Expose benchmark component ranges

[ggwpez]

Problem

The range of a benchmarking component is not written to the weight file.
Any downstream software (like substrate-weight-compare) can therefore not use this information.

Context

A benchmarking component is defined in the form:

let n in 0..100;

Substrate then samples some values in this range and measures the weight at each.
The final weight file does not contain any information about the 0..100 range from above:

fn nominate(n: u32, ) -> Weight {
		(45_566_000 as Weight)
			.saturating_add((3_063_000 as Weight).saturating_mul(n as Weight))
}

Solution

One way would be to add attribute macros. Eg:

fn nominate(#[range(min=0, max=100, step=20)] n: u32, ) -> Weight {
		(45_566_000 as Weight)
			.saturating_add((3_063_000 as Weight).saturating_mul(n as Weight))
}

... or even easier with comments:

/// n: 0..100 with step size 20
fn nominate(n: u32, ) -> Weight {
		(45_566_000 as Weight)
			.saturating_add((3_063_000 as Weight).saturating_mul(n as Weight))
}

The step size of 20 in this example is picked by Substrate.

Before implementing the doc solution, it would be wise to double-check that syn can parse this.

[ggwpez]

I tested the parsing and this works fine. It only works with triple dashes or other kind of doc comments.

/// The range of component `c` is `[1_337, 2000]`.
/// The range of component `d` is `[42, 999999]`.
fn ext(c: u32, d: u32, ) -> Weight {
	(5 as Weight)
}

are you ok with the general syntax of this @shawntabrizi ?

[shawntabrizi]

looks reasonable to me :)