You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: lib.rs
    path of file: ./repos/tectonic/crates/dep_support/src
    
    the code of the file until where you have to start completion: // Copyright 2020 the Tectonic Project
// Licensed under the MIT License.

#![deny(missing_docs)]

//! Support for locating third-party libraries ("dependencies") when building
//! Tectonic. The main point of interest is that both pkg-config and vcpkg are
//! supported as dep-finding backends. This crate does *not* deal wit
    
    fn default() -> Self {
        println!("cargo:rerun-if-env-changed=TECTONIC_DEP_BACKEND");
        println!("cargo:rerun-if-env-changed=TECTONIC_PKGCONFIG_FORCE_SEMI_STATIC");

        // This should use FromStr or whatever, but meh.
        let backend = if let Ok(dep_backend_str) = env::var("TECTONIC_DEP_BACKEND") {
            match dep_backend_str.as_ref() {
                "pkg-config" => Backend::PkgConfig,
                "vcpkg" => Backend::Vcpkg,
                "default" => Backend::default(),
                other => panic!("unrecognized TECTONIC_DEP_BACKEND setting {:?}", other),
            }
        } else {
            Backend::default()
        };

        let semi_static_mode = env::var("TECTONIC_PKGCONFIG_FORCE_SEMI_STATIC").is_ok();

        Configuration {
            backend,
            semi_static_mode,
        }
    }