Use interpolation instead of '+' concatenation for problematic expression

[Jumhyn]

Goals ⚽

Reduces the type-checking time of a debugDescription variable by at least an order of magnitude.

Implementation Details 🚧

Because type inference with + and literals has to do a lot of overload checking (for every ExpressibleBy____Literal), many expressions which use + for string concatenation with literals in contexts where type information is sparse may inadvertently cause a type checker performance explosion. In this case, we are concatenating an anonymous closure argument with a string literal, which caused a compilation time on the order of multiple seconds (on my MBP, YMMV). With this change, the type checking of this expression completes in under 100ms.

Testing Details 🔍

There's no behavior change here. We could consider adding -Xfrontend -warn-long-expression-type-checking=<limit> to the Xcode project to catch warnings about type checker performance bottlenecks.

[codecov-io]

Codecov Report

Merging #4123 into master will not change coverage.
The diff coverage is 0%.

@@          Coverage Diff           @@
##           master   #4123   +/-   ##
======================================
  Coverage    41.5%   41.5%           
======================================
  Files         119     119           
  Lines       13992   13992           
======================================
  Hits         5807    5807           
  Misses       8185    8185

Impacted Files	Coverage Δ
...Charts/Data/Implementations/ChartBaseDataSet.swift	25.82% <0%> (ø)	⬆️

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[liuxuan30]

@jjatie any feedback? Since it's just for debugging info, there is no performance change in reality.

[StephanVV]

Running xcodebuild with the following flags shows it takes a while to compile

OTHER_SWIFT_FLAGS="-Xfrontend -debug-time-function-bodies"

2346.70ms
Pods/Charts/Source/Charts/Data/Implementations/ChartBaseDataSet.swift:404:5 getter debugDescription

2180.13ms
Pods/Charts/Source/Charts/Data/Implementations/ChartBaseDataSet.swift:404:5 getter debugDescription