updating error type to use computed properties

[heckj]

Small updates to convert this error type to using computed properties instead of static properties, following the pattern at apple/swift-nio#3229

[gwynne]

Not that I have any objection, but does this pattern confer any particular benefit?

Edit: Nevermind, I should've clicked through to the PR you linked before asking 😅.

[heckj]

Heh - the tl;dr for anyone looking: Cory said it much better than I would:

Public static lets can serve a bunch of roles, but one of them is to store simple constants: integers, and other trivial types, for example. This is a nice pattern and for internal and private static lets it works well, but for public ones it produces some inefficient code.

In particular, it has two downsides. First, it allocates storage for that value. We don't actually need to allocate a few hundred extra megabytes for the various integers we want to store.

Secondly, it forces calling code to access the address and call the dispatch_once code in order to get hold of the value. For trivial types we don't need that cost: they can just know what the value is directly.

Inlinable computed vars avoid all of these costs: they have no size overhead for storage, and they are visible to all clients so their values can be directly assembled.

Result:

Better codegen, smaller memory footprints, more attributes.

[heckj]

As for the "why did I pick this particular thing to poke at?" - I'm referencing this type in some internal documentation as an example of a good way to wrangle Error types. Other examples got the above treatment, and I wanted to both have the code I'm referencing actually match what's in this project while also reflecting consistency among the examples I choose.

[fabianfett]

@heckj sorry for taking so long to come back to you!

[fabianfett]

Thank you!