API proposal: compare chars or Runes for equality under an OrdinalIgnoreCase comparer

[GrabYourPitchforks]

Full context at #32247. But the tl;dr is that going forward, it's likely that string.Equals(a, b, StringComparison.OrdinalIgnoreCase) will no longer be exactly equivalent to string.Equals(a.ToUpperInvariant(), b.ToUpperInvariant(), StringComparison.Ordinal).

This implies that to ask whether two char (or Rune) instances are equal under a case-insensitive comparison, it's no longer sufficient to query if (char.ToUpperInvariant(charA) == char.ToUpperInvariant(charB)) { /* ... */ }. We need to introduce a separate API for code paths which need to check individual char (Rune) instances for case-insensitive equality.

API proposal

namespace System
{
    struct char
    {
        public static bool Equals(char a, char b, bool ordinalIgnoreCase = false);
    }
}

namespace System.Text
{
    struct Rune
    {
        public static bool Equals(Rune a, Rune b, bool ordinalIgnoreCase = false);
    }
}

Behavior

If ordinalIgnoreCase = false, it's a straight bitwise equality comparison.

If ordinalIgnoreCase = true, we internally calculate the two "caseless" char (Rune) values in the same manner as #32247, and then we compare those caseless values for bitwise equality. This means that it would be possible to write your own implementation of string.Equals(a, b, StringComparison.OrdinalIgnoreCase) on top of these char-by-char APIs.

/cc @tarekgh

[tarekgh]

The proposal looks good to me. I suggest changing ignoreCase to ordinalIgnoreCase to indicate we'll not use cultural casing.

[GrabYourPitchforks]

Sure, no problem. I suppose if we don't want to pollute the char and Rune API surface areas we could shove these as static methods on TextInfo. Just a thought - I don't feel a great pull either way.

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