Extract array of const to static readonly field

[terrajobst]

Arrays of const passed to known methods on types like System.String (e.g. IndexOfAny(new[] { ',', '.' })) can be lifted out to static readonly fields.

Category: Performance

[jeffhandley]

Estimates:

• Analyzer: Small
• Fixer: Large

[jeffhandley]

Moving this one out of .NET 5, but this would be a good warmup analyzer for someone, and it would be a high-value analyzer in .NET 6 because it applies to some recently added APIs.

[jeffhandley]

The hardest part of this one will be coming up with the list of APIs we want this to target--using a heuristic is the recommended approach. Beyond that, we think the analyzer will be pretty easy; the fixer will have some edge cases to consider for how to rewrite the code to reflect the definition/usage of the const--simply extracting the const won't necessarily be straightforward, and there will need to be an ordered list of approaches to apply.

[jakub-klima]

Cannot optimization like this be done by the compiler?

[teo-tsirpanis]

Such optimization could be automatically done by the compiler for methods that accept a ReadOnlySpan.

[steveberdy]

@buyaa-n I would like this assigned to me. Thanks.