Special-case enums in interpolated string handlers

[stephentoub]

Once Type.IsEnum is an intrinsic (#71685) and the static Enum.TryFormat has been added (#71590), we should be able to update the in-box interpolated string handlers to special-case enums and use TryFormat rather than calling ToString on them, which at a minimum boxes and worst-case allocates a result string. This can either be a long-term solution, or it can be a stop-gap until Enum itself implements ISpanFormattable, which we've said we'll only do if the JIT is updated to avoid boxing when using its methods.

(We can similarly special-case anywhere optimized formatting would be helpful, e.g. maybe in StringBuilder, in ValueStringBuilder, etc.)

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Issue Details

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Once Type.IsEnum is an intrinsic (#71685) and the static Enum.TryFormat has been added (#71590), we should be able to update the in-box interpolated string handlers to special-case enums and use TryFormat rather than calling ToString on them, which at a minimum boxes and worst-case allocates a result string. This can either be a long-term solution, or it can be a stop-gap until Enum itself implements ISpanFormattable, which we've said we'll only do if the JIT is updated to avoid boxing when using its methods.

Author:	stephentoub
Assignees:	stephentoub
Labels:	area-System.Runtime, tenet-performance
Milestone:	8.0.0

[mburbea]

In the mean time, I've used this pattern to some success since 6.0 to special case code for enums.

typeof(T).IsValueType && default(T) is Enum;;

https://sharplab.io/#v2:C4LghgzgtgPgAgJgIwFgBQcAMACAygCzACcAHAGTACMA6AJQFcA7YASygFMBuddAbQCkWwAOLtG7IiwDGACmABPEuwD2AMxktmASi0AaQSLETpcxSvW5gkxgHMAwsqgliLCMsY7d2A6PGTZCkpqMnBImDoAuuhwAMzYiNh2ADwAKgB82ADe6Ni58XGhAGzYlMrKADbYALIyWtgAvGk5eS2B5jIpWtQAkhAAamDl9OwpZtgAZOPYACbsqmD05cAdda7YAKKM9FDcaAC+QA===

[MichalPetryka]

#71685 has been merged so the first part has been unblocked here.