bug#48841: bug#47711: [PATCH VERSION 2] Add new `completion-filter-completions` API and deferred highlighting

[Daniel Mendler]

On 8/14/21 9:01 AM, Eli Zaretskii wrote:
> Just to make sure we are on the same page: adding a text property to a
> string doesn't mutate a string.  Lisp programs that process these
> strings will not necessarily see any difference, and displaying those
> strings will also not show any difference if the property is not
> related to display.  So the assumption that seems to be made here,
> that adding a property is the same as mutating a string, is IMO
> inaccurate if not incorrect.

While I agree that adding text properties is not mutation of the string
text itself it still mutates the string data structure. Dmitry made a
good point about this - if a completion table uses obarray as backend to
the completion table you suddenly attach text properties to symbol
names. This is not a good idea. I actually had such an issue once in a
package I developed, where I accidentally attached text properties (via
the mutating put-text-property API) to some strings I didn't own. This
lead to unexpected side effects at other places where I didn't expect
the strings to have properties attached.

Daniel