proposal: slices: add `Clear` function

[Jorropo]

// Clear will zero out all elements from s[len(s):cap(s)], this is usefull if they contain
// pointers as they keep alive objects pointed to even if they are past the len.
// By zeroing this range s wont keep alive the objects pointed to anymore.
func Clear[S ~[]E, E any](s S) {
 // Implementation detail, clearer than words to explain imo.
 s = s[len(s):cap(s)]
 var zero E
 for i := range s {
  s[i] = zero
 }
}

Rational Compact* and Delete* have this comment:

// When Compact discards m elements in total, it might not modify the elements
// s[len(s)-m:len(s)]. If those elements contain pointers you might consider
// zeroing those elements so that objects they reference can be garbage collected.

// Delete might not modify the elements s[len(s)-(j-i):len(s)]. If those
// elements contain pointers you might consider zeroing those elements so that
// objects they reference can be garbage collected.

Clear would make it easy to perform this operation.

[Jorropo]

It might be usefull to add a max int parameter which stop clearing once this index is reached if this index is smaller than cap(s) so that this pattern could be used:
Can easily be done by reslicing before calling Clear.

oldLen := len(s)
s = DoDelete(s)
s = DoCompact(s)
s = DoFoo(s)
slices.Clear(s, oldLen)

This is because it can be a reasonable assumption to assume what was already free capacity was already zeroed, what would need to be rezeroed would just be the range between the old len and the new len (since we expect remanents from Delete and Compact to be remaining).

[ianlancetaylor]

I don't yet see a need for a max parameter. Can't you simply write Clear(s[:max]) ?

[Jorropo]

I somehow overlooked that, this would require s[:len(s):max] but yes that solves the problem of over zeroing traling capacity.

[magical]

There is already a builtin clear function coming in Go 1.21 (#56351, 4fe46ce). It zeroes the range 0:len(s) instead of len(s):cap(s) but you can always write clear(s[len(s):cap(s)]) to do that. Do we really need slices.Clear too?

[Jorropo]

sgtm ty