Stream cleanup

[grisumbras]

Fix #213

[codecov]

Codecov Report

Merging #551 (c1a299a) into develop (aae1863) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop     #551      +/-   ##
===========================================
- Coverage    99.10%   99.10%   -0.01%     
===========================================
  Files           67       67              
  Lines         6058     6040      -18     
===========================================
- Hits          6004     5986      -18     
  Misses          54       54              

Impacted Files	Coverage Δ
include/boost/json/basic_parser.hpp	100.00% <ø> (ø)
include/boost/json/detail/stream.hpp	100.00% <ø> (ø)
include/boost/json/serializer.hpp	100.00% <ø> (ø)
include/boost/json/impl/serializer.ipp	96.92% <100.00%> (-0.06%)	⬇️

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[cppalliance-bot]

Benchmark test results. More info at https://benchmark.cppalliance.org/jsonbenchmarks-pullrequests/551/pullrequest.html

[vinniefalco]

These affect performance

[grisumbras]

Yeah, I see that. Do you know the reason? The changes should have been purely cosmetical?

[vinniefalco]

The changes should have been purely cosmetical?

The local stream allows the compiler to assume there is no aliasing

[vinniefalco]

@sdkrystian knows more

[sdkrystian]

@grisumbras @vinniefalco is correct -- creating the local streams allows for the compiler to assume no aliasing. Without them, it must operate under the assumption that the referenced stream may be modified by writes through aliasable pointer types within the function (or ones deeper within the call stack).

[vinniefalco]

In retrospect maybe we should have left a comment on the declaration for local_stream explaining intent.