  - Include a contract test (test/contract.*) that runs OFFLINE and exits 0 exactly when the goal behavior works.
  - Pin every dependency with a lockfile so resolution is reproducible.
  - No secrets, credentials, or tokens. No real URLs (only example.com or localhost). No absolute paths.
  - No personal names, emails, company names, or project identifiers of any kind.
  - No binaries and no generated output (node_modules, dist, target, venv, .git, .env).
  - Keep it under 200 files and 256KB packed.

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  "kind": "HOW",
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  "symbols": [
    "syn::Expr"
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}

//! Clean-room verification sample for syn::Expr.

pub use syn::*;

use syn::{BinOp, Expr, ExprBinary, ExprClosure, ExprIf, ExprMatch, ExprMethodCall};

fn main() {
    // 1. syn::parse_str parses binary arithmetic expressions into Expr::Binary
    let expr: Expr = syn::parse_str("a + 42").expect("valid binary expr");
    match &expr {
        Expr::Binary(ExprBinary { op: BinOp::Add(_), .. }) => {}
        _ => panic!("expected Expr::Binary with Add"),
    }

    // 2. syn::parse_str parses method calls into Expr::MethodCall with receiver and method ident
    let expr: Expr = syn::parse_str("item.process(10)").expect("valid method call");
    match &expr {
        Expr::MethodCall(ExprMethodCall { method, args, .. }) => {
            assert_eq!(method.to_string(), "process");
            assert_eq!(args.len(), 1);
        }
        _ => panic!("expected Expr::MethodCall"),
    }

    // 3. syn::parse_str parses if-else conditional branches into Expr::If
    let expr: Expr = syn::parse_str("if ready { 1 } else { 0 }").expect("valid if expr");
    match &expr {
        Expr::If(ExprIf { cond, else_branch, .. }) => {
            assert!(else_branch.is_some());
            match cond.as_ref() {
                Expr::Path(p) => assert_eq!(p.path.segments.first().unwrap().ident.to_string(), "ready"),
                _ => panic!("expected path condition"),
            }
        }
        _ => panic!("expected Expr::If"),
    }

    // 4. syn::parse_str parses closure expressions with arguments and body into Expr::Closure
    let expr: Expr = syn::parse_str("|x, y| x * y").expect("valid closure");
    match &expr {
        Expr::Closure(ExprClosure { inputs, body, .. }) => {
            assert_eq!(inputs.len(), 2);
            assert!(matches!(body.as_ref(), Expr::Binary(_)));
        }
        _ => panic!("expected Expr::Closure"),
    }

    // 5. syn::parse_str parses match expressions into Expr::Match with arms
    let expr: Expr = syn::parse_str("match val { Some(x) => x, None => 0 }").expect("valid match");
    match &expr {
        Expr::Match(ExprMatch { arms, .. }) => {
            assert_eq!(arms.len(), 2);
        }
        _ => panic!("expected Expr::Match"),
    }

    // 6. syn::parse_str rejects invalid syntax expressions with a parse error
    let invalid = syn::parse_str::<Expr>("let x = ;");
    assert!(invalid.is_err());

    println!("All syn::Expr contract assertions passed successfully.");
}