When should I use `ConstU32` to declare a constant value for my pallet?

Question

There seems to be different approaches to declaring constants for a pallet's runtime implementation.

1. A common approach for example is:

// In pallet/example/src/lib.rs
#[pallet::config]
pub trait Config: frame_system::Config {
        // --snip --
        const SOME_CONSTANT: u32;
}

// In runtime/src/lib.rs
impl pallet_example::Config for Runtime {
        const SOME_CONSTANT: u32 = 42;
}

2. Another way is to use ConstU32. For example:

// In pallet/example/src/lib.rs
#[pallet::config]
pub trait Config: frame_system::Config {
        // --snip --
        #[pallet::constant]
        type SomeConstant: Get<u32>;
}

// In runtime/src/lib.rs
impl pallet_example::Config for Runtime {
    type SomeConstant = ConstU32<42>;
}

3. A third approach would be to use the parameter_types types macro, which is most commonly used to declare constants of custom types, but can also be used for any other type.

How do I know which approach to use? Why would I choose one over the other?

Answer 1

Associated constants are not used that much in FRAME. I mean you can use them, but it makes it harder to write tests. With the Get trait you have the possibility to "cheat" in tests by implementing it for a thread local type.

For ConstU32 vs parameter_types!. ConstU32 just came later with the stabilization of const generics. Code wise there is no real difference. However, ConstU32 for sure is "shorter".

Answer 2

Also, for things like BoundedVec, it is not currently possible to use an associated constant because const generics are not fully supported: https://github.com/paritytech/substrate/pull/9865

I think there are a lot of places we would actually use a const, but can't really do that yet.