Revert #5842

[MrAlias]

May I ask why the change, approved in #5664, was reverted? Vararg slice was allocated once and used 2/multiple times. Now it is allocated on each Add() call. There is/was even a comment, explaining the "why".

See https://go.dev/ref/spec#Passing_arguments_to_..._parameters

See https://fs.blog/chestertons-fence/

var sink []string

func abc(vals ...string) {
	sink = vals
}

func BenchmarkFuncCall(b *testing.B) {
	b.ReportAllocs()
	b.ResetTimer()
	arg := []string{"a", "b", "c"}
	for i := 0; i < b.N; i++ {
		abc(arg...)
	}
}

gives me

BenchmarkFuncCall-10    	1000000000	         0.9481 ns/op	       0 B/op	       0 allocs/op

var sink []string

func abc(vals ...string) {
	sink = vals
}

func BenchmarkFuncCall(b *testing.B) {
	b.ReportAllocs()
	b.ResetTimer()
	for i := 0; i < b.N; i++ {
		abc("a", "b", "c")
	}
}

gives me

BenchmarkFuncCall-10    	42442630	        26.69 ns/op	      48 B/op	       1 allocs/op

Put the fence back, please 😁

Originally posted by @ash2k in #5842 (comment)

[XSAM]

Should we use a more explainable fence for the comment? Something like Allocate slice once to avoid allocation for the variadic argument.

[MrAlias]

SGTM