proposal: unicode/utf8: provide generic versions of all functions

[dsnet]

I'm trying to a write a generic function that operates on either string | []byte. However, I'm unable to do so since the implementation of that function needs to depend on either utf8.DecodeRune or utf8.DecodeRuneInString, but I'm unable to express that as a simple expression without using a type switch.

I propose we add generic versions of:

func DecodeRune[Bytes []byte | string](p Bytes) (r rune, size int)
func DecodeLastRune[Bytes []byte | string](p Bytes) (r rune, size int)
func FullRune[Bytes []byte | string](p Bytes) bool
func RuneCount[Bytes []byte | string](p Bytes) int
func Valid[Bytes []byte | string](p Bytes) bool

It is unclear what the name should be since the simpler names are already taken by the non-generic variants.
Perhaps, we should have a v2 variant of utf8 that operates on either type.

[bcmills]

Given the number of existing functions that operate on []byte, and the fact that functions that accept a []byte generally cannot assume they can modify it (because that can in general cause data races), I wonder if it would be more productive to have a single generic function (perhaps in the bytes package?) like:

package bytes

func Of[Bytes ~[]byte | ~string](p Bytes) []byte

It seems to me that that would handle all of the cases where a function accepts a slice-or-string and returns only data parsed from it, including all of the utf8 functions you mentioned above. (However, it would not handle cases like TrimPrefix where the return type of the function depends on the argument type.)

bytes.Of could be implemented today using reflect, although if it were in the standard library it would be permissible for the compiler to recognize and optimize it in a different way.

[aarzilli]

What's the difference between bytes.Of and a cast to []byte?

[bcmills]

@aarzilli, []byte(s) for a string s makes a mutable copy. bytes.Of would not.

(Compare my unsafeslice.OfString, although I'm becoming increasingly convinced that that operation should not actually be considered “unsafe”...)

[aarzilli]

What would happen if someone did try to mutate the return value of bytes.Of? The cast already doesn't make copies sometimes IIRC, wouldn't it be better to improve that?

[dsnet]

@bcmills Being able to mutate a string feels pretty unsafe to me.

The motivation for this proposal comes from the fact that we can't efficiently call a non-mutating string-based argument with a []byte without allocation (or vice-versa). To some degree, this is a generalization of the type of problem in #42429. Alternatively, if the compiler allowed implicit unsafe conversions of []byte to string (or string to []byte) when calling pure (non-mutating) functions, that would obviate the need for this in many applications.

The only reason I'm trying to write a generic function that operates on both string and []byte is because of performance.

[DeedleFake]

You can almost write a function yourself to convert to a string regardless of which you started with using the unreleased Go 1.20, but if you don't want to use a type switch than it still depends on implementation details that you're not supposed to:

// toString returns a string with the same backing data as v. It is
// not safe to hold the returned string if the original was a []byte
// and might be modified.
func toString[T ~string | ~[]byte](v T) string {
	// This assumes that the first word of both the string and slice
	// header structs is a pointer to the data. Unfortunately, there's
	// no way to just get the data from either in an opaque way because
	// indexes into strings aren't addressable, for good reason.
	return unsafe.String(*(**byte)(unsafe.Pointer(&v)), len(v))
}

[dsnet]

I just filed #57072 as a compiler optimization to somewhat obviate the need for this.

[gopherbot]

Change https://go.dev/cl/469556 mentions this issue: encoding/json: unify encodeState.string and encodeState.stringBytes

[rsc]

Sounds like this is on hold for better compiler optimizations before we can even consider whether this is a good API.

[dsnet]

#20881 is also another compiler optimization that would address the need for this.
If that was fixed, then we could always do:

utf8.DecodeRune([]byte(in))

regardless of whether in was already a []byte or a string.