Prefer Span<T>.Clear() over Span<T>.Fill(default)

[GrabYourPitchforks]

(Copied from #30740 (comment).)

Consider replacing this:

Span<T> theSpan = GetSpan();
theSpan.Fill(default(T));

With this:

Span<T> theSpan = GetSpan();
theSpan.Clear();

I see a handful of uses of Fill(0) or Fill(default) throughout the codebase (see callers). The Clear() method is potentially much more optimized than the Fill method depending on the T in use.

Category: Performance

[gfoidl]

Aside from the analyzer I can submit a PR changing Fill(default) to Clear.

[Joe4evr]

Is there a rule-of-thumb on when Clear() could be more optimized than Fill(default)?

[GrabYourPitchforks]

@Joe4evr currently Clear is always more optimized than Fill(default) for all T. Maybe there's an exception when the span contains only 1 or 2 elements, but those are rarities.

[jeffhandley]

I'm going to remove this from the Roslyn Analyzers project for the time-being; we will add it back to the project once we get our triage process in place for prioritizing newly filed code-analyzer issues.