rust/src/test/compile-fail/issue-15783.rs

// Copyright 2014 The Rust Project Developers. See the COPYRIGHT
// file at the top-level directory of this distribution and at
// http://rust-lang.org/COPYRIGHT.
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.

pub fn foo(params: Option<&[&str]>) -> usize {
    params.unwrap().first().unwrap().len()
}

fn main() {
    let name = "Foo";
    let msg = foo(Some(&[name.as_slice()]));
//~^ ERROR mismatched types: expected `core::option::Option<&[&str]>`
    assert_eq!(msg, 3);
}