add either extractor

[robjtede]

PR Type

Feature

PR Checklist

• Tests for the changes have been added / updated.
• Documentation comments have been added / updated.
• A changelog entry has been made for the appropriate packages.
• Format code with the latest stable rustfmt

Overview

Add extractor helper using the existing Either<A, B> type to handle fallbacks, like Either<Json<Login>, Form<Login>> for example.

closes #1724

[jplatte]

Why A over Left if the docs and test code use left / right terminology rather than A / B?

[robjtede]

It's because this isnt a new type (i just moved the either parts out of responder.rs), it's in my head to rename the variants in next breaking release.

[codecov]

Codecov Report

Merging #1788 (d9b83ca) into master (a929209) will increase coverage by 0.11%.
The diff coverage is 70.14%.

@@            Coverage Diff             @@
##           master    #1788      +/-   ##
==========================================
+ Coverage   53.79%   53.90%   +0.11%     
==========================================
  Files         129      130       +1     
  Lines       12201    12244      +43     
==========================================
+ Hits         6563     6600      +37     
- Misses       5638     5644       +6     

Impacted Files	Coverage Δ
src/error.rs	89.18% <ø> (ø)
src/lib.rs	68.42% <ø> (ø)
src/responder.rs	87.84% <ø> (+4.59%)	⬆️
src/types/either.rs	70.14% <70.14%> (ø)
src/types/json.rs	89.85% <0.00%> (-0.97%)	⬇️
src/types/payload.rs	85.71% <0.00%> (-0.60%)	⬇️
src/service.rs	80.55% <0.00%> (-0.47%)	⬇️
src/request.rs	94.86% <0.00%> (-0.40%)	⬇️
src/middleware/logger.rs	77.34% <0.00%> (-0.33%)	⬇️
src/rmap.rs	94.84% <0.00%> (-0.03%)	⬇️
... and 13 more

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[fakeshadow]

lgtm

[fakeshadow]

I like this extractor. The only thing I would like to improve on is avoid using another box.
Maybe something like this https://gist.github.com/fakeshadow/7f42a2b82ab46f7ae417a6233764dba7.
It's not necessary or make things better by any means though.

[robjtede]

We'll leave it for now. A box is very little considering the rest of this change.

One thing I'd just like an explicit thought on is the choice to return the error for A in the case of both failing. Currently the B error is never made available. The alternatives are to only show B's error or to return both errors in a tuple.