bug#72689: 31.0.50; Proposal to improve string-pixel-width

[David Ponce via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

On 18/08/2024 8:12 AM, Jim Porter wrote:
> On 8/17/2024 3:03 PM, David Ponce via Bug reports for GNU Emacs, the Swiss army knife of text editors wrote:
>> I propose the attached patch to make string-pixel-width faster while
>> using less memory, as shown by the following results of a basic
>> benchmark run in emacs -Q to compare the current implementation and this
>> proposal:[snip]
> 
>> -      (insert (propertize string 'line-prefix nil 'wrap-prefix nil))
>> +      (insert string)
>> +      ;; Prefer `remove-text-properties' to `propertize' to avoid
>> +      ;; creating a new string on each call.
>> +      (remove-text-properties
>> +       (point-min) (point-max) '(line-prefix nil wrap-prefix nil))
> 
> Is this change safe? I suppose most of the time, this wouldn't matter, but I could imagine a case where a caller wanted the pixel-width of a string that had one of those properties set, and they'd want those properties to be preserved (e.g. if the string was to be inserted into a buffer later).
> 
> Maybe this code could just check for the presence of 'line-prefix' or 'wrap-prefix' properties and only call 'propertize' if they're in the string? (Or maybe it's even possible to leave the string as-is and compute the width in some way that accounts for these properties in the correct way...)
> 
> I'm not sure how much this matters, but I always get a bit nervous about a function changing something about one of its arguments when it's not obvious.

My implementation proposal does not change the behavior of the
function. It is just an attempt to improve its performance.

As far as I am concerned I do not think that the values ​​of
line-prefix, wrap prefix and display-line-numbers should be taken into
account because the number of pixels of a string should not depend
on the position where this string is displayed, but only on its own
display attributes like faces and other display properties.