Avoid boxing in FrugalList

[stephentoub]

Description

All comparisons are being performed with object.Equals(object). If T is a value type, every comparison ends up boxing the argument.

Customer Impact

Less boxing means less garbage generated and less time spent on collections.

Regression

No

Testing

Just CI

Risk

Minimal. EqualityComparer<T>.Default.Equals will use object.Equals if the type doesn't provide an IEquatable<T> implementation.

[lindexi]

The other PR: #4220

[ThomasGoulet73]

Would it be worth to do something like Dictionary to devirtualize when it's a value type and cache when it's a reference type ?
https://github.com/dotnet/runtime/blob/main/src/libraries/System.Private.CoreLib/src/System/Collections/Generic/Dictionary.cs#L274-L298

[stephentoub]

Would it be worth to do something like Dictionary to devirtualize when it's a value type and cache when it's a reference type ?

I don't think so. We don't do so in the vast majority of use cases. The difference is tiny and only relevant in the hottest of hot paths when the rest of the overheads are tiny. It's also the kind of thing that is temporarily an optimization but could easily become a deoptimization as the JIT improves, e.g. it should become unnecessary in dictionary once dotnet/runtime#50446 is merged.