Migrate to edition 2021

[Philippe-Cholet]

I noted 5 months ago that edition 2021 requires rustc 1.56+ so it's now possible. Is there any reason to not update?

[codecov[bot]]

Codecov Report

:x: Patch coverage is 94.44444% with 1 line in your changes missing coverage. Please review. :white_check_mark: Project coverage is 93.65%. Comparing base (6814180) to head (fde1ad9). :warning: Report is 168 commits behind head on master.

Files with missing lines	Patch %	Lines
src/adaptors/mod.rs	91.66%	1 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #967      +/-   ##
==========================================
- Coverage   94.38%   93.65%   -0.74%     
==========================================
  Files          48       50       +2     
  Lines        6665     6323     -342     
==========================================
- Hits         6291     5922     -369     
- Misses        374      401      +27     

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[Philippe-Cholet]

I added some marginal changes:

• MSRV automated in the crate documentation (valid for a MSRV >= 1.63.0).
• Fix some additional clippy lints, mostly related to the recent MSRV/edition changes.
• Use ControlFlow?!

@jswrenn I remember your https://github.com/rust-itertools/itertools/pull/780#discussion_r1352664093 where you prefered ControlFlow over Result, I replaced some, left some aside for now. Can you elaborate on the reason?

[jswrenn]

I remember your #780 (comment) where you prefered ControlFlow over Result, I replaced some, left some aside for now. Can you elaborate on the reason?

Result::Err denotes "something went wrong"; ControlFlow::Break doesn't. Favor ControlFlow when we're trying to encode conditional iteration without judgement about error-ness, and favor Result when you're specifically dealing with an early return due to error.

[jswrenn]

@Philippe-Cholet is this good to merge?

[Natural-selection1]

haha, it's been nearly ten months since the 2024 Edition was released. Maybe we can migrate a bit more