bug#65105: Reusing the same string as 'display on consecutive characters evades display

[JD Smith <jdtsmith@gmail.com>]

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> On Aug 5, 2023, at 3:03 PM, Eli Zaretskii <eliz@gnu.org> wrote:
> 
> tags 65105 notabug
> thanks
> 
>> From: JD Smith <jdtsmith@gmail.com>
>> Date: Sat, 5 Aug 2023 14:35:23 -0400
>> 
>> Evaluate:
>> 
>> (let ((s1 "test1")
>>      (s2 "test2"))
>>  (insert "\n"
>>          (propertize " " 'display s1)
>>          (propertize " " 'display s1)
>>          (propertize " " 'display s2)
>>          (propertize " " 'display s1)))
>> 
>> 
>> The first space display does not take effect, since the s1 string is used for two consecutive characters.  This has a practical impact for font-lock backends that use the ‘display text-property and would like to minimize string allocation.  
> 
> Emacs cannot distinguish between two consecutive characters having
> each a text property with the same value, and two characters having
> the same property.  If you think about this for a moment, you will
> understand why: we use intervals for text properties, so two adjacent
> intervals with the identical property values and one interval with
> that same value are indistinguishable (and in fact Emacs optimizes
> this during GC by making just one interval from these two).
> 
> This is not a bug.

Aha, thanks.  It does make sense from an optimization standpoint to “gang” properties in this manner.  Are you aware of any approach that allow re-using a string for ‘display, but permits consecutive intervals to remain distinct?

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