Fix assertion rewriting involving Starred + side-effects

[asottile]

Resolves #4412

[codecov]

Codecov Report

Merging #4414 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #4414      +/-   ##
==========================================
+ Coverage   95.89%   95.92%   +0.02%     
==========================================
  Files         111      111              
  Lines       25002    25006       +4     
  Branches     2438     2438              
==========================================
+ Hits        23975    23986      +11     
+ Misses        726      723       -3     
+ Partials      301      297       -4

Flag	Coverage Δ
#docs	29.92% <0%> (+0.09%)	⬆️
#doctesting	29.92% <0%> (+0.09%)	⬆️
#linting	29.92% <0%> (+0.09%)	⬆️
#linux	95.74% <100%> (+0.02%)	⬆️
#nobyte	92.34% <20%> (-0.02%)	⬇️
#numpy	93.12% <20%> (-0.03%)	⬇️
#pexpect	41.73% <0%> (-0.01%)	⬇️
#py27	94.04% <20%> (ø)	⬆️
#py34	92.16% <20%> (+0.05%)	⬆️
#py35	92.18% <100%> (+0.06%)	⬆️
#py36	92.2% <100%> (+0.06%)	⬆️
#py37	94.12% <100%> (+0.02%)	⬆️
#trial	93.12% <20%> (-0.03%)	⬇️
#windows	94.12% <100%> (ø)	⬆️
#xdist	93.94% <100%> (ø)	⬆️

Impacted Files	Coverage Δ
src/_pytest/assertion/rewrite.py	95.7% <100%> (ø)	⬆️
testing/test_assertrewrite.py	83.46% <100%> (+0.07%)	⬆️
src/_pytest/capture.py	93.87% <0%> (+0.45%)	⬆️
src/_pytest/terminal.py	93.02% <0%> (+0.85%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 4c9cde7...690a63b. Read the comment docs.

[RonnyPfannschmidt]

@asottile great job, that one must have been very puzzling to find, the original "wrong" code looks so "right"

[asottile]

<3 I've spent way more than my fair share staring at ast.NodeVisitor / ast.NodeTransformers 😆