[API Proposal]: String.Manipulation generic overloads with ReadOnlySpan<T>

[LeaFrock]

Background and motivation

I'm wondering why there are no generic overloads for String.Manipulation APIs like String.Concat or String.Join that accept ReadOnlySpan<T>. Is this due to concerns about the complexity of overload resolution for the compiler?

Recently I've been working on params T[] upgrading to params ReadOnlySpan<T>. For exmple,

private static string CombineCacheKey(string prefix, string str) => $"{prefix}:{str}";

private static string CombineCacheKey(string prefix, params int[] ids) => CombineCacheKey(prefix, string.Join(':', ids));

private static string CombineCacheKey(string prefix, params long[] ids) => CombineCacheKey(prefix, string.Join(':', ids));

They are totally blocked without string.Join<T> accepting ReadOnlySpan<T>.

One workaround is merging them as,

private static string CombineCacheKey(string prefix, params ReadOnlySpan<object> ids) => CombineCacheKey(prefix, string.Join(':', ids));.

However, since the methods are called frequently and most of the inputs are value type, I'm not sure if it could cause boxing each time.

API Proposal

public class String
{
    public static string Concat<T>(char separator, ReadOnlySpan<T> values);
    
    public static string Concat<T>(string? separator, ReadOnlySpan<T> values);

    public static string Join<T>(char separator, ReadOnlySpan<T> values);

    public static string Join<T>(string? separator, ReadOnlySpan<T> values);
}

API Usage

private static string CombineCacheKey(params ReadOnlySpan<int> ids) => string.Join<int>(':', ids);

Alternative Designs

No response

Risks

No response

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