proposal: bytes,strings: add LastIndexRune

[gqgs]

Proposal Details

Currently these are the functions that currently exist for the Index operation

Index(s, substr string) int
IndexAny(s, chars string) int
IndexByte(s string, c byte) int
IndexFunc(s string, f func(rune) bool) int
IndexRune(s string, r rune) int

And these are the operations for the Last operation

LastIndex(s, substr string
LastIndexAny(s, chars string) int
LastIndexByte(s string, c byte) int
LastIndexFunc(s string, f func(rune) bool) int

As you can see all Last methods have a associated Index method expect for LastIndexRune.
I propose the implementation of LastIndexRune to make the association 1:1 between both of these operation sets.

[gabyhelp]

Related Issues

• proposal: slices: add LastIndex and LastIndexFunc #63128

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[adonovan]

LastRune would be equivalent to strings.LastIndexFunc(s, func(rune) bool { return true }). I agree it's not symmetric, but it also seems like something that is almost never needed, since runes usually want to be processed in order. I have no objection to adding it though.

[rothelius]

Isn't it rather a LastIndexRune that you're suggesting to add?

[gqgs]

@rothelius Yes. Thanks. I fixed the description.

But yes. I believe the initial implementation for these helpers could be as trivial as mentioned by @adonovan

// strings/strings.go

func LastIndexRune(s string, r rune) int {
	return LastIndexFunc(s, func(r2 rune) bool {
		return r2 == r
	})
}

// bytes/bytes.go

func LastIndexRune(b []byte, r rune) int {
	return LastIndexFunc(b, func(r2 rune) bool {
		return r2 == r
	})
}

Certainly simple enough to implement manually but I don't see a detriment in adding it to the standard library and hence making the APIs symmetric.