bug#20153: 24.4.91; destructive add-face-text-property and string deep copying

[Lars Ingebrigtsen]

Eli Zaretskii <eliz@gnu.org> writes:

>> Instead I've now changed add_properties (and add_text_properties_1) to
>> take a bool parameter to say whether they're allowed to be destructive
>> or not, and make the add-face-text-property call that with false as the
>> parameter if the object is a string.  This fixes the test case for me
>> and should hopefully have no measurable performance impact.
>
> Isn't that a backward-incompatible change?

It is, but the previous behaviour was a bug.  If you have a copy of a
string and modify the copy, the original string shouldn't change.

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