syn 3.0.3: syn::ExprMatch

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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::ExprMatch 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::ExprMatch

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.
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  - Keep it under 200 files and 256KB packed.

{"case":{"caseId":"case:sha256:04ccf32c4f7d8d3be4ac89d2b2e45612574275968c23778839fb90b1947ccf5e","contract":["syn::parse_str parses a basic match expression into syn::ExprMatch with match target and arms","syn::parse_str parses match arms with pattern guards into syn::ExprMatch with Pat::Guard","syn::parse_str parses match expressions with outer and inner attributes into syn::ExprMatch","syn::parse_str parses match arms with multiple alternatives into Pat::Or","syn::parse_str parses an empty match expression into syn::ExprMatch with empty arms","syn::parse_str returns an error when match expression syntax is invalid"],"goal":"verify syn::ExprMatch in pkg:cargo/syn@3.0.3","kind":"HOW","packages":["pkg:cargo/syn@3.0.3"],"schemaVersion":1,"symbols":["syn::ExprMatch"]},"contractCommand":["cargo","run","--offline"],"environment":{"arch":"x64","distro":"alpine","ecosystem":"cargo","libc":"musl","os":"linux","packageManager":"cargo","schemaVersion":1},"license":"MIT-0","packages":["pkg:cargo/syn@3.0.3"],"schemaVersion":1,"subject":"pkg:cargo/syn@3.0.3","symbols":["syn::ExprMatch"],"verifierAdapter":"cargo@1"}

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  "goal": "verify syn::ExprMatch in pkg:cargo/syn@3.0.3",
  "kind": "HOW",
  "packages": [
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  ],
  "symbols": [
    "syn::ExprMatch"
  ]
}

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

pub use syn::*;

use syn::{parse_str, AttrStyle, Expr, ExprMatch, Pat};

fn main() {
    // 1. syn::parse_str parses a basic match expression into syn::ExprMatch with match target and arms
    let match_code = r#"
        match status {
            200 => "OK",
            404 => "Not Found",
            _ => "Unknown",
        }
    "#;
    let parsed_match: ExprMatch = parse_str(match_code).expect("failed to parse ExprMatch");
    assert_eq!(parsed_match.arms.len(), 3);
    if let Expr::Path(path) = &*parsed_match.expr {
        assert_eq!(path.path.segments[0].ident.to_string(), "status");
    } else {
        panic!("expected Expr::Path for match target");
    }

    // 2. syn::parse_str parses match arms with pattern guards into syn::ExprMatch with Pat::Guard
    let guarded_code = r#"
        match value {
            Some(x) if x > 0 => x,
            _ => 0,
        }
    "#;
    let parsed_match: ExprMatch = parse_str(guarded_code).expect("failed to parse guarded match");
    assert_eq!(parsed_match.arms.len(), 2);
    let first_arm = &parsed_match.arms[0];
    if let Pat::Guard(pat_guard) = &first_arm.pat {
        if let Pat::TupleStruct(inner_pat) = &*pat_guard.pat {
            assert_eq!(inner_pat.path.segments[0].ident.to_string(), "Some");
        } else {
            panic!("expected tuple struct inside guard pattern");
        }
    } else {
        panic!("expected Pat::Guard for guarded match arm");
    }

    // 3. syn::parse_str parses match expressions with outer and inner attributes into syn::ExprMatch
    let attr_code = r#"
        #[must_use]
        match flag {
            #![allow(unreachable_patterns)]
            true => 1,
            false => 0,
        }
    "#;
    let parsed_match: ExprMatch = parse_str(attr_code).expect("failed to parse match with attributes");
    assert_eq!(parsed_match.attrs.len(), 2);
    match &parsed_match.attrs[0].style {
        AttrStyle::Outer => {}
        _ => panic!("expected outer attribute"),
    }
    match &parsed_match.attrs[1].style {
        AttrStyle::Inner(_) => {}
        _ => panic!("expected inner attribute"),
    }

    // 4. syn::parse_str parses match arms with multiple alternatives into Pat::Or
    let or_code = r#"
        match num {
            1 | 2 | 3 => true,
            _ => false,
        }
    "#;
    let parsed_match: ExprMatch = parse_str(or_code).expect("failed to parse or-pattern match");
    assert_eq!(parsed_match.arms.len(), 2);
    if let Pat::Or(pat_or) = &parsed_match.arms[0].pat {
        assert_eq!(pat_or.cases.len(), 3);
    } else {
        panic!("expected Pat::Or for multi-case pattern");
    }

    // 5. syn::parse_str parses an empty match expression into syn::ExprMatch with empty arms
    let empty_code = "match val {}";
    let parsed_match: ExprMatch = parse_str(empty_code).expect("failed to parse empty match");
    assert!(parsed_match.arms.is_empty());

    // 6. syn::parse_str returns an error when match expression syntax is invalid
    let invalid_code = "match { => }";
    let parse_result = parse_str::<ExprMatch>(invalid_code);
    assert!(parse_result.is_err(), "expected syntax error for invalid match expression");

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