slices: add Concat

[earthboundkid]

Fixes #56353

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[6543]

my implementation uses copy instead of append

func MergeSlices[T any](slices ...[]T) []T {
	sl := 0
	for i := range slices {
		sl += len(slices[i])
	}
	result := make([]T, sl)
	cp := 0
	for _, s := range slices {
		if sLen := len(s); sLen != 0 {
			copy(result[cp:], s)
			cp += sLen
		}
	}
	return result
}

this should result in no new mem allocs if Im right

[6543]

should I send new pull or work on this?

[randall77]

I don't think your implementation is any different than https://go-review.googlesource.com/c/go/+/504882
They both do exactly one allocation of the result size.

[6543]

as there was no work done since ... I submitted my own impl. adjusted and simplified to meet the criteria -> #61817

[earthboundkid]

AFAIK, the CL is just on hold because Go 1.21 isn't out quite yet. It should be reviewed and merged soon, then you can do a CL to add new tests or benchmarks.

[6543]

ah that's why you did not work on it - sorry

@carlmjohnson should I add you as co-author to my patch or do you update yours ?!?

[earthboundkid]

I don't see a reason to update to match yours without a benchmark that shows that it's faster. Write a benchmark and then open a new CL that stacks on top of mine.

[6543]

well this need a patch anyway ... as your pull has this api
func Concat[S ~[]E, E any](slices ...S) S {}

and proposed is
func Concat[S ~[]E, E any](dst S, slices ...S) S {

not sure what has to be done when an accepted proposal has to be adjusted ?!?

[6543]

PS: I'll write some benchmarks :)

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[earthboundkid]

well this need a patch anyway ... as your pull has this api func Concat[S ~[]E, E any](slices ...S) S {}

Read the full discussion. This was the accepted API. The dst slice turned out to be unworkable because of the aliasing problem.

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Patch Set 4: Run-TryBot+1

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Patch Set 5: Run-TryBot+1

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Patch Set 5: Auto-Submit+1 Code-Review+2 Run-TryBot+1

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[earthboundkid]

@6543 slices.Concat has been added. Feel free to open a performance boost CL. Those don't need to go through the proposal process. You just need to demonstrate that it's faster, and it can be merged.