code review 72160044: spec: clarify when constant slice indices must be in range

spec: clarify when constant slice indices must be in range

This documents the status quo for most implementations,
with one exception: gc generates a run-time error for
constant but out-of-range indices when slicing a constant
string. See issue 7200 for a detailed discussion.

[gri, 2014-03-06 22:02:02]

Hello r@golang.org, rsc@golang.org, iant@golang.org, ken@golang.org (cc:
golang-codereviews@googlegroups.com),

I'd like you to review this change to
https://code.google.com/p/go/

[gri, 2014-03-06 22:03:14]

I'm proposing this spec change for 1.3, and conversion of issue 7200 into a
gc issue for 1.4 for the one case where it's inconsistent.
- gri


On Thu, Mar 6, 2014 at 2:02 PM, <gri@golang.org> wrote:

> Reviewers: r, rsc, iant, ken2,
>
> Message:
> Hello r@golang.org, rsc@golang.org, iant@golang.org, ken@golang.org (cc:
> golang-codereviews@googlegroups.com),
>
> I'd like you to review this change to
> https://code.google.com/p/go/
>
>
> Description:
> spec: clarify when constant slice indices must be in range
>
> This documents the status quo for most implementations,
> with one exception: gc generates a run-time error for
> constant but out-of-range indices when slicing a constant
> string. See issue 7200 for a detailed discussion.
>
> Please review this at https://codereview.appspot.com/72160044/
>
> Affected files (+3, -3 lines):
>   M doc/go_spec.html
>
>
> Index: doc/go_spec.html
> ===================================================================
> --- a/doc/go_spec.html
> +++ b/doc/go_spec.html
> @@ -1,6 +1,6 @@
>  <!--{
>         "Title": "The Go Programming Language Specification",
> -       "Subtitle": "Version of March 6, 2014",
> +       "Subtitle": "Version of March 7, 2014",
>         "Path": "/ref/spec"
>  }-->
>
> @@ -2691,7 +2691,7 @@
>  otherwise they are <i>out of range</i>.
>  For slices, the upper index bound is the slice capacity
> <code>cap(a)</code> rather than the length.
>  A <a href="#Constants">constant</a> index must be non-negative and
> representable by a value of type
> -<code>int</code>.
> +<code>int</code>; for arrays or constant strings, constant indices must
> also be in range.
>  If both indices are constant, they must satisfy <code>low &lt;=
> high</code>.
>  If the indices are out of range at run time, a <a
> href="#Run_time_panics">run-time panic</a> occurs.
>  </p>
> @@ -2751,7 +2751,7 @@
>  The indices are <i>in range</i> if <code>0 &lt;= low &lt;= high &lt;= max
> &lt;= cap(a)</code>,
>  otherwise they are <i>out of range</i>.
>  A <a href="#Constants">constant</a> index must be non-negative and
> representable by a value of type
> -<code>int</code>.
> +<code>int</code>; for arrays, constant indices must also be in range.
>  If multiple indices are constant, the constants that are present must be
> in range relative to each
>  other.
>  If the indices are out of range at run time, a <a
> href="#Run_time_panics">run-time panic</a> occurs.
>
>
>

[r, 2014-03-06 23:10:11]

LGTM

[gri, 2014-03-07 01:11:27]

*** Submitted as https://code.google.com/p/go/source/detail?r=8a975cf68c20 ***

spec: clarify when constant slice indices must be in range

This documents the status quo for most implementations,
with one exception: gc generates a run-time error for
constant but out-of-range indices when slicing a constant
string. See issue 7200 for a detailed discussion.

LGTM=r
R=r, rsc, iant, ken
CC=golang-codereviews
https://codereview.appspot.com/72160044

[gobot, 2014-03-07 02:15:49]

This CL appears to have broken the linux-arm-cheney-panda builder.