Request: dummy_impl macro

[dhardy]

We can already use macros for a "dummy implementation" of an expression:

fn do_something_complex() -> T {
    todo!()
}

Now and again I've wanted a "dummy trait object". So:

trait T {
    type Q;
    fn ask(q: &Q) -> bool;
}

struct S;
dummy_impl!(T<Q = ()> for S);
// generates:
// impl T for S {
//     type Q = ();
//     fn ask(q: &Q) -> bool { todo!() }
// }

Here we specify Q = (); in this example it is not strictly necessary but in cases where the associated type has trait bounds it may be.

The biggest issue: macros can't do this due to lack of introspection for the trait T (even with proc-macros).

So, questions:

• is such a thing possible with compiler built-ins?
• is it desirable enough to be worth it?
• should it be exposed in std?

Motivation:

• writing tests on bounded generic functions may require a concrete type
• writing the impl by hand can be tedious, especially if the trait is frequently modified

[matklad]

While I don't usually suggest "just use an IDE", it seems like, in this case, this might be the right solution?

[H2CO3]

@dhardy

is such a thing possible with compiler built-ins?

Yes, probably.

@dhardy

is it desirable enough to be worth it?

No, probably.

Doesn't copy-pasting solve this, even if you don't use an IDE?

[dhardy]

In this particular case, I wanted to generate a dummy trait object for use in a doc test demonstrating usage that would be completely unrelated if not for a type bound. Wasting (hidden) lines on the doc-test and having to update them should the underlying trait change (likely) is undesirable.

Not a big use-case, but copy+paste / IDE auto-complete don't actually solve it.

[matklad]

For that sepecific case, the following works

/// ```
/// # fn give_me_T<S: T>(S: S) {
///    your docs here
/// # }
/// ```