Benchmarks 'how-to guide' uses outdated syntax

[Wiezzel]

In am referring to this guide. In section 2. Add benchmarking to your runtime, point 3. says:

Then, in addition to your normal runtime configuration, you also need to update the benchmarking section of your runtime. To add our new benchmarks, we simply add a new line with the add_benchmark! macro

As far as I know, this syntax (add_benchmark!) has been deprecated in favour of a more convenient and easier define_benchmarks! in this PR. I'd like to request updating this section of the guide to use the new syntax.

[lisa-parity]

@ggwpez , Can you provide some guidance on what to change?
It seems like the add_benchmark! macro is still valid based on this:
https://paritytech.github.io/substrate/master/frame_benchmarking/index.html
and
Adding Benchmarks
The benchmarks included with each pallet are not automatically added to your node. To actually execute these benchmarks, you need to implement the frame_benchmarking::Benchmark trait. You can see an example of how to do this in the included Substrate node.

Assuming there are already some benchmarks set up on your node, you just need to add another instance of the add_benchmark! macro:

/// configuration for running benchmarks
/// | name of your pallet's crate (as imported)
/// v v
add_benchmark!(params, batches, pallet_balances, Balances);
/// ^ ^
/// where all benchmark results are saved |
/// the struct created for your pallet by construct_runtime!

I wouldn't be at all surprised that this doc is out-of-date, but the How-to guide is pretty thin on details so I'm not sure what needs to change (well, actually, it probably needs to be completely rewritten at some point ;-D).

[ggwpez]

Thanks for spotting it!
The old syntax was in the form of:

add_benchmark!(params, batches, frame_benchmarking, BaselineBench::<Runtime>);
add_benchmark!(params, batches, pallet_assets, Assets);
add_benchmark!(params, batches, pallet_babe, Babe);
…

and then in another function, one had to write the same thing again which is error prone:

list_benchmark!(list, extra, frame_benchmarking, BaselineBench::<Runtime>);
list_benchmark!(list, extra, pallet_assets, Assets);
list_benchmark!(list, extra, pallet_babe, Babe);
…

these two got unified into define_benchmarks! which allows a developer to only write it once.

define_benchmarks!(
	[frame_benchmarking, BaselineBench::<Runtime>]
	[pallet_assets, Assets]
	[pallet_babe, Babe]
	…
);

This can be seen here (and following lines). The old syntax is still valid and it is possible to mix the old and new syntax, but I would suggest to only mention the new one.
The developer now only has to add the new benchmark to the define_benchmarks! macro invocation.
In case an alias is needed, they still need to be defined twice eg. here and here.

Looks like this article also mentions the old syntax.

[lisa-parity]

Thanks, @ggwpez ! I took a stab at updating the guide.