code review 7587045: bytes: add Capped function to set a byte slice's capacity

bytes: add Capped function to set a byte slice's capacity

Proposal. New function to set a []byte's capacity down to
its length.

This isn't a general purpose fix to Issue 1642, but it's
one of the (most?) common uses.

I'd like to be able to use this on App Engine, which doesn't
permit untrusted code from using the unsafe package.

Update issue 1642

[bradfitz, 2013-03-20 06:39:12]

Hello golang-dev@googlegroups.com,

I'd like you to review this change to
https://go.googlecode.com/hg/

[0xjnml, 2013-03-20 09:14:47]

I don't like the idea, sorry. `cap(slice)` would become either 'capacity of
slice' or any arbitrary lower number if slice was 'Capped'. That also subverts
the logic of 'append'.

I have no proof (concrete example), only a gut feeling that it has the potential
to break some existing code in mysterious ways.

[bradfitz, 2013-03-20 14:04:29]

The whole point is to subvert append.

On Wed, Mar 20, 2013 at 2:14 AM, <0xjnml@gmail.com> wrote:

> I don't like the idea, sorry. `cap(slice)` would become either 'capacity
> of slice' or any arbitrary lower number if slice was 'Capped'. That also
> subverts the logic of 'append'.
>
> I have no proof (concrete example), only a gut feeling that it has the
> potential to break some existing code in mysterious ways.
>
>
https://codereview.appspot.**com/7587045/<https://codereview.appspot.com/7587...
>

[rsc, 2013-03-20 14:54:30]

Not now please. Too late for new API, especially this fundamental.