Asserting static properties in Rust

If you write a generic library in Rust, you will at some point run into the issue that you have to assert properties of your exported types that are implicitly derived by the compiler.

Implicit derivation makes a trade-off: it removes a lot of boilerplate in exchange for the possibility to break things silently by accident.

The three most common things falling in that category are:

• Send
• Sync
• Object safety

But we can test these statically, by using them in our test suite. If one of the conditions we have breaks, compilation will fail.

For Send and Sync, we can use the following functions:

struct MyType {
  inner: i32
}

fn _assert_send<T: Send>() {}
fn _assert_sync<T: Sync>() {}

fn _assertions() {
    _assert_send::<MyType>();
    _assert_sync::<MyType>();
}

This article used to have an approach to test object safety here that didn’t work.

Finally, check out the compile-fail crate for proper integration of this into your test suite.

Notable mentions: The pattern for _assert_send/sync is lifted from Alex Crichtons futures-cpupool crate.