Testing Rust Code Without Adding Test-Only Traits
A common pattern in Rust unit tests looks like this: You have a small function that calls the filesystem, a clock, a socket, an FFI function, or some other dependency. The production code is simple. Then you try to unit test an error path. Suddenly you are introducing a trait, adding a generic parameter, threading an implementation through several layers, and maintaining an abstraction that…
Unit testing in Rust often involves dealing with dependencies such as the filesystem, clock, sockets, or foreign function interfaces (FFI). Normally, testing error paths requires introducing traits, adding generic parameters, or threading implementations through multiple layers, which can be cumbersome and unnecessary. ShimForge aims to address this issue by allowing tests to replace Rust functions without modifying the production code.
To use ShimForge, simply add it as a development dependency using Cargo: `cargo add --dev shimforge`. The `claim_slot` function, which attempts to create a directory, can be tested by mocking the `fs::create_dir_all` function. In the success path test, ShimForge verifies that the function was called once with the expected path and returns `Ok(())` when successful.
Similarly, the error path can be tested by mocking the function to return an `Err` with a specific error message, ensuring that the function returns the expected error when the directory creation fails. This approach eliminates the need for temporary directories, filesystem setup, or test-only traits, providing a straightforward way to test functions while keeping the production code unchanged.
ShimForge supports mocking normal functions, methods, generic function instantiations, unsafe functions, and even external C or system functions.
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