Update inventory requirement from 0.1.10 to 0.2.0

[dependabot]

Updates the requirements on inventory to permit the latest version.

Release notes

Sourced from inventory's releases.

0.2.0

• Enforce that elements given to inventory::submit are const constructible (#43) and Sync (#42)

Commits

• 71b6c1f Release 0.2.0
• 7d81709 Document compiler requirement of 1.37
• 8291011 Add ui test of nonconst construction
• b499293 Merge pull request #43 from dtolnay/const
• b853350 Require inventory element type to be const constructed
• 762b5ce Merge pull request #42 from dtolnay/sync
• ea173f9 Add ui test of sync bound
• e1e347d Require inventory element type to be Sync
• 95501ae Merge pull request #41 from dtolnay/macrorules
• 4a6f321 Import inventory-impl macros into inventory crate
• Additional commits viewable in compare view

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[tyranron]

cc @ilslv

[ilslv]

@tyranron this became quite a rabbit hole.

For now we are generating 3 structs (each for given, when and then) inside WorldInit macros and implement StepConstructor trait on them

pub trait StepConstructor<W> {
    fn new(_: step::Location, _: Regex, _: Step<W>) -> Self;
    fn inner(&self) -> (step::Location, Regex, Step<W>);
}

This is done to have an ability to construct those struct using only World type and inventory::submit!() them.
Since 0.2 inventory::submit!() requires const, so we have to remove fn new() from StepConstructor and use simple impl pub const fn new(). But there is a catch: function pointers cannot appear in constant functions (Playground). To be fair I don't understand why this is the case, but ok.

Then I've tried ditching const fn new() and create step struct with <World as WorldInit>::Given { .. }, but this syntax isn't supported yet: rust-lang/rust#86935

So the only way I see this can be possible on stable Rust is to mem::transmute into *const () on both sides. But wait, there is one more catch. From 0.2 step structs have to be Sync.
To overcome this we'll need a bit more unsafety

#[derive(Clone, Copy)]
#[repr(C)]
pub struct SyncFriend(*const ());

#[allow(unsafe_code)]
unsafe impl Sync for SyncFriend {}

All this does work, but unfortunately we have to expand code with unsafe into users codebase. All this while not having obvious benefits from upgrading to the latest version of inventory. Maybe we should postpone this upgrade until rust-lang/rust#86935 is stabilised?

[tyranron]

@ilslv

Then I've tried ditching const fn new() and create step struct with <World as WorldInit>::Given { .. }, but this syntax isn't supported yet: rust-lang/rust#86935

Maybe we can workaround it with introducing in-place type alias?

[ilslv]

@tyranron in-place type alias still runs into function pointers cannot appear in constant functions, but it looks like I've figured out a way to do it: Playground
It looks like it's forbidden to cast fn items to fn pointers in const functions, while in basic consts it's ok.

[tyranron]

@ilslv ahaha... brilliant! 🙈

What we love Rust for - is squating 😁

[tyranron]

@ilslv so, is it ready for review then? If so, request the one please.

[ilslv]

@tyranron not yet, as I've just discovered this trick. I'll request review, once it's done

[ilslv]

FCM

Upgrade `inventory` to 0.2 (#158)

Additionally:
- simplify code examples in docs with using `#[tokio::main]` macro