Extend Regex alternation source generator optimization to small sets

[stephentoub]

In the Regex source generator, we special-case alternations where every branch begins with a different char, preferring to generate a switch statement, e.g. "abc|def" yields a switch statement with a case for 'a' and a case for 'd', as the implementation sees that all branches begin with a different character and thus there's no need to fall through from one case to the next, nor backtrack. However, we don't currently extend this to small sets, e.g. "[Aa]bc|[Dd]ef" could similarly benefit from a switch. This will be even more impactful now that IgnoreCase produces sets for all case-equivalent variations of a letter, e.g. "abc|def" under IgnoreCase producing the equivalent of "[Aa][Bb][Cc]|[Dd][Ee][Ff]".

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Issue Details

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In the Regex source generator, we special-case alternations where every branch begins with a different char, preferring to generate a switch statement, e.g. "abc|def" yields a switch statement with a case for 'a' and a case for 'd', as the implementation sees that all branches begin with a different character and thus there's no need to fall through from one case to the next, nor backtrack. However, we don't currently extend this to small sets, e.g. "[Aa]bc|[Dd]ef" could similarly benefit from a switch. This will be even more impactful now that IgnoreCase produces sets for all case-equivalent variations of a letter, e.g. "abc|def" under IgnoreCase producing the equivalent of "[Aa][Bb][Cc]|[Dd][Ee][Ff]".

Author:	stephentoub
Assignees:	-
Labels:	area-System.Text.RegularExpressions, tenet-performance
Milestone:	7.0.0