dyn compatibility (formerly object safety) can be verified by using

struct Foo {
    // If this compiles then `Trait` is dyn compatible.
    p: Box<dyn Trait>,
}

We should add such a test to any api test file (e.g. units/tests/api.rs).

Before closing this issue it must be done for all traits in all leaf crates

• units (units: Test for dyn compatibility #3926)
• [ ] io (Make Read and BufRead dyn compatible #3917)
• hashes (done as part of hashes: Add an API test module #3927)
• primitives (api.rs file still todo: primitives: Add an API test module #3928)

ref:

• https://rust-lang.github.io/api-guidelines/flexibility.html#c-object
• https://quinedot.github.io/rust-learning/dyn-safety.html