How do I check if a trait is implemented at compile time in Rust?

Question

I want to print a variable with the Display trait if it is implemented, or with the Debug trait otherwise. All the types are known at compile time.

let display = MyDisplayType::new(); // implements Display
let debug = MyDebugType::new(); // implements Debug

output!(display); // uses Display a.k.a. println!("{}", display);
output!(debug); // uses Debug a.k.a. println!("{:?}", debug);

What is the cleanest way?

I believe your question is answered by the answers of [Is it possible to check if an object implements a trait at runtime?](https://stackoverflow.com/q/30274091/155423). If you disagree, please [edit] your question to explain the differences. Otherwise, we can mark this question as already answered. — Shepmaster, Aug 12 '18 at 19:29
You may also be interested in [Rust generic trait implementations for trait](https://stackoverflow.com/q/42426239/155423). — Shepmaster, Aug 12 '18 at 19:32
As well as [How can I implement a function differently depending on if a generic type implements a trait or not?](https://stackoverflow.com/q/51141200/155423). — Shepmaster, Aug 12 '18 at 19:36
I think the other questions are not related. I need only a macro (not a function) that prints a variable with either Display or Debug if Display is not implemented. — hedgar2017, Aug 12 '18 at 19:53
Macros expand to code and at the time they are expanded, type information isn't available yet. So a macro wouldn't help. Any ways, if you want to format using either `Display` or `Debug`, it probably means you just want `Debug`. — mcarton, Aug 12 '18 at 20:01
@mcarton, I have just created separate macros. I needed to implement stuff described above for DHCP options. There are around 150 of them, so I won't overload each, especially when the majority of them are complex types like `Vec<(IpV4Addr, IpV4Addr>`. — hedgar2017, Aug 13 '18 at 07:40
I'd say this is not a duplicate. Compile time != run time. I'm trying to figure out how to assert at compile time that a variable implements a trait. Which is similar, but not quite the same question. :( — LordCecil, May 20 '19 at 06:52

score 1 · Answer 1 · answered Jan 07 '22 at 15:48

It's possible if you want to assert that a type, at compile time, implements a trait. For example in a unit test you can create a function bounded for the trait. If the type doesn't implement the trait, the code will not compile.

fn implements_the_trait<T: MyTrait>() {}

#[test]
fn implements_my_trait() {
  implements_the_trait::<MyType>();
}

How do I check if a trait is implemented at compile time in Rust?

1 Answers1