Associated types in benchmark `where` bound

Question

I'm trying to constrain the value of an associated type of a trait inside a benchmark's where bound, like this:

#[benchmarks(where T: currency::Config<Balance = u128>)]
pub mod benchmarks {
    use super::*;

    #[benchmark]
    pub fn set_nomination_enabled() {
        #[extrinsic_call]
        _(RawOrigin::Root, true);
    }
}

However, this results in an error:

error[E0284]: type annotations needed
    |
122 | #[benchmarks(where T: currency::Config<Balance = u128>)]
    |                                        ^^^^^^^^^^^^^^ cannot infer type
    |
    = note: cannot satisfy `<T as currency::Config>::Balance == _`

The type annotations needed error does not make any sense to me, and I suspect it may be because of some cleverness by the macro.. Am I doing something wrong?

Answer 1

Instead of requiring an exact type, you can use a trait like From<u128>, or better: From<u32>.
Keep in mind that these bounds should be as general as possible.
Otherwise it artificially restricts the runtimes that your pallet can be deployed in.
For example when forcing it to u128, it wont work anymore with runtimes that use u64.

The correct bound would be:

#[benchmarks(where <T as pallet_balances::Config>::Balance: From<u32>)]

Then you can use .into() to consturct it like let balance = 10u32.into().