im 12.3.4

Immutable collection datatypes
Documentation 
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// This Source Code Form is subject to the terms of the Mozilla Public
// License, v. 2.0. If a copy of the MPL was not distributed with this
// file, You can obtain one at http://mozilla.org/MPL/2.0/.

extern crate rustc_version;

use rustc_version::{version_meta, Channel};
use std::env;

fn main() {
    println!("cargo:rerun-if-changed=build.rs");
    match version_meta().unwrap().channel {
        Channel::Nightly => {
            println!("cargo:rustc-cfg=has_specialisation");
        }
        _ => (),
    }
    let pkgname = env::var("CARGO_PKG_NAME").expect("Cargo didn't set the CARGO_PKG_NAME env var!");
    let test_rc = env::var("IM_TEST_RC").is_ok();
    match pkgname.as_str() {
        "im" => if !test_rc {
            println!("cargo:rustc-cfg=threadsafe")
        },
        "im-rc" => {}
        _ => panic!("unexpected package name!"),
    }
}