Add web::ReqData<T> 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

Superceeds #1729 but includes original commits by @jplatte.

Adds a ReqData extractor for request data / request extensions, analogous to web::Data.

Changes from original:

• remove DerefMut impl
  • i see this as a footgun, devs may infer that data is mutated in place (in the extensions map)
• make inner field private
  • matches Data
  • can always be make pub but not the other way without breaking change
• prints a failed extraction's type name (inspired by Print unconfigured Data<T> type when attempting extraction #1743)
• provide an into_inner
• improve docs
  • adding an example
  • clarify that data is cloned
• couple words changed on app data extractor to distingush from req data
  • plus formatting tweaks

Future work:

• support unsized
• mention possibility of Rc<RefCell<T>> in docs for in-place mutation (theres a test demonstrating this)

[codecov-io]

Codecov Report

Merging #1748 into master will increase coverage by 0.22%.
The diff coverage is 90.74%.

@@            Coverage Diff             @@
##           master    #1748      +/-   ##
==========================================
+ Coverage   53.60%   53.83%   +0.22%     
==========================================
  Files         126      127       +1     
  Lines       11996    12050      +54     
==========================================
+ Hits         6431     6487      +56     
+ Misses       5565     5563       -2     

Impacted Files	Coverage Δ
src/data.rs	93.51% <ø> (ø)
src/lib.rs	68.42% <ø> (ø)
src/web.rs	85.71% <ø> (ø)
src/request_data.rs	90.74% <90.74%> (ø)
src/service.rs	81.01% <0.00%> (+1.38%)	⬆️
src/request.rs	95.25% <0.00%> (+1.58%)	⬆️

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[jplatte]

can always be make pub but not the other way without breaking change

This isn't true due to Deref behavior, as we have seen with making the Path extractors inner value public. That said, I'm fine with this.