bb8: Make parking_lot an optional feature enabled by default

[r-ml]

Rust 1.62 improved std::sync::Mutex speed and 1.63 constified its constructor. It can now be used in the same contexts parking_lot's implementation can.

Currently, bb8 is enabling the parking_lot tokio feature for its users. A library should probably not do that without a good reason. With the recent-ish improvements to std::sync::Mutex, I believe we can drop parking_lot from the dependency tree entirely.

[djc]

Well, bb8 currently has an MSRV of 1.57, so we cannot rely on the const constructor. On top of that, the 1.62 release notes only discuss Linux, not any of the other platforms. So I don't think this makes sense as a default. I'd be okay to take this as a Cargo feature that is enabled by default, though.

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.16 🎉

Comparison is base (5737736) 74.11% compared to head (04ee589) 74.28%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #152      +/-   ##
==========================================
+ Coverage   74.11%   74.28%   +0.16%     
==========================================
  Files           6        6              
  Lines         537      556      +19     
==========================================
+ Hits          398      413      +15     
- Misses        139      143       +4     

Impacted Files	Coverage Δ
bb8/src/internals.rs	94.92% <ø> (ø)
bb8/src/lib.rs	100.00% <100.00%> (ø)
bb8/src/api.rs	62.42% <0.00%> (-1.33%)	⬇️
bb8/src/inner.rs	86.24% <0.00%> (+0.07%)	⬆️

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[r-ml]

Well, bb8 currently has an MSRV of 1.57, so we cannot rely on the const constructor. On top of that, the 1.62 release notes only discuss Linux, not any of the other platforms. So I don't think this makes sense as a default. I'd be okay to take this as a Cargo feature that is enabled by default, though.

The pool constructor isn't const anyway.

To put it behind a feature flag I'll check what other libraries do in this case, because std::sync::Mutex::lock does not have a compatible signature to parking_lot's impl.

[r-ml]

Done. Please tell me whether the mod is a good enough way to implement this or would you prefer I sprinkle #[cfg(feature ...)] everywhere?

[r-ml]

Well, cargo's dep: feature was added in Rust 1.60 sigh...

[r-ml]

I installed 1.57 and I can work around the issue by naming the feature parking-lot instead of parking_lot. Let me know whether that's good enough.

Another thing, compilation failed here with "there is no argument named err". This appears to be fixed with 1.58. Want me to make a PR to make it compile? CI should have caught that though 🤔

[djc]

Okay, let's bump the MSRV to 1.60 (in a separate commit) so we can use dep:?

The CI setup, as I recall it, is a bit strange because the redis crate has a newer MSRV than the core bb8 crates. But bumping the MSRV should fix this anyway.

[r-ml]

Done. (For some reason I did not get notified by Github of your comment)

[djc]

Thanks!