Automating Dependency Download with a Rust Build Script

Rust build scripts allow you to execute arbitrary code during the build process, enabling tasks like downloading dependencies, generating code, or running external tools. This challenge focuses on implementing a build script that automatically downloads a pre-built library from a remote URL and places it in a location accessible to the compiler. This is useful for incorporating external, non-Rust libraries into your project, especially when those libraries are not available through crates.io.

Problem Description

You are tasked with creating a Rust build script (build.rs) that downloads a pre-built static library (libmylibrary.a) from a specified URL and places it in the target/my_library directory. The URL and the target directory are configurable via environment variables. The build script should:

1. Read Configuration: Read the URL of the library from the MY_LIBRARY_URL environment variable and the target directory from the MY_LIBRARY_TARGET environment variable. If either environment variable is not set, the script should print an error message to stderr and exit with a non-zero exit code.
2. Create Target Directory: Create the target directory (target/my_library) if it doesn't already exist.
3. Download Library: Download libmylibrary.a from the provided URL and save it to the target directory.
4. Link Library: Tell the compiler to link against the downloaded library. This is done by printing a line to cargo:rustc-link-search and cargo:rustc-link-lib.

Examples

Example 1:

Input:
MY_LIBRARY_URL=https://example.com/libmylibrary.a
MY_LIBRARY_TARGET=target/my_library

Output (printed to stdout by build.rs):

cargo:rustc-link-search=target/my_library
cargo:rustc-link-lib=mylibrary

Explanation: The build script downloads libmylibrary.a from the specified URL, places it in target/my_library, and instructs the compiler to search for libraries in that directory and link against mylibrary.

Example 2:

Input:
MY_LIBRARY_URL=https://example.com/libmylibrary.a
MY_LIBRARY_TARGET=target/my_library

(Assume the URL is invalid and the download fails) Output (printed to stderr by build.rs):

Error: Failed to download libmylibrary.a from https://example.com/libmylibrary.a

Explanation: The build script attempts to download the library but fails. It prints an error message to stderr and exits with a non-zero exit code.

Example 3: (Edge Case - Missing Environment Variables)

Input:
(No environment variables set)

Output (printed to stderr by build.rs):

Error: MY_LIBRARY_URL environment variable not set.

Explanation: The MY_LIBRARY_URL environment variable is not set. The build script prints an error message to stderr and exits with a non-zero exit code.

Constraints

• The URL will be a valid HTTP(S) URL.
• The target directory will be relative to the project's root directory.
• The build script must handle potential download errors gracefully.
• The build script must exit with a non-zero exit code if any error occurs.
• The library name to link against is simply the filename without the "lib" prefix and extension (e.g., "mylibrary" for "libmylibrary.a").

Notes

• You'll need to use the std::env module to access environment variables.
• You'll need to use a crate like reqwest to perform the HTTP download. Add reqwest = { version = "0.11", features = ["blocking"] } to your Cargo.toml under [build-dependencies].
• Use std::process::exit() to exit the build script with a non-zero exit code.
• Remember to print the cargo:rustc-link-search and cargo:rustc-link-lib lines to stdout. These are special directives that Cargo understands.
• Error handling is crucial. Consider what might go wrong during the download process (e.g., network errors, invalid URLs, file not found).
• The std::fs module can be used to create directories.